Queue/Definition
< Queue
Implement a FIFO queue.
You are encouraged to solve this task according to the task description, using any language you may know.
Data Structure
This illustrates a data structure, a means of storing data within a program.
- Task
Elements are added at one side and popped from the other in the order of insertion.
Operations:
- push (aka enqueue) - add element
- pop (aka dequeue) - pop first element
- empty - return truth value when empty
Errors:
- handle the error of trying to pop from an empty queue (behavior depends on the language and platform)
- See
- Queue/Usage for the built-in FIFO or queue of your language or standard library.
- See also
- Array
- Associative array: Creation, Iteration
- Collections
- Compound data type
- Doubly-linked list: Definition, Element definition, Element insertion, List Traversal, Element Removal
- Linked list
- Queue: Definition, Usage
- Set
- Singly-linked list: Element definition, Element insertion, List Traversal, Element Removal
- Stack
11l
T FIFO
[Int] contents
F push(item)
.contents.append(item)
F pop()
R .contents.pop(0)
F empty()
R .contents.empty
V f = FIFO()
f.push(3)
f.push(2)
f.push(1)
L !f.empty()
print(f.pop())- Output:
3 2 1
AArch64 Assembly
/* ARM assembly AARCH64 Raspberry PI 3B */
/* program defqueue64.s */
/*******************************************/
/* Constantes file */
/*******************************************/
/* for this file see task include a file in language AArch64 assembly*/
.include "../includeConstantesARM64.inc"
.equ NBMAXIELEMENTS, 100
/*******************************************/
/* Structures */
/********************************************/
/* example structure for value of item */
.struct 0
value_ident: // ident
.struct value_ident + 8
value_value1: // value 1
.struct value_value1 + 8
value_value2: // value 2
.struct value_value2 + 8
value_fin:
/* example structure for queue */
.struct 0
queue_ptdeb: // begin pointer of item
.struct queue_ptdeb + 8
queue_ptfin: // end pointer of item
.struct queue_ptfin + 8
queue_stvalue: // structure of value item
.struct queue_stvalue + (value_fin * NBMAXIELEMENTS)
queue_fin:
/*********************************/
/* Initialized data */
/*********************************/
.data
szMessEmpty: .asciz "Empty queue. \n"
szMessNotEmpty: .asciz "Not empty queue. \n"
szMessError: .asciz "Error detected !!!!. \n"
szMessResult: .asciz "Ident : @ value 1 : @ value 2 : @ \n" // message result
szCarriageReturn: .asciz "\n"
/*********************************/
/* UnInitialized data */
/*********************************/
.bss
.align 4
Queue1: .skip queue_fin // queue memory place
stItem: .skip value_fin // value item memory place
sZoneConv: .skip 100
/*********************************/
/* code section */
/*********************************/
.text
.global main
main: // entry of program
ldr x0,qAdrQueue1 // queue structure address
bl isEmpty
cmp x0,#0
beq 1f
ldr x0,qAdrszMessEmpty
bl affichageMess // display message empty
b 2f
1:
ldr x0,qAdrszMessNotEmpty
bl affichageMess // display message not empty
2:
// init item 1
ldr x0,qAdrstItem
mov x1,#1
str x1,[x0,#value_ident]
mov x1,#11
str x1,[x0,#value_value1]
mov x1,#12
str x1,[x0,#value_value2]
ldr x0,qAdrQueue1 // queue structure address
ldr x1,qAdrstItem
bl pushQueue // add item in queue
cmp x0,#-1 // error ?
beq 99f
// init item 2
ldr x0,qAdrstItem
mov x1,#2
str x1,[x0,#value_ident]
mov x1,#21
str x1,[x0,#value_value1]
mov x1,#22
str x1,[x0,#value_value2]
ldr x0,qAdrQueue1 // queue structure address
ldr x1,qAdrstItem
bl pushQueue // add item in queue
cmp x0,#-1
beq 99f
ldr x0,qAdrQueue1 // queue structure address
bl isEmpty
cmp x0,#0 // not empty
beq 3f
ldr x0,qAdrszMessEmpty
bl affichageMess // display message empty
b 4f
3:
ldr x0,qAdrszMessNotEmpty
bl affichageMess // display message not empty
4:
ldr x0,qAdrQueue1 // queue structure address
bl popQueue // return address item
cmp x0,#-1 // error ?
beq 99f
mov x2,x0 // save item pointer
ldr x0,[x2,#value_ident]
ldr x1,qAdrsZoneConv // conversion ident
bl conversion10S // decimal conversion
ldr x0,qAdrszMessResult
ldr x1,qAdrsZoneConv
bl strInsertAtCharInc // insert result at first @ character
mov x5,x0
ldr x0,[x2,#value_value1]
ldr x1,qAdrsZoneConv // conversion value 1
bl conversion10S // decimal conversion
mov x0,x5
ldr x1,qAdrsZoneConv
bl strInsertAtCharInc // insert result at Second @ character
mov x5,x0
ldr x0,[x2,#value_value2]
ldr x1,qAdrsZoneConv // conversion value 2
bl conversion10S // decimal conversion
mov x0,x5
ldr x1,qAdrsZoneConv
bl strInsertAtCharInc // insert result at third @ character
bl affichageMess // display message final
b 4b // loop
99: // error
ldr x0,qAdrszMessError
bl affichageMess
100: // standard end of the program
mov x0,0 // return code
mov x8,EXIT // request to exit program
svc 0 // perform the system call
qAdrQueue1: .quad Queue1
qAdrstItem: .quad stItem
qAdrszMessError: .quad szMessError
qAdrszMessEmpty: .quad szMessEmpty
qAdrszMessNotEmpty: .quad szMessNotEmpty
qAdrszMessResult: .quad szMessResult
qAdrszCarriageReturn: .quad szCarriageReturn
qAdrsZoneConv: .quad sZoneConv
/******************************************************************/
/* test if queue empty */
/******************************************************************/
/* x0 contains the address of queue structure */
/* x0 returns 0 if not empty, 1 if empty */
isEmpty:
stp x1,lr,[sp,-16]! // save registers
stp x2,x3,[sp,-16]! // save registers
ldr x1,[x0,#queue_ptdeb] // begin pointer
ldr x2,[x0,#queue_ptfin] // begin pointer
cmp x1,x2
bne 1f
mov x0,#1 // empty queue
b 2f
1:
mov x0,#0 // not empty
2:
ldp x2,x3,[sp],16 // restaur 2 registers
ldp x1,lr,[sp],16 // restaur 2 registers
ret // return to address lr x30
/******************************************************************/
/* add item in queue */
/******************************************************************/
/* x0 contains the address of queue structure */
/* x1 contains the address of item */
pushQueue:
stp x1,lr,[sp,-16]! // save registers
stp x2,x3,[sp,-16]! // save registers
add x2,x0,#queue_stvalue // address of values structure
ldr x3,[x0,#queue_ptfin] // end pointer
add x2,x2,x3 // free address of queue
ldr x4,[x1,#value_ident] // load ident item
str x4,[x2,#value_ident] // and store in queue
ldr x4,[x1,#value_value1] // idem
str x4,[x2,#value_value1]
ldr x4,[x1,#value_value2]
str x4,[x2,#value_value2]
add x3,x3,#value_fin
cmp x3,#value_fin * NBMAXIELEMENTS
beq 99f
str x3,[x0,#queue_ptfin] // store new end pointer
b 100f
99:
mov x0,#-1 // error
100:
ldp x2,x3,[sp],16 // restaur 2 registers
ldp x1,lr,[sp],16 // restaur 2 registers
ret // return to address lr x30
/******************************************************************/
/* pop queue */
/******************************************************************/
/* x0 contains the address of queue structure */
popQueue:
stp x1,lr,[sp,-16]! // save registers
stp x2,x3,[sp,-16]! // save registers
mov x1,x0 // control if empty queue
bl isEmpty
cmp x0,#1 // yes -> error
beq 99f
mov x0,x1
ldr x1,[x0,#queue_ptdeb] // begin pointer
add x2,x0,#queue_stvalue // address of begin values item
add x2,x2,x1 // address of item
add x1,x1,#value_fin
str x1,[x0,#queue_ptdeb] // store nex begin pointer
mov x0,x2 // return pointer item
b 100f
99:
mov x0,#-1 // error
100:
ldp x2,x3,[sp],16 // restaur 2 registers
ldp x1,lr,[sp],16 // restaur 2 registers
ret // return to address lr x30
/********************************************************/
/* File Include fonctions */
/********************************************************/
/* for this file see task include a file in language AArch64 assembly */
.include "../includeARM64.inc"- Output:
Empty queue. Not empty queue. Ident : +1 value 1 : +11 value 2 : +12 Ident : +2 value 1 : +21 value 2 : +22 Error detected !!!!.
ACL2
(defun enqueue (x xs)
(cons x xs))
(defun dequeue (xs)
(declare (xargs :guard (and (consp xs)
(true-listp xs))))
(if (or (endp xs) (endp (rest xs)))
(mv (first xs) nil)
(mv-let (x ys)
(dequeue (rest xs))
(mv x (cons (first xs) ys)))))
(defun empty (xs)
(endp xs))
Action!
Static memory
Following solution uses fixed array as a buffer for the queue.
DEFINE MAXSIZE="200"
BYTE ARRAY queue(MAXSIZE)
BYTE queueFront=[0],queueRear=[0]
BYTE FUNC IsEmpty()
IF queueFront=queueRear THEN
RETURN (1)
FI
RETURN (0)
PROC Push(BYTE v)
BYTE rear
rear=queueRear+1
IF rear=MAXSIZE THEN
rear=0
FI
IF rear=queueFront THEN
PrintE("Error: queue is full!")
Break()
FI
queue(queueRear)=v
queueRear=rear
RETURN
BYTE FUNC Pop()
BYTE v
IF IsEmpty() THEN
PrintE("Error: queue is empty!")
Break()
FI
v=queue(queueFront)
queueFront==+1
IF queueFront=MAXSIZE THEN
queueFront=0
FI
RETURN (v)
PROC TestIsEmpty()
IF IsEmpty() THEN
PrintE("Queue is empty")
ELSE
PrintE("Queue is not empty")
FI
RETURN
PROC TestPush(BYTE v)
PrintF("Push: %B%E",v)
Push(v)
RETURN
PROC TestPop()
BYTE v
Print("Pop: ")
v=Pop()
PrintBE(v)
RETURN
PROC Main()
TestIsEmpty()
TestPush(10)
TestIsEmpty()
TestPush(31)
TestPop()
TestIsEmpty()
TestPush(5)
TestPop()
TestPop()
TestPop()
RETURNDynamic memory
Following solution uses module for dynamic memory allocation. The user must type in the monitor the following command after compilation and before running the program!
SET EndProg=*
CARD EndProg ;required for ALLOCATE.ACT
INCLUDE "D2:ALLOCATE.ACT" ;from the Action! Tool Kit. You must type 'SET EndProg=*' from the monitor after compiling, but before running this program!
DEFINE PTR="CARD"
DEFINE NODE_SIZE="3"
TYPE QueueNode=[BYTE data PTR nxt]
QueueNode POINTER queueFront,queueRear
BYTE FUNC IsEmpty()
IF queueFront=0 THEN
RETURN (1)
FI
RETURN (0)
PROC Push(BYTE v)
QueueNode POINTER node
node=Alloc(NODE_SIZE)
node.data=v
node.nxt=0
IF IsEmpty() THEN
queueFront=node
ELSE
queueRear.nxt=node
FI
queueRear=node
RETURN
BYTE FUNC Pop()
QueueNode POINTER node
BYTE v
IF IsEmpty() THEN
PrintE("Error: queue is empty!")
Break()
FI
node=queueFront
v=node.data
queueFront=node.nxt
Free(node,NODE_SIZE)
RETURN (v)
PROC TestIsEmpty()
IF IsEmpty() THEN
PrintE("Queue is empty")
ELSE
PrintE("Queue is not empty")
FI
RETURN
PROC TestPush(BYTE v)
PrintF("Push: %B%E",v)
Push(v)
RETURN
PROC TestPop()
BYTE v
Print("Pop: ")
v=Pop()
PrintBE(v)
RETURN
PROC Main()
AllocInit(0)
queueFront=0
queueRear=0
Put(125) PutE() ;clear screen
TestIsEmpty()
TestPush(10)
TestIsEmpty()
TestPush(31)
TestPop()
TestIsEmpty()
TestPush(5)
TestPop()
TestPop()
TestPop()
RETURN- Output:
Error at the end of the program is intentional. Screenshot from Atari 8-bit computer
Queue is empty Push: 10 Queue is not empty Push: 31 Pop: 10 Queue is not empty Push: 5 Pop: 31 Pop: 5 Pop: Error: queue is empty! RETURN Error: 128
Ada
The first example below demonstrates a FIFO created for single-threaded computing. This version has the advantage of using a minimum of memory per FIFO element, and being very fast.
The interface specification for a FIFO is described in the package specification.
generic
type Element_Type is private;
package Fifo is
type Fifo_Type is private;
procedure Push(List : in out Fifo_Type; Item : in Element_Type);
procedure Pop(List : in out Fifo_Type; Item : out Element_Type);
function Is_Empty(List : Fifo_Type) return Boolean;
Empty_Error : exception;
private
type Fifo_Element;
type Fifo_Ptr is access Fifo_Element;
type Fifo_Type is record
Head : Fifo_Ptr := null;
Tail : Fifo_Ptr := null;
end record;
type Fifo_Element is record
Value : Element_Type;
Next : Fifo_Ptr := null;
end record;
end Fifo;
The FIFO implementation is described in the package body:
with Ada.Unchecked_Deallocation;
package body Fifo is
----------
-- Push --
----------
procedure Push (List : in out Fifo_Type; Item : in Element_Type) is
Temp : Fifo_Ptr := new Fifo_Element'(Item, null);
begin
if List.Tail = null then
List.Tail := Temp;
end if;
if List.Head /= null then
List.Head.Next := Temp;
end if;
List.Head := Temp;
end Push;
---------
-- Pop --
---------
procedure Pop (List : in out Fifo_Type; Item : out Element_Type) is
procedure Free is new Ada.Unchecked_Deallocation(Fifo_Element, Fifo_Ptr);
Temp : Fifo_Ptr := List.Tail;
begin
if List.Head = null then
raise Empty_Error;
end if;
Item := List.Tail.Value;
List.Tail := List.Tail.Next;
if List.Tail = null then
List.Head := null;
end if;
Free(Temp);
end Pop;
--------------
-- Is_Empty --
--------------
function Is_Empty (List : Fifo_Type) return Boolean is
begin
return List.Head = null;
end Is_Empty;
end Fifo;
A "main" procedure for this program is:
with Fifo;
with Ada.Text_Io; use Ada.Text_Io;
procedure Fifo_Test is
package Int_Fifo is new Fifo(Integer);
use Int_Fifo;
My_Fifo : Fifo_Type;
Val : Integer;
begin
for I in 1..10 loop
Push(My_Fifo, I);
end loop;
while not Is_Empty(My_Fifo) loop
Pop(My_Fifo, Val);
Put_Line(Integer'Image(Val));
end loop;
end Fifo_Test;
The following implementation produces equivalent functionality by deriving from the standard Ada Container type Doubly_Linked_Lists.
This example needs fewer lines of code on the part of the application programmer, but the implementation is less efficient than the previous example. Each element has all the data members needed for a doubly linked list. It also links in all the functionality of a doubly linked list. Most of that functionality is unneeded in a FIFO.
with Ada.Containers.Doubly_Linked_Lists;
generic
type Element_Type is private;
package Generic_Fifo is
type Fifo_Type is tagged private;
procedure Push(The_Fifo : in out Fifo_Type; Item : in Element_Type);
procedure Pop(The_Fifo : in out Fifo_Type; Item : out Element_Type);
Empty_Error : Exception;
private
package List_Pkg is new Ada.Containers.Doubly_Linked_Lists(Element_Type);
use List_Pkg;
Type Fifo_Type is new List with null record;
end Generic_Fifo;
package body Generic_Fifo is
----------
-- Push --
----------
procedure Push (The_Fifo : in out Fifo_Type; Item : in Element_Type) is
begin
The_Fifo.Prepend(Item);
end Push;
---------
-- Pop --
---------
procedure Pop (The_Fifo : in out Fifo_Type; Item : out Element_Type) is
begin
if Is_Empty(The_Fifo) then
raise Empty_Error;
end if;
Item := The_Fifo.Last_Element;
The_Fifo.Delete_Last;
end Pop;
end Generic_Fifo;
with Generic_Fifo;
with Ada.Text_Io; use Ada.Text_Io;
procedure Generic_Fifo_Test is
package Int_Fifo is new Generic_Fifo(Integer);
use Int_Fifo;
My_Fifo : Fifo_Type;
Val : Integer;
begin
for I in 1..10 loop
My_Fifo.Push(I);
end loop;
while not My_Fifo.Is_Empty loop
My_Fifo.Pop(Val);
Put_Line(Integer'Image(Val));
end loop;
end Generic_Fifo_Test;
The function Is_Empty is inherited from the Lists type.
The next two examples provide simple FIFO functionality for concurrent tasks. The buffer in each example holds a single value. When running concurrent tasks, one writing to the buffer, and one reading from the buffer, either the writer will be faster than the reader, or the reader will be faster than the writer. If the writer is faster a dynamic FIFO will grow to consume all available memory on the computer. If the reader is faster the FIFO will either contain a single value or it will be empty. In either case, no implementation is more efficient than a single element buffer.
If we wish for the reader to read every value written by the writer we must synchronize the tasks. The writer can only write a new value when the buffer contains a stale value. The reader can only read a value when the value is fresh. This synchronization forces the two tasks to run at the same speed.
generic
type Element_Type is private;
package Synchronous_Fifo is
protected type Fifo is
entry Push(Item : Element_Type);
entry Pop(Item : out Element_Type);
private
Value : Element_Type;
Is_New : Boolean := False;
end Fifo;
end Synchronous_Fifo;
package body Synchronous_Fifo is
----------
-- Fifo --
----------
protected body Fifo is
---------
-- Push --
---------
entry Push (Item : Element_Type) when not Is_New is
begin
Value := Item;
Is_New := True;
end Push;
---------
-- Pop --
---------
entry Pop (Item : out Element_Type) when Is_New is
begin
Item := Value;
Is_New := False;
end Pop;
end Fifo;
end Synchronous_Fifo;
with Synchronous_Fifo;
with Ada.Text_Io; use Ada.Text_Io;
procedure Synchronous_Fifo_Test is
package Int_Fifo is new Synchronous_Fifo(Integer);
use Int_Fifo;
Buffer : Fifo;
task Writer is
entry Stop;
end Writer;
task body Writer is
Val : Positive := 1;
begin
loop
select
accept Stop;
exit;
else
select
Buffer.Push(Val);
Val := Val + 1;
or
delay 1.0;
end select;
end select;
end loop;
end Writer;
task Reader is
entry Stop;
end Reader;
task body Reader is
Val : Positive;
begin
loop
select
accept Stop;
exit;
else
select
Buffer.Pop(Val);
Put_Line(Integer'Image(Val));
or
delay 1.0;
end select;
end select;
end loop;
end Reader;
begin
delay 0.1;
Writer.Stop;
Reader.Stop;
end Synchronous_Fifo_Test;
Another choice is to cause the two tasks to run independently. The writer can write whenever it is scheduled. The reader reads whenever it is scheduled, after the writer writes the first valid value.
In this example the writer writes several values before the reader reads a value. The reader will then read that same value several times before the writer is scheduled to write more values.
In a fully asynchronous system the reader only samples the values written by the writer. There is no control over the number of values not sampled by the reader, or over the number of times the reader reads the same value.
generic
type Element_Type is private;
package Asynchronous_Fifo is
protected type Fifo is
procedure Push(Item : Element_Type);
entry Pop(Item : out Element_Type);
private
Value : Element_Type;
Valid : Boolean := False;
end Fifo;
end Asynchronous_Fifo;
You may notice that the protected type specification is remarkably similar to the synchronous example above. The only important difference is that Push is declared to be an Entry in the synchronous example while it is a procedure in the asynchronous example. Entries only execute when their boundary condition evaluates to TRUE. Procedures execute unconditionally.
package body Asynchronous_Fifo is
----------
-- Fifo --
----------
protected body Fifo is
----------
-- Push --
----------
procedure Push (Item : Element_Type) is
begin
Value := Item;
Valid := True;
end Push;
---------
-- Pop --
---------
entry Pop (Item : out Element_Type) when Valid is
begin
Item := Value;
end Pop;
end Fifo;
end Asynchronous_Fifo;
with Asynchronous_Fifo;
with Ada.Text_Io; use Ada.Text_Io;
procedure Asynchronous_Fifo_Test is
package Int_Fifo is new Asynchronous_Fifo(Integer);
use Int_Fifo;
Buffer : Fifo;
task Writer is
entry Stop;
end Writer;
task body Writer is
Val : Positive := 1;
begin
loop
select
accept Stop;
exit;
else
Buffer.Push(Val);
Val := Val + 1;
end select;
end loop;
end Writer;
task Reader is
entry Stop;
end Reader;
task body Reader is
Val : Positive;
begin
loop
select
accept Stop;
exit;
else
Buffer.Pop(Val);
Put_Line(Integer'Image(Val));
end select;
end loop;<syntaxhighlight lang="ada">
end Reader;
begin
delay 0.1;
Writer.Stop;
Reader.Stop;
end Asynchronous_Fifo_Test;
ALGOL 68
File: prelude/queue_base.a68
# -*- coding: utf-8 -*- #
CO REQUIRES:
MODE OBJLINK = STRUCT(
REF OBJLINK next,
REF OBJLINK prev,
OBJVALUE value # ... etc. required #
);
PROC obj link new = REF OBJLINK: ~;
PROC obj link free = (REF OBJLINK free)VOID: ~
END CO
# actually a pointer to the last LINK, there ITEMs are ADDED/get #
MODE OBJQUEUE = REF OBJLINK;
OBJQUEUE obj queue empty = NIL;
BOOL obj queue par = FALSE; # make code thread safe #
SEMA obj queue sema = LEVEL ABS obj queue par;
# Warning: 1 SEMA for all queues of type obj, i.e. not 1 SEMA per queue #
PROC obj queue init = (REF OBJQUEUE self)REF OBJQUEUE:
self := obj queue empty;
PROC obj queue put = (REF OBJQUEUE self, OBJVALUE obj)REF OBJQUEUE: (
REF OBJLINK out = obj link new;
IF obj queue par THEN DOWN obj queue sema FI;
IF self IS obj queue empty THEN
out := (out, out, obj) # self referal #
ELSE # join into list #
out := (self, prev OF self, obj);
next OF prev OF out := prev OF next OF out := out
FI;
IF obj queue par THEN UP obj queue sema FI;
self := out
);
# define a useful prepend/put/plusto (+=:) operator... #
PROC obj queue plusto = (OBJVALUE obj, REF OBJQUEUE self)OBJQUEUE: obj queue put(self,obj);
OP +=: = (OBJVALUE obj, REF OBJQUEUE self)REF OBJQUEUE: obj queue put(self,obj);
# a potential append/plusab (+:=) operator...
OP (REF OBJQUEUE, OBJVALUE)OBJQUEUE +:= = obj queue plusab;
#
# see if the program/coder wants the OBJ problem mended... #
PROC (REF OBJQUEUE #self#)BOOL obj queue index error mended
:= (REF OBJQUEUE self)BOOL: (abend("obj queue index error"); stop);
PROC on obj queue index error = (REF OBJQUEUE self, PROC(REF OBJQUEUE #self#)BOOL mended)VOID:
obj queue index error mended := mended;
PROC obj queue get = (REF OBJQUEUE self)OBJVALUE: (
# DOWN obj queue sema; #
IF self IS obj queue empty THEN
IF NOT obj queue index error mended(self) THEN abend("obj stack index error") FI FI;
OBJQUEUE old tail = prev OF self;
IF old tail IS self THEN # free solo member #
self := obj queue empty
ELSE # free self/tail member #
OBJQUEUE new tail = prev OF old tail;
next OF new tail := self;
prev OF self := new tail
FI;
#;UP obj queue sema #
OBJVALUE out = value OF old tail;
# give a recovery hint to the garbage collector #
obj link free(old tail);
out
);
PROC obj queue is empty = (REF OBJQUEUE self)BOOL:
self IS obj queue empty;
SKIPSee also: Queue/Usage
ALGOL W
begin
% define a Queue type that will hold StringQueueElements %
record StringQueue ( reference(StringQueueElement) front, back );
% define the StringQueueElement type %
record StringQueueElement ( string(8) element
; reference(StringQueueElement) next
);
% we would need separate types for other element types %
% adds s to the end of the StringQueue q %
procedure pushString ( reference(StringQueue) value q
; string(8) value e
) ;
begin
reference(StringQueueElement) newElement;
newElement := StringQueueElement( e, null );
if front(q) = null then begin
% adding to an empty queue %
front(q) := newElement;
back(q) := newElement
end
else begin
% the queue is not empty %
next(back(q)) := newElement;
back(q) := newElement
end
end pushString ;
% removes an element from the front of the StringQueue q %
% asserts the queue is not empty, which will stop the %
% program if it is %
string(8) procedure popString ( reference(StringQueue) value q ) ;
begin
string(8) v;
assert( not isEmptyStringQueue( q ) );
v := element(front(q));
front(q) := next(front(q));
if front(q) = null then % just popped the last element % back(q) := null;
v
end popStringQueue ;
% returns true if the StringQueue q is empty, false otherwise %
logical procedure isEmptyStringQueue ( reference(StringQueue) value q ) ; front(q) = null;
begin % test the StringQueue operations %
reference(StringQueue) q;
q := StringQueue( null, null );
pushString( q, "fred" );
pushString( q, "whilma" );
pushString( q, "betty" );
pushString( q, "barney" );
while not isEmptyStringQueue( q ) do write( popString( q ) )
end
end.- Output:
fred whilma betty barney
Applesoft BASIC
0 DEF FN E(MPTY) = SP = FIRST
10 GOSUB 150EMPTY
20 LET A$ = "A": GOSUB 100PUSH
30 LET A$ = "B": GOSUB 100PUSH
40 GOSUB 150EMPTY
50 GOSUB 120PULL FIRST
60 GOSUB 120PULL FIRST
70 GOSUB 150EMPTY
80 GOSUB 120PULL FIRST
90 END
100 PRINT "PUSH "A$
110 LET S$(SP) = A$:SP = SP + 1: RETURN
120 GOSUB 130: PRINT "POP "A$: RETURN
130 IF FN E(0) THEN PRINT "POPPING FROM EMPTY QUEUE";: STOP
140 A$ = S$(FI): FI = FI + 1 : RETURN
150 PRINT "EMPTY? " MID$ ("YESNO",4 ^ FN E(0),3): RETURNARM Assembly
/* ARM assembly Raspberry PI */
/* program defqueue.s */
/* Constantes */
.equ STDOUT, 1 @ Linux output console
.equ EXIT, 1 @ Linux syscall
.equ WRITE, 4 @ Linux syscall
.equ NBMAXIELEMENTS, 100
/*******************************************/
/* Structures */
/********************************************/
/* example structure for value of item */
.struct 0
value_ident: @ ident
.struct value_ident + 4
value_value1: @ value 1
.struct value_value1 + 4
value_value2: @ value 2
.struct value_value2 + 4
value_fin:
/* example structure for queue */
.struct 0
queue_ptdeb: @ begin pointer of item
.struct queue_ptdeb + 4
queue_ptfin: @ end pointer of item
.struct queue_ptfin + 4
queue_stvalue: @ structure of value item
.struct queue_stvalue + (value_fin * NBMAXIELEMENTS)
queue_fin:
/*********************************/
/* Initialized data */
/*********************************/
.data
szMessEmpty: .asciz "Empty queue. \n"
szMessNotEmpty: .asciz "Not empty queue. \n"
szMessError: .asciz "Error detected !!!!. \n"
szMessResult: .ascii "Ident :" @ message result
sMessIdent: .fill 11, 1, ' '
.ascii " value 1 :"
sMessValue1: .fill 11, 1, ' '
.ascii " value 2 :"
sMessValue2: .fill 11, 1, ' '
.asciz "\n"
szCarriageReturn: .asciz "\n"
/*********************************/
/* UnInitialized data */
/*********************************/
.bss
.align 4
Queue1: .skip queue_fin @ queue memory place
stItem: .skip value_fin @ value item memory place
/*********************************/
/* code section */
/*********************************/
.text
.global main
main: @ entry of program
ldr r0,iAdrQueue1 @ queue structure address
bl isEmpty
cmp r0,#0
beq 1f
ldr r0,iAdrszMessEmpty
bl affichageMess @ display message empty
b 2f
1:
ldr r0,iAdrszMessNotEmpty
bl affichageMess @ display message not empty
2:
@ init item 1
ldr r0,iAdrstItem
mov r1,#1
str r1,[r0,#value_ident]
mov r1,#11
str r1,[r0,#value_value1]
mov r1,#12
str r1,[r0,#value_value2]
ldr r0,iAdrQueue1 @ queue structure address
ldr r1,iAdrstItem
bl pushQueue @ add item in queue
cmp r0,#-1 @ error ?
beq 99f
@ init item 2
ldr r0,iAdrstItem
mov r1,#2
str r1,[r0,#value_ident]
mov r1,#21
str r1,[r0,#value_value1]
mov r1,#22
str r1,[r0,#value_value2]
ldr r0,iAdrQueue1 @ queue structure address
ldr r1,iAdrstItem
bl pushQueue @ add item in queue
cmp r0,#-1
beq 99f
ldr r0,iAdrQueue1 @ queue structure address
bl isEmpty
cmp r0,#0 @ not empty
beq 3f
ldr r0,iAdrszMessEmpty
bl affichageMess @ display message empty
b 4f
3:
ldr r0,iAdrszMessNotEmpty
bl affichageMess @ display message not empty
4:
ldr r0,iAdrQueue1 @ queue structure address
bl popQueue @ return address item
cmp r0,#-1 @ error ?
beq 99f
mov r2,r0 @ save item pointer
ldr r0,[r2,#value_ident]
ldr r1,iAdrsMessIdent @ display ident
bl conversion10 @ decimal conversion
ldr r0,[r2,#value_value1]
ldr r1,iAdrsMessValue1 @ display value 1
bl conversion10 @ decimal conversion
ldr r0,[r2,#value_value2]
ldr r1,iAdrsMessValue2 @ display value 2
bl conversion10 @ decimal conversion
ldr r0,iAdrszMessResult
bl affichageMess @ display message
b 4b @ loop
99:
@ error
ldr r0,iAdrszMessError
bl affichageMess
100: @ standard end of the program
mov r0, #0 @ return code
mov r7, #EXIT @ request to exit program
svc #0 @ perform the system call
iAdrQueue1: .int Queue1
iAdrstItem: .int stItem
iAdrszMessError: .int szMessError
iAdrszMessEmpty: .int szMessEmpty
iAdrszMessNotEmpty: .int szMessNotEmpty
iAdrszMessResult: .int szMessResult
iAdrszCarriageReturn: .int szCarriageReturn
iAdrsMessIdent: .int sMessIdent
iAdrsMessValue1: .int sMessValue1
iAdrsMessValue2: .int sMessValue2
/******************************************************************/
/* test if queue empty */
/******************************************************************/
/* r0 contains the address of queue structure */
isEmpty:
push {r1,r2,lr} @ save registres
ldr r1,[r0,#queue_ptdeb] @ begin pointer
ldr r2,[r0,#queue_ptfin] @ begin pointer
cmp r1,r2
moveq r0,#1 @ empty queue
movne r0,#0 @ not empty
pop {r1,r2,lr} @ restaur registers
bx lr @ return
/******************************************************************/
/* add item in queue */
/******************************************************************/
/* r0 contains the address of queue structure */
/* r1 contains the address of item */
pushQueue:
push {r1-r4,lr} @ save registres
add r2,r0,#queue_stvalue @ address of values structure
ldr r3,[r0,#queue_ptfin] @ end pointer
add r2,r3 @ free address of queue
ldr r4,[r1,#value_ident] @ load ident item
str r4,[r2,#value_ident] @ and store in queue
ldr r4,[r1,#value_value1] @ idem
str r4,[r2,#value_value1]
ldr r4,[r1,#value_value2]
str r4,[r2,#value_value2]
add r3,#value_fin
cmp r3,#value_fin * NBMAXIELEMENTS
moveq r0,#-1 @ error
beq 100f
str r3,[r0,#queue_ptfin] @ store new end pointer
100:
pop {r1-r4,lr} @ restaur registers
bx lr @ return
/******************************************************************/
/* pop queue */
/******************************************************************/
/* r0 contains the address of queue structure */
popQueue:
push {r1,r2,lr} @ save registres
mov r1,r0 @ control if empty queue
bl isEmpty
cmp r0,#1 @ yes -> error
moveq r0,#-1
beq 100f
mov r0,r1
ldr r1,[r0,#queue_ptdeb] @ begin pointer
add r2,r0,#queue_stvalue @ address of begin values item
add r2,r1 @ address of item
add r1,#value_fin
str r1,[r0,#queue_ptdeb] @ store nex begin pointer
mov r0,r2 @ return pointer item
100:
pop {r1,r2,lr} @ restaur registers
bx lr @ return
/******************************************************************/
/* display text with size calculation */
/******************************************************************/
/* r0 contains the address of the message */
affichageMess:
push {r0,r1,r2,r7,lr} @ save registres
mov r2,#0 @ counter length
1: @ loop length calculation
ldrb r1,[r0,r2] @ read octet start position + index
cmp r1,#0 @ if 0 its over
addne r2,r2,#1 @ else add 1 in the length
bne 1b @ and loop
@ so here r2 contains the length of the message
mov r1,r0 @ address message in r1
mov r0,#STDOUT @ code to write to the standard output Linux
mov r7, #WRITE @ code call system "write"
svc #0 @ call systeme
pop {r0,r1,r2,r7,lr} @ restaur registers */
bx lr @ return
/******************************************************************/
/* Converting a register to a decimal */
/******************************************************************/
/* r0 contains value and r1 address area */
.equ LGZONECAL, 10
conversion10:
push {r1-r4,lr} @ save registers
mov r3,r1
mov r2,#LGZONECAL
1: @ start loop
bl divisionpar10 @ r0 <- dividende. quotient ->r0 reste -> r1
add r1,#48 @ digit
strb r1,[r3,r2] @ store digit on area
cmp r0,#0 @ stop if quotient = 0
subne r2,#1 @ previous position
bne 1b @ else loop
@ end replaces digit in front of area
mov r4,#0
2:
ldrb r1,[r3,r2]
strb r1,[r3,r4] @ store in area begin
add r4,#1
add r2,#1 @ previous position
cmp r2,#LGZONECAL @ end
ble 2b @ loop
mov r1,#' '
3:
strb r1,[r3,r4]
add r4,#1
cmp r4,#LGZONECAL @ end
ble 3b
100:
pop {r1-r4,lr} @ restaur registres
bx lr @return
/***************************************************/
/* division par 10 signé */
/* Thanks to http://thinkingeek.com/arm-assembler-raspberry-pi/*
/* and http://www.hackersdelight.org/ */
/***************************************************/
/* r0 dividende */
/* r0 quotient */
/* r1 remainder */
divisionpar10:
/* r0 contains the argument to be divided by 10 */
push {r2-r4} @ save registers */
mov r4,r0
mov r3,#0x6667 @ r3 <- magic_number lower
movt r3,#0x6666 @ r3 <- magic_number upper
smull r1, r2, r3, r0 @ r1 <- Lower32Bits(r1*r0). r2 <- Upper32Bits(r1*r0)
mov r2, r2, ASR #2 @ r2 <- r2 >> 2
mov r1, r0, LSR #31 @ r1 <- r0 >> 31
add r0, r2, r1 @ r0 <- r2 + r1
add r2,r0,r0, lsl #2 @ r2 <- r0 * 5
sub r1,r4,r2, lsl #1 @ r1 <- r4 - (r2 * 2) = r4 - (r0 * 10)
pop {r2-r4}
bx lr @ return- Output:
Empty queue.
Not empty queue.
Ident :1 value 1 :11 value 2 :12
Ident :2 value 1 :21 value 2 :22
Error detected !!!!.
Arturo
define :queue [][
init: [
this\items: []
]
]
empty?: function [this :queue][
zero? this\items
]
push: function [this :queue, item][
this\items: this\items ++ item
]
pop: function [this :queue][
ensure -> not? empty? this
result: this\items\0
this\items: remove.index this\items 0
return result
]
ATS
A common theme in these examples is that there is no runtime error for popping from an empty queue. Instead, you simply cannot compile a program that tries to pop from an empty queue. The type of a queue depends on its size, and you will get a type error if that size is not proven to be nonzero.
One way to get such a proof is with an assertion that calls the is_empty predicate. But the compiler does not insert that check for you, and the example programs do not need it.
A linear linked list as a queue
(*------------------------------------------------------------------*)
#define ATS_DYNLOADFLAG 0
#include "share/atspre_staload.hats"
staload UN = "prelude/SATS/unsafe.sats"
(*------------------------------------------------------------------*)
vtypedef queue_vt (vt : vt@ype+, n : int) =
(* A list that forms the queue, and a pointer to its last node. *)
@(list_vt (vt, n), ptr)
#define NIL list_vt_nil ()
#define :: list_vt_cons
fn {}
queue_vt_nil {vt : vt@ype}
() :
queue_vt (vt, 0) =
@(NIL, the_null_ptr)
fn {}
queue_vt_is_empty
{n : int}
{vt : vt@ype}
(q : !queue_vt (vt, n)) :
[is_empty : bool | is_empty == (n == 0)]
bool is_empty =
case+ q.0 of
| NIL => true
| _ :: _ => false
fn {vt : vt@ype}
queue_vt_enqueue
{n : int}
(q : queue_vt (vt, n),
x : vt) :
(* Returns the new queue. *)
[m : int | 1 <= m; m == n + 1]
queue_vt (vt, m) =
let
val @(lst, tail_ptr) = q
prval _ = lemma_list_vt_param lst
in
case+ lst of
| ~ NIL =>
let
val lst = x :: NIL
val tail_ptr = $UN.castvwtp1{ptr} lst
in
@(lst, tail_ptr)
end
| _ :: _ =>
let
val old_tail = $UN.castvwtp0{list_vt (vt, 1)} tail_ptr
val+ @ (hd :: tl) = old_tail
(* Extend the list by one node, at its tail end. *)
val new_tail : list_vt (vt, 1) = x :: NIL
val tail_ptr = $UN.castvwtp1{ptr} new_tail
prval _ = $UN.castvwtp0{void} tl
val _ = tl := new_tail
prval _ = fold@ old_tail
prval _ = $UN.castvwtp0{void} old_tail
(* Let us cheat and simply *assert* (rather than prove) that
the list has grown by one node. *)
val lst = $UN.castvwtp0{list_vt (vt, n + 1)} lst
in
@(lst, tail_ptr)
end
end
(* The dequeue routine simply CANNOT BE CALLED with an empty queue.
It requires a queue of type queue_vt (vt, n) where n is positive. *)
fn {vt : vt@ype}
queue_vt_dequeue
{n : int | 1 <= n}
(q : queue_vt (vt, n)) :
(* Returns a tuple: the dequeued element and the new queue. *)
[m : int | 0 <= m; m == n - 1]
@(vt, queue_vt (vt, m)) =
case+ q.0 of
| ~ (x :: lst) => @(x, @(lst, q.1))
(* queue_vt is a linear type that must be freed. *)
extern fun {vt : vt@ype}
queue_vt$element_free : vt -> void
fn {vt : vt@ype}
queue_vt_free {n : int}
(q : queue_vt (vt, n)) :
void =
let
fun
loop {n : nat} .<n>. (lst : list_vt (vt, n)) : void =
case+ lst of
| ~ NIL => begin end
| ~ (hd :: tl) =>
begin
queue_vt$element_free<vt> hd;
loop tl
end
prval _ = lemma_list_vt_param (q.0)
in
loop (q.0)
end
(*------------------------------------------------------------------*)
(* An example: a queue of nonlinear strings. *)
vtypedef strq_vt (n : int) = queue_vt (string, n)
fn {} (* A parameterless template, for efficiency. *)
strq_vt_nil () : strq_vt 0 =
queue_vt_nil ()
fn {} (* A parameterless template, for efficiency. *)
strq_vt_is_empty {n : int} (q : !strq_vt n) :
[is_empty : bool | is_empty == (n == 0)] bool is_empty =
queue_vt_is_empty<> q
fn
strq_vt_enqueue {n : int} (q : strq_vt n, x : string) :
[m : int | 1 <= m; m == n + 1] strq_vt m =
queue_vt_enqueue<string> (q, x)
fn (* Impossible to... VVVVVV ...call with an empty queue. *)
strq_vt_dequeue {n : int | 1 <= n} (q : strq_vt n) :
[m : int | 0 <= m; m == n - 1] @(string, strq_vt m) =
queue_vt_dequeue<string> q
fn
strq_vt_free {n : int} (q : strq_vt n) : void =
let
implement
queue_vt$element_free<string> x =
(* A nonlinear string will be allowed to leak. (It might be
collected as garbage, however.) *)
begin end
in
queue_vt_free<string> q
end
macdef qnil = strq_vt_nil ()
overload iseqz with strq_vt_is_empty
overload << with strq_vt_enqueue
overload pop with strq_vt_dequeue
overload free with strq_vt_free
implement
main0 () =
{
val q = qnil
val _ = println! ("val q = qnil")
val _ = println! ("iseqz q = ", iseqz q)
val _ = println! ("val q = q << \"one\" << \"two\" << \"three\"")
val q = q << "one" << "two" << "three"
val _ = println! ("iseqz q = ", iseqz q)
val _ = println! ("val (x, q) = pop q")
val (x, q) = pop q
val _ = println! ("x = ", x)
val _ = println! ("val (x, q) = pop q")
val (x, q) = pop q
val _ = println! ("x = ", x)
val _ = println! ("val q = q << \"four\"")
val q = q << "four"
val _ = println! ("val (x, q) = pop q")
val (x, q) = pop q
val _ = println! ("x = ", x)
val _ = println! ("val (x, q) = pop q")
val (x, q) = pop q
val _ = println! ("x = ", x)
val _ = println! ("iseqz q = ", iseqz q)
//val (x, q) = pop q // If you uncomment this you cannot compile!
val _ = free q
}
(*------------------------------------------------------------------*)- Output:
$ patscc -O2 -DATS_MEMALLOC_GCBDW queues-postiats.dats -lgc && ./a.out val q = qnil iseqz q = true val q = q << "one" << "two" << "three" iseqz q = false val (x, q) = pop q x = one val (x, q) = pop q x = two val q = q << "four" val (x, q) = pop q x = three val (x, q) = pop q x = four iseqz q = true
A nonlinear circular queue with an automatically resizing buffer
(*------------------------------------------------------------------*)
(*
The following implementation prevents us from trying to dequeue
from an empty queue. A program that tries to do so cannot be
compiled.
However, it does not prove there are no buffer overruns.
It contains much embedded C code, for which I used the quick and
dirty "$extfcall" method.
*)
(*------------------------------------------------------------------*)
#define ATS_DYNLOADFLAG 0
#include "share/atspre_staload.hats"
(*------------------------------------------------------------------*)
(* For the demonstration, let us set BUFSIZE_INITIAL to the minimum
possible. If you try setting it any lower, though, you cannot
compile the program. *)
#define BUFSIZE_INITIAL 2
prval _ = prop_verify {2 <= BUFSIZE_INITIAL} ()
datatype queue_t (t : t@ype+,
n : int) =
| queue_t_empty (t, 0) of (size_t, ptr)
| {1 <= n}
queue_t_nonempty (t, n) of
(size_t, ptr, size_t n, size_t, size_t)
fn
queue_t_new {t : t@ype}
() : queue_t (t, 0) =
queue_t_empty (i2sz 0, the_null_ptr)
fn
queue_t_is_empty
{n : int}
{t : t@ype}
(q : queue_t (t, n)) :
[b : bool | b == (n == 0)]
bool b =
case+ q of
| queue_t_empty _ => true
| queue_t_nonempty _ => false
fn {t : t@ype}
queue_t_enqueue
{n : int}
(q : queue_t (t, n),
x : t) :
[m : int | 1 <= m; m == n + 1]
queue_t (t, m) =
let
macdef tsz = sizeof<t>
macdef zero = i2sz 0
macdef one = i2sz 1
var xvar = x
val px = addr@ xvar
in
case+ q of
| queue_t_empty (bufsize, pbuf) =>
if bufsize = zero then
let
val bufsize = i2sz BUFSIZE_INITIAL
val pbuf =
$extfcall (ptr, "ATS_MALLOC", bufsize * tsz)
val _ = $extfcall (ptr, "memcpy", pbuf, px, tsz)
in
queue_t_nonempty (bufsize, pbuf, one, zero, one)
end
else
let
val _ = $extfcall (ptr, "memcpy", pbuf, px, tsz)
in
queue_t_nonempty (bufsize, pbuf, one, zero, one)
end
| queue_t_nonempty (bufsize, pbuf, n, ihead, itail) =>
if n = bufsize then
let
(* Resize the buffer. *)
val bsize = i2sz 2 * bufsize
val _ = assertloc (itail = ihead) (* Sanity check. *)
val _ = assertloc (bufsize < bsize) (* Overflow? *)
val p = $extfcall (ptr, "ATS_MALLOC", bsize * tsz)
val _ = $extfcall (ptr, "memcpy", p,
ptr_add<t> (pbuf, ihead),
(bufsize - ihead) * tsz)
val _ = $extfcall (ptr, "memcpy",
ptr_add<t> (p, bufsize - ihead),
pbuf, ihead * tsz)
val _ = $extfcall (ptr, "memcpy", ptr_add<t> (p, n),
px, tsz)
in
queue_t_nonempty (bsize, p, succ n, zero, succ n)
end
else
let
val _ = $extfcall (ptr, "memcpy", ptr_add<t> (pbuf, itail),
px, tsz)
val itail = (succ itail) mod bufsize
in
queue_t_nonempty (bufsize, pbuf, succ n, ihead, itail)
end
end
fn {t : t@ype}
queue_t_dequeue
{n : int | 1 <= n}
(q : queue_t (t, n)) :
[m : int | m == n - 1]
@(t, queue_t (t, m)) =
let
macdef tsz = sizeof<t>
macdef zero = i2sz 0
macdef one = i2sz 1
var xvar : t
val px = addr@ xvar
val queue_t_nonempty (bufsize, pbuf, n, ihead, itail) = q
val _ = $extfcall (ptr, "memcpy", px, ptr_add<t> (pbuf, ihead),
tsz)
val ihead = (succ ihead) mod bufsize
val x = $UNSAFE.cast{t} xvar
in
if n = one then
@(x, queue_t_empty (bufsize, pbuf))
else
@(x, queue_t_nonempty (bufsize, pbuf, pred n, ihead, itail))
end
(*------------------------------------------------------------------*)
(* An example: a queue of strings. *)
vtypedef strq_t (n : int) = queue_t (string, n)
fn
strq_t_new () : strq_t 0 =
queue_t_new ()
fn {} (* A parameterless template, for efficiency. *)
strq_t_is_empty {n : int} (q : strq_t n) :
[is_empty : bool | is_empty == (n == 0)] bool is_empty =
queue_t_is_empty q
fn
strq_t_enqueue {n : int} (q : strq_t n, x : string) :
[m : int | 1 <= m; m == n + 1] strq_t m =
queue_t_enqueue<string> (q, x)
fn (* Impossible to... VVVVVV ...call with an empty queue. *)
strq_t_dequeue {n : int | 1 <= n} (q : strq_t n) :
[m : int | 0 <= m; m == n - 1] @(string, strq_t m) =
queue_t_dequeue<string> q
overload strq with strq_t_new
overload iseqz with strq_t_is_empty
overload << with strq_t_enqueue
overload pop with strq_t_dequeue
implement
main0 () =
{
val q = strq ()
val _ = println! ("val q = strq ()")
val _ = println! ("iseqz q = ", iseqz q)
val _ = println! ("val q = q << \"one\" << \"two\" << \"three\"")
val q = q << "one" << "two" << "three"
val _ = println! ("val q = q << \"ett\" << \"två\" << \"tre\"")
val q = q << "ett" << "två" << "tre"
val _ = println! ("iseqz q = ", iseqz q)
val _ = println! ("val (x, q) = pop q")
val (x, q) = pop q
val _ = println! ("x = ", x)
val _ = println! ("val (x, q) = pop q")
val (x, q) = pop q
val _ = println! ("x = ", x)
val _ = println! ("val q = q << \"four\"")
val q = q << "four"
val _ = println! ("val (x, q) = pop q")
val (x, q) = pop q
val _ = println! ("x = ", x)
val _ = println! ("val (x, q) = pop q")
val (x, q) = pop q
val _ = println! ("x = ", x)
val _ = println! ("val q = q << \"fyra\"")
val q = q << "fyra"
val _ = println! ("val (x, q) = pop q")
val (x, q) = pop q
val _ = println! ("x = ", x)
val _ = println! ("val (x, q) = pop q")
val (x, q) = pop q
val _ = println! ("x = ", x)
val _ = println! ("val (x, q) = pop q")
val (x, q) = pop q
val _ = println! ("x = ", x)
val _ = println! ("val (x, q) = pop q")
val (x, q) = pop q
val _ = println! ("x = ", x)
val _ = println! ("iseqz q = ", iseqz q)
//val (x, q) = pop q // If you uncomment this you cannot compile!
}
(*------------------------------------------------------------------*)- Output:
$ patscc -O2 -DATS_MEMALLOC_GCBDW circular_queues-postiats.dats -lgc && ./a.out val q = strq () iseqz q = true val q = q << "one" << "two" << "three" val q = q << "ett" << "två" << "tre" iseqz q = false val (x, q) = pop q x = one val (x, q) = pop q x = two val q = q << "four" val (x, q) = pop q x = three val (x, q) = pop q x = ett val q = q << "fyra" val (x, q) = pop q x = två val (x, q) = pop q x = tre val (x, q) = pop q x = four val (x, q) = pop q x = fyra iseqz q = true
AutoHotkey
push("qu", 2), push("qu", 44), push("qu", "xyz") ; TEST
MsgBox % "Len = " len("qu") ; Number of entries
While !empty("qu") ; Repeat until queue is not empty
MsgBox % pop("qu") ; Print popped values (2, 44, xyz)
MsgBox Error = %ErrorLevel% ; ErrorLevel = 0: OK
MsgBox % pop("qu") ; Empty
MsgBox Error = %ErrorLevel% ; ErrorLevel = -1: popped too much
MsgBox % "Len = " len("qu") ; Number of entries
push(queue,_) { ; push _ onto queue named "queue" (!=_), _ string not containing |
Global
%queue% .= %queue% = "" ? _ : "|" _
}
pop(queue) { ; pop value from queue named "queue" (!=_,_1,_2)
Global
RegExMatch(%queue%, "([^\|]*)\|?(.*)", _)
Return _1, ErrorLevel := -(%queue%=""), %queue% := _2
}
empty(queue) { ; check if queue named "queue" is empty
Global
Return %queue% = ""
}
len(queue) { ; number of entries in "queue"
Global
StringReplace %queue%, %queue%, |, |, UseErrorLevel
Return %queue% = "" ? 0 : ErrorLevel+1
}
AWK
#!/usr/bin/awk -f
BEGIN {
delete q
print "empty? " emptyP()
print "push " push("a")
print "push " push("b")
print "empty? " emptyP()
print "pop " pop()
print "pop " pop()
print "empty? " emptyP()
print "pop " pop()
}
function push(n) {
q[length(q)+1] = n
return n
}
function pop() {
if (emptyP()) {
print "Popping from empty queue."
exit
}
r = q[length(q)]
delete q[length(q)]
return r
}
function emptyP() {
return length(q) == 0
}
- Output:
empty? 1 push a push b empty? 0 pop b pop a empty? 1 Popping from empty queue.
BASIC
BBC BASIC
FIFOSIZE = 1000
FOR n = 3 TO 5
PRINT "Push ";n : PROCenqueue(n)
NEXT
PRINT "Pop " ; FNdequeue
PRINT "Push 6" : PROCenqueue(6)
REPEAT
PRINT "Pop " ; FNdequeue
UNTIL FNisempty
PRINT "Pop " ; FNdequeue
END
DEF PROCenqueue(n) : LOCAL f%
DEF FNdequeue : LOCAL f% : f% = 1
DEF FNisempty : LOCAL f% : f% = 2
PRIVATE fifo(), rptr%, wptr%
DIM fifo(FIFOSIZE-1)
CASE f% OF
WHEN 0:
wptr% = (wptr% + 1) MOD FIFOSIZE
IF rptr% = wptr% ERROR 100, "Error: queue overflowed"
fifo(wptr%) = n
WHEN 1:
IF rptr% = wptr% ERROR 101, "Error: queue empty"
rptr% = (rptr% + 1) MOD FIFOSIZE
= fifo(rptr%)
WHEN 2:
= (rptr% = wptr%)
ENDCASE
ENDPROC
- Output:
Push 3 Push 4 Push 5 Pop 3 Push 6 Pop 4 Pop 5 Pop 6 Pop Error: queue empty
Batch File
This solution uses an environment variable naming convention to implement a queue as a pseudo object containing a pseudo dynamic array and head and tail attributes, as well as an empty "method" that is a sort of macro. The implementation depends on delayed expansion being enabled at the time of each call to a queue function. More complex variations can be written that remove this limitation.
@echo off
setlocal enableDelayedExpansion
:::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::
:: FIFO queue usage
:: Define the queue
call :newQueue myQ
:: Populate the queue
for %%A in (value1 value2 value3) do call :enqueue myQ %%A
:: Test if queue is empty by examining the tail "attribute"
if myQ.tail==0 (echo myQ is empty) else (echo myQ is NOT empty)
:: Peek at the head of the queue
call:peekQueue myQ val && echo a peek at the head of myQueue shows !val!
:: Process the first queue value
call :dequeue myQ val && echo dequeued myQ value=!val!
:: Add some more values to the queue
for %%A in (value4 value5 value6) do call :enqueue myQ %%A
:: Process the remainder of the queue
:processQueue
call :dequeue myQ val || goto :queueEmpty
echo dequeued myQ value=!val!
goto :processQueue
:queueEmpty
:: Test if queue is empty using the empty "method"/"macro". Use of the
:: second IF statement serves to demonstrate the negation of the empty
:: "method". A single IF could have been used with an ELSE clause instead.
if %myQ.empty% echo myQ is empty
if not %myQ.empty% echo myQ is NOT empty
exit /b
:::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::
:: FIFO queue definition
:newQueue qName
set /a %~1.head=1, %~1.tail=0
:: Define an empty "method" for this queue as a sort of macro
set "%~1.empty=^!%~1.tail^! == 0"
exit /b
:enqueue qName value
set /a %~1.tail+=1
set %~1.!%~1.tail!=%2
exit /b
:dequeue qName returnVar
:: Sets errorlevel to 0 if success
:: Sets errorlevel to 1 if failure because queue was empty
if !%~1.tail! equ 0 exit /b 1
for %%N in (!%~1.head!) do (
set %~2=!%~1.%%N!
set %~1.%%N=
)
if !%~1.head! == !%~1.tail! (set /a "%~1.head=1, %~1.tail=0") else set /a %~1.head+=1
exit /b 0
:peekQueue qName returnVar
:: Sets errorlevel to 0 if success
:: Sets errorlevel to 1 if failure because queue was empty
if !%~1.tail! equ 0 exit /b 1
for %%N in (!%~1.head!) do set %~2=!%~1.%%N!
exit /b 0
BQN
Queues are already straightforward to make in BQN via its convenient builtins. This object is made for demonstration of BQN's object oriented features. It would generally be much simpler to apply the related functions to an array instead of creating a big object.
queue ← {
data ← ⟨⟩
Push ⇐ {data∾˜↩𝕩}
Pop ⇐ {
𝕊𝕩:
0=≠data ? •Show "Cannot pop from empty queue";
(data↓˜↩¯1)⊢⊑⌽data
}
Empty ⇐ {𝕊𝕩: 0=≠data}
Display ⇐ {𝕊𝕩: •Show data}
}
q1 ← queue
•Show q1.Empty@
q1.Push 3
q1.Push 4
q1.Display@
•Show q1.Pop@
q1.Display@
1
⟨ 4 3 ⟩
3
⟨ 4 ⟩
It's also possible to build a queue out of linked node objects, an approach discussed in this section of the BQN documentation. While much slower to traverse, this approach opens up new possibilities, such as constant time deletion and insertion at an arbitrary node, that aren't available with plain arrays.
Bracmat
Below, queue is the name of a class with a data member list and three methods enqueue, dequeue and empty.
No special provision is implemented to "throw and exception" in case you try to dequeue from and empty queue, because, in Bracmat, evaluation of an expression, besides resulting in an evaluated expression, always also either "succeeds" or "fails". (There is, in fact, a third possibility, "ignore", telling Bracmat to close an eye even though an evaluation didn't succeed.) So in the example below, the last dequeue operation fails and the program continues on the right hand side of the bar (|) operator
( queue
= (list=)
(enqueue=.(.!arg) !(its.list):?(its.list))
( dequeue
= x
. !(its.list):?(its.list) (.?x)
& !x
)
(empty=.!(its.list):)
)Normally you would seldom use a class as depicted above, because the operations are so simple that you probably use them directly. Bracmat lists allow prepending as well as appending elements, and single elements can be removed from the beginning or from the end of a list.
Appending an element to a long list and removing an element from the end of a long list are quite expensive operations, with a cost O(n), where n is the number of elements in the queue.
C
Dynamic array
Dynamic array working as a circular buffer.
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
typedef int DATA; /* type of data to store in queue */
typedef struct {
DATA *buf;
size_t head, tail, alloc;
} queue_t, *queue;
queue q_new()
{
queue q = malloc(sizeof(queue_t));
q->buf = malloc(sizeof(DATA) * (q->alloc = 4));
q->head = q->tail = 0;
return q;
}
int empty(queue q)
{
return q->tail == q->head;
}
void enqueue(queue q, DATA n)
{
if (q->tail >= q->alloc) q->tail = 0;
q->buf[q->tail++] = n;
// Fixed bug where it failed to resizes
if (q->tail == q->alloc) { /* needs more room */
q->buf = realloc(q->buf, sizeof(DATA) * q->alloc * 2);
if (q->head) {
memcpy(q->buf + q->head + q->alloc, q->buf + q->head,
sizeof(DATA) * (q->alloc - q->head));
q->head += q->alloc;
} else
q->tail = q->alloc;
q->alloc *= 2;
}
}
int dequeue(queue q, DATA *n)
{
if (q->head == q->tail) return 0;
*n = q->buf[q->head++];
if (q->head >= q->alloc) { /* reduce allocated storage no longer needed */
q->head = 0;
if (q->alloc >= 512 && q->tail < q->alloc / 2)
q->buf = realloc(q->buf, sizeof(DATA) * (q->alloc/=2));
}
return 1;
}
Doubly linked list
#include <stdio.h>
#include <stdlib.h>
typedef struct node_t node_t, *node, *queue;
struct node_t { int val; node prev, next; };
#define HEAD(q) q->prev
#define TAIL(q) q->next
queue q_new()
{
node q = malloc(sizeof(node_t));
q->next = q->prev = 0;
return q;
}
int empty(queue q)
{
return !HEAD(q);
}
void enqueue(queue q, int n)
{
node nd = malloc(sizeof(node_t));
nd->val = n;
if (!HEAD(q)) HEAD(q) = nd;
nd->prev = TAIL(q);
if (nd->prev) nd->prev->next = nd;
TAIL(q) = nd;
nd->next = 0;
}
int dequeue(queue q, int *val)
{
node tmp = HEAD(q);
if (!tmp) return 0;
*val = tmp->val;
HEAD(q) = tmp->next;
if (TAIL(q) == tmp) TAIL(q) = 0;
free(tmp);
return 1;
}
Test code This main function works with both implementions above.
int main()
{
int i, n;
queue q = q_new();
for (i = 0; i < 100000000; i++) {
n = rand();
if (n > RAND_MAX / 2) {
// printf("+ %d\n", n);
enqueue(q, n);
} else {
if (!dequeue(q, &n)) {
// printf("empty\n");
continue;
}
// printf("- %d\n", n);
}
}
while (dequeue(q, &n));// printf("- %d\n", n);
return 0;
}
Of the above two programs, for int types the array method is about twice as fast for the test code given. The doubly linked list is marginally faster than the sys/queue.h below.
sys/queue.h
Using the sys/queue.h, which is not POSIX.1-2001 (but it is BSD). The example allows to push/pop int values, but instead of int one can use void * and push/pop any kind of "object" (of course changes to the commodity functions m_queue and m_dequeue are needed)
#include <stdio.h>
#include <stdlib.h>
#include <stdbool.h>
#include <sys/queue.h>
struct entry {
int value;
TAILQ_ENTRY(entry) entries;
};
typedef struct entry entry_t;
TAILQ_HEAD(FIFOList_s, entry);
typedef struct FIFOList_s FIFOList;
bool m_enqueue(int v, FIFOList *l)
{
entry_t *val;
val = malloc(sizeof(entry_t));
if ( val != NULL ) {
val->value = v;
TAILQ_INSERT_TAIL(l, val, entries);
return true;
}
return false;
}
bool m_dequeue(int *v, FIFOList *l)
{
entry_t *e = l->tqh_first;
if ( e != NULL ) {
*v = e->value;
TAILQ_REMOVE(l, e, entries);
free(e);
return true;
}
return false;
}
bool isQueueEmpty(FIFOList *l)
{
if ( l->tqh_first == NULL ) return true;
return false;
}
C#
Compatible with C# 3.0 specification, requires System library for exceptions (from either .Net or Mono). A FIFO class in C# using generics and nodes.
public class FIFO<T>
{
class Node
{
public T Item { get; set; }
public Node Next { get; set; }
}
Node first = null;
Node last = null;
public void push(T item)
{
if (empty())
{
//Uses object initializers to set fields of new node
first = new Node() { Item = item, Next = null };
last = first;
}
else
{
last.Next = new Node() { Item = item, Next = null };
last = last.Next;
}
}
public T pop()
{
if (first == null)
throw new System.Exception("No elements");
if (last == first)
last = null;
T temp = first.Item;
first = first.Next;
return temp;
}
public bool empty()
{
return first == null;
}
}
C++
C++ already has a class queue in the standard library, however the following is a simple implementation based on a singly linkes list. Note that an empty queue is internally represented by head == 0, therefore it doesn't matter that the tail value is invalid in that case.
namespace rosettacode
{
template<typename T> class queue
{
public:
queue();
~queue();
void push(T const& t);
T pop();
bool empty();
private:
void drop();
struct node;
node* head;
node* tail;
};
template<typename T> struct queue<T>::node
{
T data;
node* next;
node(T const& t): data(t), next(0) {}
};
template<typename T>
queue<T>::queue():
head(0)
{
}
template<typename T>
inline void queue<T>::drop()
{
node* n = head;
head = head->next;
delete n;
}
template<typename T>
queue<T>::~queue()
{
while (!empty())
drop();
}
template<typename T>
void queue<T>::push(T const& t)
{
node*& next = head? tail->next : head;
next = new node(t);
tail = next;
}
template<typename T>
T queue<T>::pop()
{
T tmp = head->data;
drop();
return tmp;
}
template<typename T>
bool queue<T>::empty()
{
return head == 0;
}
}
Clojure
Clojure has a built-in persistent FIFO queue which can be accessed by referring to clojure.lang.PersistentQueue/EMPTY. Queues are manipulated similarly to Clojure's stacks using peek and pop.
user=> (def empty-queue clojure.lang.PersistentQueue/EMPTY)
#'user/empty-queue
user=> (def aqueue (atom empty-queue))
#'user/aqueue
; Check if queue is empty
user=> (empty? @aqueue)
true
; As with other Clojure data structures, you can add items using conj and into
user=> (swap! aqueue conj 1)
user=> (swap! aqueue into [2 3 4])
user=> (pprint @aqueue)
<-(1 2 3 4)-<
; You can read the head of the queue with peek
user=> (peek @aqueue)
1
; You can remove the head producing a new queue using pop
user=> (pprint (pop @aqueue))
<-(2 3 4)-<
; pop returns a new queue, the original is still intact
user=> (pprint @aqueue)
<-(1 2 3 4)-<
; you can treat a queue as a sequence
user=> (into [] @aqueue)
[1 2 3 4]
; but remember that using rest or next converts the queue to a seq. Compare:
user=> (-> @aqueue rest (conj 5) pprint)
(5 2 3 4)
; with:
user=> (-> @aqueue pop (conj 5) pprint)
<-(2 3 4 5)-<
Here's a link with further documentation Queues in Clojure
CoffeeScript
# Implement a fifo as an array of arrays, to
# greatly amortize dequeue costs, at some expense of
# memory overhead and insertion time. The speedup
# depends on the underlying JS implementation, but
# it's significant on node.js.
Fifo = ->
max_chunk = 512
arr = [] # array of arrays
count = 0
self =
enqueue: (elem) ->
if count == 0 or arr[arr.length-1].length >= max_chunk
arr.push []
count += 1
arr[arr.length-1].push elem
dequeue: (elem) ->
throw Error("queue is empty") if count == 0
val = arr[0].shift()
count -= 1
if arr[0].length == 0
arr.shift()
val
is_empty: (elem) ->
count == 0
# test
do ->
max = 5000000
q = Fifo()
for i in [1..max]
q.enqueue
number: i
console.log q.dequeue()
while !q.is_empty()
v = q.dequeue()
console.log v
- Output:
> time coffee fifo.coffee
{ number: 1 }
{ number: 5000000 }
real 0m2.394s
user 0m2.089s
sys 0m0.265s
Common Lisp
This defines a queue structure that stores its items in a list, and maintains a tail pointer (i.e., a pointer to the last cons in the list). Note that dequeuing the last item in the queue does not clear the tail pointer—enqueuing into the resulting empty queue will correctly reset the tail pointer.
(defstruct (queue (:constructor %make-queue))
(items '() :type list)
(tail '() :type list))
(defun make-queue ()
"Returns an empty queue."
(%make-queue))
(defun queue-empty-p (queue)
"Returns true if the queue is empty."
(endp (queue-items queue)))
(defun enqueue (item queue)
"Enqueue item in queue. Returns the queue."
(prog1 queue
(if (queue-empty-p queue)
(setf (queue-items queue) (list item)
(queue-tail queue) (queue-items queue))
(setf (cdr (queue-tail queue)) (list item)
(queue-tail queue) (cdr (queue-tail queue))))))
(defun dequeue (queue)
"Dequeues an item from queue. Signals an error if queue is empty."
(if (queue-empty-p queue)
(error "Cannot dequeue from empty queue.")
(pop (queue-items queue))))
Component Pascal
BlackBox Component Builder
MODULE Queue;
IMPORT
Boxes;
TYPE
Instance* = POINTER TO LIMITED RECORD
size: LONGINT;
first,last: LONGINT;
_queue: POINTER TO ARRAY OF Boxes.Box;
END;
PROCEDURE (self: Instance) Initialize(capacity: LONGINT),NEW;
BEGIN
self.size := 0;
self.first := 0;
self.last := 0;
NEW(self._queue,capacity)
END Initialize;
PROCEDURE New*(capacity: LONGINT): Instance;
VAR
aQueue: Instance;
BEGIN
NEW(aQueue);aQueue.Initialize(capacity);RETURN aQueue
END New;
PROCEDURE (self: Instance) IsEmpty*(): BOOLEAN, NEW;
BEGIN
RETURN self.size = 0;
END IsEmpty;
PROCEDURE (self: Instance) Capacity*(): LONGINT, NEW;
BEGIN
RETURN LEN(self._queue)
END Capacity;
PROCEDURE (self: Instance) Size*(): LONGINT, NEW;
BEGIN
RETURN self.size
END Size;
PROCEDURE (self: Instance) IsFull*(): BOOLEAN, NEW;
BEGIN
RETURN self.size = self.Capacity()
END IsFull;
PROCEDURE (self: Instance) Push*(b: Boxes.Box), NEW;
VAR
i, j, newCapacity, oldSize: LONGINT;
queue: POINTER TO ARRAY OF Boxes.Box;
BEGIN
INC(self.size);
self._queue[self.last] := b;
self.last := (self.last + 1) MOD self.Capacity();
IF self.IsFull() THEN
(* grow queue *)
newCapacity := self.Capacity() + (self.Capacity() DIV 2);
(* new queue *)
NEW(queue,newCapacity);
(* move data from old to new queue *)
i := self.first; j := 0; oldSize := self.Capacity() - self.first + self.last;
WHILE (j < oldSize) & (j < newCapacity - 1) DO
queue[j] := self._queue[i];
i := (i + 1) MOD newCapacity;INC(j)
END;
self._queue := queue;self.first := 0;self.last := j
END
END Push;
PROCEDURE (self: Instance) Pop*(): Boxes.Box, NEW;
VAR
b: Boxes.Box;
BEGIN
ASSERT(~self.IsEmpty());
DEC(self.size);
b := self._queue[self.first];
self._queue[self.first] := NIL;
self.first := (self.first + 1) MOD self.Capacity();
RETURN b
END Pop;
END Queue.
Interface extracted from implementation
DEFINITION Queue;
IMPORT Boxes;
TYPE
Instance = POINTER TO LIMITED RECORD
(self: Instance) Capacity (): LONGINT, NEW;
(self: Instance) IsEmpty (): BOOLEAN, NEW;
(self: Instance) IsFull (): BOOLEAN, NEW;
(self: Instance) Pop (): Boxes.Box, NEW;
(self: Instance) Push (b: Boxes.Box), NEW;
(self: Instance) Size (): LONGINT, NEW
END;
PROCEDURE New (capacity: LONGINT): Instance;
END Queue.
Cowgol
This code should be put in a file called queue.coh, to be used with the
Cowgol program at Queue/Usage. The queue is implemented by means of a linked list.
include "strings.coh";
include "malloc.coh";
# Define types. The calling code is expected to provide a QueueData type.
record QueueItem is
data: QueueData;
next: [QueueItem];
end record;
record QueueMeta is
head: [QueueItem];
tail: [QueueItem];
end record;
typedef Queue is [QueueMeta];
const Q_NONE := 0 as [QueueItem];
# Allocate and free the queue datastructure.
sub MakeQueue(): (q: Queue) is
q := Alloc(@bytesof QueueMeta) as Queue;
q.head := Q_NONE;
q.tail := Q_NONE;
end sub;
sub FreeQueue(q: Queue) is
var cur := q.head;
while cur != Q_NONE loop
var next := cur.next;
Free(cur as [uint8]);
cur := next;
end loop;
Free(q as [uint8]);
end sub;
# Check if queue is empty.
sub QueueEmpty(q: Queue): (r: uint8) is
r := 0;
if q.head == Q_NONE then
r := 1;
end if;
end sub;
# Enqueue and dequeue data. Cowgol has no exceptions, so the calling code
# should check QueueEmpty first.
sub Enqueue(q: Queue, d: QueueData) is
var item := Alloc(@bytesof QueueItem) as [QueueItem];
item.data := d;
item.next := Q_NONE;
if q.head == Q_NONE then
q.head := item;
else
q.tail.next := item;
end if;
q.tail := item;
end sub;
sub Dequeue(q: Queue): (d: QueueData) is
d := q.head.data;
var cur := q.head;
q.head := q.head.next;
Free(cur as [uint8]);
if q.head == Q_NONE then
q.tail := Q_NONE;
end if;
end sub;D
See code here: http://rosettacode.org/wiki/Queue/Usage#D
Déjà Vu
This uses a dictionary to have a sort of circular buffer of infinite size.
queue:
{ :start 0 :end 0 }
enqueue q item:
set-to q q!end item
set-to q :end ++ q!end
dequeue q:
if empty q:
Raise :value-error "popping from empty queue"
q! q!start
delete-from q q!start
set-to q :start ++ q!start
empty q:
= q!start q!endDelphi
program QueueDefinition;
{$APPTYPE CONSOLE}
uses
System.Generics.Collections;
type
TQueue = System.Generics.Collections.TQueue<Integer>;
TQueueHelper = class helper for TQueue
function Empty: Boolean;
function Pop: Integer;
procedure Push(const NewItem: Integer);
end;
{ TQueueHelper }
function TQueueHelper.Empty: Boolean;
begin
Result := count = 0;
end;
function TQueueHelper.Pop: Integer;
begin
Result := Dequeue;
end;
procedure TQueueHelper.Push(const NewItem: Integer);
begin
Enqueue(NewItem);
end;
var
Queue: TQueue;
i: Integer;
begin
Queue := TQueue.Create;
for i := 1 to 1000 do
Queue.push(i);
while not Queue.Empty do
Write(Queue.pop, ' ');
Writeln;
Queue.Free;
Readln;
end.
E
This uses a linked list representation of queues, hanging onto both ends of the list, except that the next-link reference is an E promise rather than a mutable slot.
Also, according to E design principles, the read and write ends of the queue are separate objects. This has two advantages; first, it implements POLA by allowing only the needed end of the queue to be handed out to its users; second, if the reader end is garbage collected the contents of the queue automatically will be as well (rather than accumulating if the writer continues writing).
def makeQueue() {
def [var head, var tail] := Ref.promise()
def writer {
to enqueue(value) {
def [nh, nt] := Ref.promise()
tail.resolve([value, nh])
tail := nt
}
}
def reader {
to empty() { return !Ref.isResolved(head) }
to dequeue(whenEmpty) {
if (Ref.isResolved(head)) {
def [value, next] := head
head := next
return value
} else {
throw.eject(whenEmpty, "pop() of empty queue")
}
}
}
return [reader, writer]
}EasyLang
A double-linked list is used, which is implemented via an expandable array.
prefix qu_
global q[] head tail .
#
proc enq n . .
if tail = 0
head = 1
else
q[tail + 2] = len q[] + 1
.
q[] &= n
q[] &= tail
q[] &= 0
tail = len q[] - 2
.
func deq .
if head = 0
return 0 / 0
.
r = q[head]
old = head
head = q[head + 2]
last = len q[]
prev = q[last - 1]
if prev <> 0
q[prev + 2] = old
.
next = q[last]
if next <> 0
q[next + 1] = old
else
tail = old
.
q[old] = q[last - 2]
q[old + 1] = q[last - 1]
q[old + 2] = q[last]
len q[] -3
if head = len q[] + 1
head = old
.
if head <> 0
q[head + 1] = 0
else
tail = 0
.
return r
.
func empty .
return if head = 0
.
prefix
#
qu_enq 2
qu_enq 5
qu_enq 7
while qu_empty = 0
print qu_deq
.
EchoLisp
There is no native queue type in EchoLisp. make-Q implements queues in message passing style, using vector operations. Conversions from-to lists are also provided.
;; put info string in permanent storage for later use
(info 'make-Q
"usage: (define q (make-Q)) ; (q '[top | empty? | pop | push value | to-list | from-list list])")
;; make-Q
(define (make-Q)
(let ((q (make-vector 0)))
(lambda (message . args)
(case message
((empty?) (vector-empty? q))
((top) (if (vector-empty? q) (error 'Q:top:empty q) (vector-ref q 0)))
((push) (vector-push q (car args)))
((pop) (if (vector-empty? q) (error 'Q:pop:empty q) (vector-shift q)))
((to-list) (vector->list q))
((from-list) (set! q (list->vector (car args))) q )
(else (info 'make-Q) (error "Q:bad message:" message )))))) ; display info if unknown message
;;
(define q (make-Q))
(q 'empty?) → #t
(q 'push 'first) → first
(q 'push 'second) → second
(q 'pop) → first
(q 'pop) → second
(q 'top)
"💬 error: Q:top:empty #()"
(q 'from-list '( 6 7 8)) → #( 6 7 8)
(q 'top) → 6
(q 'pop) → 6
(q 'to-list)→ (7 8)
(q 'delete)
"💭 error: Q:bad message: delete"
;; save make-Q
(local-put 'make-Q)
Elena
ELENA 6.x :
import extensions;
class queue<T>
{
T[] theArray;
int theTop;
int theTale;
constructor()
{
theArray := new T[](8);
theTop := 0;
theTale := 0;
}
bool empty()
= theTop == theTale;
push(T object)
{
if (theTale > theArray.Length)
{
theArray := theArray.reallocate(theTale)
};
theArray[theTale] := object;
theTale += 1
}
T pop()
{
if (theTale == theTop)
{ InvalidOperationException.new("Queue is empty").raise() };
T item := theArray[theTop];
theTop += 1;
^ item
}
}
public program()
{
queue<int> q := new queue<int>();
q.push(1);
q.push(2);
q.push(3);
console.printLine(q.pop());
console.printLine(q.pop());
console.printLine(q.pop());
console.printLine("a queue is ", q.empty().iif("empty","not empty"));
console.print("Trying to pop:");
try
{
q.pop()
}
catch(Exception e)
{
console.printLine(e.Message)
}
}- Output:
1 2 3 a queue is empty Trying to pop:Queue is empty
Elisa
This is a generic Queue component based on bi-directional lists. See how in Elisa these lists are defined.
component GenericQueue ( Queue, Element );
type Queue;
Queue (MaxLength = integer) -> Queue;
Length( Queue ) -> integer;
Empty ( Queue ) -> boolean;
Full ( Queue ) -> boolean;
Push ( Queue, Element) -> nothing;
Pull ( Queue ) -> Element;
begin
Queue (MaxLength) = Queue:[ MaxLength; length:=0; list=alist(Element) ];
Length ( queue ) = queue.length;
Empty ( queue ) = (queue.length <= 0);
Full ( queue ) = (queue.length >= queue.MaxLength);
Push ( queue, element ) =
[ exception (Full(queue), "Queue Overflow");
queue.length:= queue.length + 1;
add (queue.list, element)];
Pull ( queue ) =
[ exception (Empty(queue), "Queue Underflow");
queue.length:= queue.length - 1;
remove(first(queue.list))];
end component GenericQueue;In the following tests we will also show how the internal structure of the queue can be made visible to support debugging.
use GenericQueue (QueueofPersons, Person);
type Person = text;
Q = QueueofPersons(25);
Push (Q, "Peter");
Push (Q, "Alice");
Push (Q, "Edward");
Q?
QueueofPersons:[MaxLength = 25;
length = 3;
list = { "Peter",
"Alice",
"Edward"}]
Pull (Q)?
"Peter"
Pull (Q)?
"Alice"
Pull (Q)?
"Edward"
Q?
QueueofPersons:[MaxLength = 25;
length = 0;
list = { }]
Pull (Q)?
***** Exception: Queue UnderflowElixir
defmodule Queue do
def new, do: {Queue, [], []}
def push({Queue, input, output}, x), do: {Queue, [x|input], output}
def pop({Queue, [], []}), do: (raise RuntimeError, message: "empty Queue")
def pop({Queue, input, []}), do: pop({Queue, [], Enum.reverse(input)})
def pop({Queue, input, [h|t]}), do: {h, {Queue, input, t}}
def empty?({Queue, [], []}), do: true
def empty?({Queue, _, _}), do: false
end
Example:
iex(1)> c("queue.ex")
[Queue]
iex(2)> q = Queue.new
{Queue, [], []}
iex(3)> Queue.empty?(q)
true
iex(4)> q2 = Queue.push(q,1)
{Queue, [1], []}
iex(5)> q3 = Queue.push(q2,2)
{Queue, [2, 1], []}
iex(6)> Queue.empty?(q3)
false
iex(7)> Queue.pop(q3)
{1, {Queue, [], [2]}}
iex(8)> {popped, ^q} = Queue.pop(q2)
{1, {Queue, [], []}}
iex(9)> Queue.pop(Queue.new)
** (RuntimeError) empty Queue
queue.ex:6: Queue.pop/1
Erlang
The standard way to manage fifo in functional programming is to use a pair of list for the fifo queue, one is the input, the other is the output. When the output is empty just take the input list and reverse it.
-module(fifo).
-export([new/0, push/2, pop/1, empty/1]).
new() -> {fifo, [], []}.
push({fifo, In, Out}, X) -> {fifo, [X|In], Out}.
pop({fifo, [], []}) -> erlang:error('empty fifo');
pop({fifo, In, []}) -> pop({fifo, [], lists:reverse(In)});
pop({fifo, In, [H|T]}) -> {H, {fifo, In, T}}.
empty({fifo, [], []}) -> true;
empty({fifo, _, _}) -> false.
Note that there exists a 'queue' module in the standard library handling this for you in the first place
ERRE
With ERRE 3.0 you can use a class to define the task (in C-64 version you can simply use procedures):
PROGRAM CLASS_DEMO
CLASS QUEUE
LOCAL SP
LOCAL DIM STACK[100]
FUNCTION ISEMPTY()
ISEMPTY=(SP=0)
END FUNCTION
PROCEDURE INIT
SP=0
END PROCEDURE
PROCEDURE POP(->XX)
XX=STACK[SP]
SP=SP-1
END PROCEDURE
PROCEDURE PUSH(XX)
SP=SP+1
STACK[SP]=XX
END PROCEDURE
END CLASS
NEW PILA:QUEUE
BEGIN
PILA_INIT ! constructor
FOR N=1 TO 4 DO ! push 4 numbers
PRINT("Push";N)
PILA_PUSH(N)
END FOR
FOR I=1 TO 5 DO ! pop 5 numbers
IF NOT PILA_ISEMPTY() THEN
PILA_POP(->N)
PRINT("Pop";N)
ELSE
PRINT("Queue is empty!")
END IF
END FOR
PRINT("* End *")
END PROGRAM- Output:
Push 1 Push 2 Push 3 Push 4 Pop 4 Pop 3 Pop 2 Pop 1 Queue is empty! * End *
Factor
USING: accessors kernel ;
IN: rosetta-code.queue-definition
TUPLE: queue head tail ;
TUPLE: node value next ;
: <queue> ( -- queue ) queue new ;
: <node> ( obj -- node ) node new swap >>value ;
: empty? ( queue -- ? ) head>> >boolean not ;
: enqueue ( obj queue -- )
[ <node> ] dip 2dup dup empty?
[ head<< ] [ tail>> next<< ] if tail<< ;
: dequeue ( queue -- obj )
dup empty? [ "Cannot dequeue empty queue." throw ] when
[ head>> value>> ] [ head>> next>> ] [ head<< ] tri ;
Fantom
class Queue
{
List queue := [,]
public Void push (Obj obj)
{
queue.add (obj) // add to right of list
}
public Obj pop ()
{
if (queue.isEmpty)
throw (Err("queue is empty"))
else
{
return queue.removeAt(0) // removes left-most item
}
}
public Bool isEmpty ()
{
queue.isEmpty
}
}Forth
This is a FIFO implemented as a circular buffer, as is often found between communicating processes such the interrupt and user parts of a device driver. In practice, the get/put actions would block instead of aborting if the queue is empty/full.
1024 constant size
create buffer size cells allot
here constant end
variable head buffer head !
variable tail buffer tail !
variable used 0 used !
: empty? used @ 0= ;
: full? used @ size = ;
: next ( ptr -- ptr )
cell+ dup end = if drop buffer then ;
: put ( n -- )
full? abort" buffer full"
\ begin full? while pause repeat
tail @ ! tail @ next tail ! 1 used +! ;
: get ( -- n )
empty? abort" buffer empty"
\ begin empty? while pause repeat
head @ @ head @ next head ! -1 used +! ;
Linked list version
Using Forth-2012 structure words and ALLOCATE/FREE. In spirit quite similar to the Java variant below, with one difference: Here we use addresses of fields (not possible in Java), which often makes things simpler than in Java (fewer special cases at boundaries), but in this case it does not. Where the Java version has a special case on enqueue, this version has a special case on dequeue:
0
field: list-next
field: list-val
constant list-struct
: insert ( x list-addr -- )
list-struct allocate throw >r
swap r@ list-val !
dup @ r@ list-next !
r> swap ! ;
: remove ( list-addr -- x )
>r r@ @ ( list-node )
r@ @ dup list-val @ ( list-node x )
swap list-next @ r> !
swap free throw ;
0
field: queue-last \ points to the last entry (head of the list)
field: queue-nextaddr \ points to the pointer to the next-inserted entry
constant queue-struct
: init-queue ( queue -- )
>r 0 r@ queue-last !
r@ queue-last r> queue-nextaddr ! ;
: make-queue ( -- queue )
queue-struct allocate throw dup init-queue ;
: empty? ( queue -- f )
queue-last @ 0= ;
: enqueue ( x queue -- )
dup >r queue-nextaddr @ insert
r@ queue-nextaddr @ @ list-next r> queue-nextaddr ! ;
: dequeue ( queue -- x )
dup empty? abort" dequeue applied to an empty queue"
dup queue-last remove ( queue x )
over empty? if
over init-queue then
nip ;
Fortran
See FIFO (usage) for an example of fifo_nodes
module FIFO
use fifo_nodes
! fifo_nodes must define the type fifo_node, with the two field
! next and valid, for queue handling, while the field datum depends
! on the usage (see [[FIFO (usage)]] for an example)
! type fifo_node
! integer :: datum
! ! the next part is not variable and must be present
! type(fifo_node), pointer :: next
! logical :: valid
! end type fifo_node
type fifo_head
type(fifo_node), pointer :: head, tail
end type fifo_head
contains
subroutine new_fifo(h)
type(fifo_head), intent(out) :: h
nullify(h%head)
nullify(h%tail)
end subroutine new_fifo
subroutine fifo_enqueue(h, n)
type(fifo_head), intent(inout) :: h
type(fifo_node), intent(inout), target :: n
if ( associated(h%tail) ) then
h%tail%next => n
h%tail => n
else
h%tail => n
h%head => n
end if
nullify(n%next)
end subroutine fifo_enqueue
subroutine fifo_dequeue(h, n)
type(fifo_head), intent(inout) :: h
type(fifo_node), intent(out), target :: n
if ( associated(h%head) ) then
n = h%head
if ( associated(n%next) ) then
h%head => n%next
else
nullify(h%head)
nullify(h%tail)
end if
n%valid = .true.
else
n%valid = .false.
end if
nullify(n%next)
end subroutine fifo_dequeue
function fifo_isempty(h) result(r)
logical :: r
type(fifo_head), intent(in) :: h
if ( associated(h%head) ) then
r = .false.
else
r = .true.
end if
end function fifo_isempty
end module FIFO
Free Pascal
program queue;
{$IFDEF FPC}{$MODE DELPHI}{$IFDEF WINDOWS}{$APPTYPE CONSOLE}{$ENDIF}{$ENDIF}
{$ASSERTIONS ON}
uses Generics.Collections;
var
lQueue: TQueue<Integer>;
begin
lQueue := TQueue<Integer>.Create;
try
lQueue.EnQueue(1);
lQueue.EnQueue(2);
lQueue.EnQueue(3);
Write(lQueue.DeQueue:2); // 1
Write(lQueue.DeQueue:2); // 2
Writeln(lQueue.DeQueue:2); // 3
Assert(lQueue.Count = 0, 'Queue is not empty'); // should be empty
finally
lQueue.Free;
end;
end.
Output: 1 2 3
FreeBASIC
We first use a macro to define a generic Queue type :
' FB 1.05.0 Win64
' queue_rosetta.bi
' simple generic Queue type
#Define Queue(T) Queue_##T
#Macro Declare_Queue(T)
Type Queue(T)
Public:
Declare Constructor()
Declare Destructor()
Declare Property capacity As Integer
Declare Property count As Integer
Declare Property empty As Boolean
Declare Property front As T
Declare Function pop() As T
Declare Sub push(item As T)
Private:
a(any) As T
count_ As Integer = 0
Declare Function resize(size As Integer) As Integer
End Type
Constructor Queue(T)()
Redim a(0 To 0) '' create a default T instance for various purposes
End Constructor
Destructor Queue(T)()
Erase a
End Destructor
Property Queue(T).capacity As Integer
Return UBound(a)
End Property
Property Queue(T).count As Integer
Return count_
End Property
Property Queue(T).empty As Boolean
Return count_ = 0
End Property
Property Queue(T).front As T
If count_ > 0 Then
Return a(1)
End If
Print "Error: Attempted to access 'front' element of an empty queue"
Return a(0) '' return default element
End Property
Function Queue(T).pop() As T
If count_ > 0 Then
Dim value As T = a(1)
If count_ > 1 Then '' move remaining elements to fill space vacated
For i As Integer = 2 To count_
a(i - 1) = a(i)
Next
End If
a(count_) = a(0) '' zero last element
count_ -= 1
Return value
End If
Print "Error: Attempted to remove 'front' element of an empty queue"
Return a(0) '' return default element
End Function
Sub Queue(T).push(item As T)
Dim size As Integer = UBound(a)
count_ += 1
If count_ > size Then
size = resize(size)
Redim Preserve a(0 to size)
End If
a(count_) = item
End Sub
Function Queue(T).resize(size As Integer) As Integer
If size = 0 Then
size = 4
ElseIf size <= 32 Then
size = 2 * size
Else
size += 32
End If
Return size
End Function
#EndMacro
We now use this type to create a Queue of Cat instances :
' FB 1.05.0 Win64
#Include "queue_rosetta.bi"
Type Cat
name As String
age As Integer
Declare Constructor
Declare Constructor(name_ As string, age_ As integer)
Declare Operator Cast() As String
end type
Constructor Cat '' default constructor
End Constructor
Constructor Cat(name_ As String, age_ As Integer)
name = name_
age = age_
End Constructor
Operator Cat.Cast() As String
Return "[" + name + ", " + Str(age) + "]"
End Operator
Declare_Queue(Cat) '' expand Queue type for Cat instances
Dim CatQueue As Queue(Cat)
Var felix = Cat("Felix", 8)
Var sheba = Cat("Sheba", 4)
Var fluffy = Cat("Fluffy", 2)
With CatQueue '' push these Cat instances into the Queue
.push(felix)
.push(sheba)
.push(fluffy)
End With
Print "Number of Cats in the Queue :" ; CatQueue.count
Print "Capacity of Cat Queue :" ; CatQueue.capacity
Print "Front Cat : "; CatQueue.front
CatQueue.pop()
Print "Front Cat now : "; CatQueue.front
Print "Number of Cats in the Queue :" ; CatQueue.count
CatQueue.pop()
Print "Front Cat now : "; CatQueue.front
Print "Number of Cats in the Queue :" ; CatQueue.count
Print "Is Queue empty now : "; CatQueue.empty
catQueue.pop()
Print "Number of Cats in the Queue :" ; CatQueue.count
Print "Is Queue empty now : "; CatQueue.empty
catQueue.pop()
Print
Print "Press any key to quit"
Sleep
- Output:
Number of Cats in the Queue : 3 Capacity of Cat Queue : 4 Front Cat : [Felix, 8] Front Cat now : [Sheba, 4] Number of Cats in the Queue : 2 Front Cat now : [Fluffy, 2] Number of Cats in the Queue : 1 Is Queue empty now : false Number of Cats in the Queue : 0 Is Queue empty now : true Error: Attempted to remove 'front' element of an empty queue
GAP
Enqueue := function(v, x)
Add(v[1], x);
end;
Dequeue := function(v)
if IsEmpty(v[2]) then
if IsEmpty(v[1]) then
return fail;
else
v[2] := Reversed(v[1]);
v[1] := [];
fi;
fi;
return Remove(v[2]);
end;
# a new queue
v := [[], []];
Enqueue(v, 3);
Enqueue(v, 4);
Enqueue(v, 5);
Dequeue(v);
# 3
Enqueue(v, 6);
Dequeue(v);
# 4
Dequeue(v);
# 5
Dequeue(v);
# 6
Dequeue(v);
# fail
Go
Hard coded to be a queue of strings. Implementation is a circular buffer which grows as needed.
package queue
// int queue
// the zero object is a valid queue ready to be used.
// items are pushed at tail, popped at head.
// tail = -1 means queue is full
type Queue struct {
b []string
head, tail int
}
func (q *Queue) Push(x string) {
switch {
// buffer full. reallocate.
case q.tail < 0:
next := len(q.b)
bigger := make([]string, 2*next)
copy(bigger[copy(bigger, q.b[q.head:]):], q.b[:q.head])
bigger[next] = x
q.b, q.head, q.tail = bigger, 0, next+1
// zero object. make initial allocation.
case len(q.b) == 0:
q.b, q.head, q.tail = make([]string, 4), 0 ,1
q.b[0] = x
// normal case
default:
q.b[q.tail] = x
q.tail++
if q.tail == len(q.b) {
q.tail = 0
}
if q.tail == q.head {
q.tail = -1
}
}
}
func (q *Queue) Pop() (string, bool) {
if q.head == q.tail {
return "", false
}
r := q.b[q.head]
if q.tail == -1 {
q.tail = q.head
}
q.head++
if q.head == len(q.b) {
q.head = 0
}
return r, true
}
func (q *Queue) Empty() bool {
return q.head == q.tail
}
Groovy
Solution:
class Queue {
private List buffer
public Queue(List buffer = new LinkedList()) {
assert buffer != null
assert buffer.empty
this.buffer = buffer
}
def push (def item) { buffer << item }
final enqueue = this.&push
def pop() {
if (this.empty) throw new NoSuchElementException('Empty Queue')
buffer.remove(0)
}
final dequeue = this.&pop
def getEmpty() { buffer.empty }
String toString() { "Queue:${buffer}" }
}
Test:
def q = new Queue()
assert q.empty
['Crosby', 'Stills'].each { q.push(it) }
assert !q.empty
['Nash', 'Young'].each { q.enqueue(it) }
println q
assert !q.empty
assert q.pop() == 'Crosby'
println q
assert !q.empty
assert q.dequeue() == 'Stills'
println q
assert !q.empty
assert q.pop() == 'Nash'
println q
assert !q.empty
q.push('Crazy Horse')
println q
assert q.dequeue() == 'Young'
println q
assert !q.empty
assert q.pop() == 'Crazy Horse'
println q
assert q.empty
try { q.pop() } catch (NoSuchElementException e) { println e }
try { q.dequeue() } catch (NoSuchElementException e) { println e }
- Output:
Queue:[Crosby, Stills, Nash, Young] Queue:[Stills, Nash, Young] Queue:[Nash, Young] Queue:[Young] Queue:[Young, Crazy Horse] Queue:[Crazy Horse] Queue:[] java.util.NoSuchElementException: Empty Queue java.util.NoSuchElementException: Empty Queue
Haskell
The standard way to manage fifo in functional programming is to use a pair of list for the fifo queue, one is the input, the other is the output. When the output is empty just take the input list and reverse it.
data Fifo a = F [a] [a]
emptyFifo :: Fifo a
emptyFifo = F [] []
push :: Fifo a -> a -> Fifo a
push (F input output) item = F (item:input) output
pop :: Fifo a -> (Maybe a, Fifo a)
pop (F input (item:output)) = (Just item, F input output)
pop (F [] [] ) = (Nothing, F [] [])
pop (F input [] ) = pop (F [] (reverse input))
isEmpty :: Fifo a -> Bool
isEmpty (F [] []) = True
isEmpty _ = False
Icon and Unicon
Icon
The following works in both Icon and Unicon:
- Output:
Popped value: 1 Popped value: 2 Popped value: 3 Popped value: 4 Popped value: 5 empty queue empty queue
Unicon
Unicon also provides classes:
# Use a class to hold a Queue, with a list as the concrete implementation
class Queue (items)
method push (item)
put (items, item)
end
# if the queue is empty, this will 'fail' and return nothing
method take ()
return pop (items)
end
method is_empty ()
return *items = 0
end
initially () # initialises the field on creating an instance
items := []
end
procedure main ()
queue := Queue ()
every (item := 1 to 5) do
queue.push (item)
every (1 to 6) do {
write ("Popped value: " || queue.take ())
if queue.is_empty () then write ("empty queue")
}
end
Produces the same output as above.
J
Object oriented technique, using mutable state:
queue_fifo_=: ''
pop_fifo_=: verb define
r=. {. ::] queue
queue=: }.queue
r
)
push_fifo_=: verb define
queue=: queue,y
y
)
isEmpty_fifo_=: verb define
0=#queue
)
Function-level technique, with no reliance on mutable state:
pop =: ( {.^:notnull ; }. )@: > @: ] /
push =: ( '' ; ,~ )& > /
tell_atom =: >& {.
tell_queue =: >& {:
is_empty =: '' -: 1 tell_queue
make_empty =: a: , a: [ ]
onto =: [ ; }.@]
notnull =: 0 ~: #
See also FIFO (usage)#J
Java
This task could be done using a LinkedList from java.util, but here is a user-defined version with generics:
public class Queue<E>{
Node<E> head = null, tail = null;
static class Node<E>{
E value;
Node<E> next;
Node(E value, Node<E> next){
this.value= value;
this.next= next;
}
}
public Queue(){
}
public void enqueue(E value){ //standard queue name for "push"
Node<E> newNode= new Node<E>(value, null);
if(empty()){
head= newNode;
}else{
tail.next = newNode;
}
tail= newNode;
}
public E dequeue() throws java.util.NoSuchElementException{//standard queue name for "pop"
if(empty()){
throw new java.util.NoSuchElementException("No more elements.");
}
E retVal= head.value;
head= head.next;
return retVal;
}
public boolean empty(){
return head == null;
}
}
JavaScript
Most of the time, the built-in Array suffices. However, if you explicitly want to limit the usage to FIFO operations, it's easy to implement such a constructor.
Using built-in Array
var fifo = [];
fifo.push(42); // Enqueue.
fifo.push(43);
var x = fifo.shift(); // Dequeue.
alert(x); // 42
Custom constructor function
function FIFO() {
this.data = new Array();
this.push = function(element) {this.data.push(element)}
this.pop = function() {return this.data.shift()}
this.empty = function() {return this.data.length == 0}
this.enqueue = this.push;
this.dequeue = this.pop;
}
jq
Since jq is a purely functional language, the entities chosen to represent queues must somehow be presented to the functions that operate on them. The approach taken here is to use a JSON object with a key named "queue" to hold the contents of the queue. This allows us to "pop" a queue by modifying .queue while returning the popped item in the same object under a different key.
There are three possibilities for defining `pop` on an empty queue:
- Do not make a special case of it, which in our case would mean that `{queue: []} | pop` would emit `{queue: [], item: null}`
- Raise an error
- Emit nothing
Here (1), is questionable as the queue might contain null, so here we define `pop_or_error`, which raises an error when given an empty queue, and `pop`, which emits the empty stream when given an empty queue. In order to facilitate observing the evolving states of queues during processing, we use the same `observe` function defined at Stack.
# Input: an object
# Output: the updated object with .emit filled in from `update|emit`.
# `emit` may produce a stream of values, which need not be strings.
def observe(update; emit):
def s(stream): reduce stream as $_ (null;
if $_ == null then .
elif . == null then "\($_)"
else . + "\n\($_)"
end);
.emit as $x
| update
| .emit = s($x // null, emit);
def fifo: {queue: []};
# Is the input an object that represents the empty queue?
def isempty:
type == "object"
and (.queue | length == 0); # so .queue == null and .queue == [] are equivalent
def push(e): .queue += [e];
def pop: if isempty then empty else .item = .queue[0] | .queue |= .[1:] end;
def pop_or_error: if isempty then error("pop_or_error") else pop end;
# Examples
# fifo | pop // "nothing" # produces the string "nothing"
fifo
| observe(push(42); "length after pushing: \(.queue | length)" )
| observe(push(43); "length after pushing: \(.queue | length)" )
| pop # dequeue
| .emit, .itemOutput
length after pushing: 1 length after pushing: 2 42
Julia
Julia provides a variety of queue-like methods for vectors, making the solution to this task rather straightforward. Define a Queue in terms of a one dimensional array, and provide its methods using the appropriate vector operations. To adhere to Julia naming conventions, the queue operations are named "push!", "pop!" and "isempty" rather than "push", "pop" and "empty".
struct Queue{T}
a::Array{T,1}
end
Queue() = Queue(Any[])
Queue(a::DataType) = Queue(a[])
Queue(a) = Queue(typeof(a)[])
Base.isempty(q::Queue) = isempty(q.a)
function Base.pop!(q::Queue{T}) where {T}
!isempty(q) || error("queue must be non-empty")
pop!(q.a)
end
function Base.push!(q::Queue{T}, x::T) where {T}
pushfirst!(q.a, x)
return q
end
function Base.push!(q::Queue{Any}, x::T) where {T}
pushfirst!(q.a, x)
return q
end
- Output:
It is easiest to use the REPL to show a Queue in action.
julia> q = Queue()
Queue{Any}(Any[])
julia> isempty(q)
true
julia> push!(q, 1)
Queue{Any}(Any[1])
julia> isempty(q)
false
julia> push!(q, "two")
Queue{Any}(Any["two",1])
julia> push!(q, 3.0)
Queue{Any}(Any[3.0,"two",1])
julia> push!(q, false)
Queue{Any}(Any[false,3.0,"two",1])
julia> pop!(q)
1
julia> pop!(q)
"two"
julia> pop!(q)
3.0
julia> pop!(q)
false
julia> pop!(q)
ERROR: queue must be non-empty
Stacktrace:
[1] error(s::String)
@ Base ./error.jl:33
[2] pop!(q::Queue{Any})
@ Main /tmp/cmdline_1648668849_nacnudus/lines.jl:3
[3] top-level scope
@ REPL[11]:1
Klingphix
{ include ..\Utilitys.tlhy }
"..\Utilitys.tlhy" load
:push! { l i -- l&i }
0 put
;
:empty? { l -- flag }
len not { len 0 equal }
;
:pop! { l -- l-1 }
empty? (
["Empty"]
[pop swap]
) if
;
( ) { empty queue }
1 push! 2 push! 3 push!
pop! ? pop! ? pop! ? pop! ?
"End " input- Output:
1 2 3 Empty End
Kotlin
// version 1.1.2
import java.util.LinkedList
class Queue<E> {
private val data = LinkedList<E>()
val size get() = data.size
val empty get() = size == 0
fun push(element: E) = data.add(element)
fun pop(): E {
if (empty) throw RuntimeException("Can't pop elements from an empty queue")
return data.removeFirst()
}
val top: E
get() {
if (empty) throw RuntimeException("Empty queue can't have a top element")
return data.first()
}
fun clear() = data.clear()
override fun toString() = data.toString()
}
fun main(args: Array<String>) {
val q = Queue<Int>()
(1..5).forEach { q.push(it) }
println(q)
println("Size of queue = ${q.size}")
print("Popping: ")
(1..3).forEach { print("${q.pop()} ") }
println("\nRemaining in queue: $q")
println("Top element is now ${q.top}")
q.clear()
println("After clearing, queue is ${if(q.empty) "empty" else "not empty"}")
try {
q.pop()
}
catch (e: Exception) {
println(e.message)
}
}
- Output:
[1, 2, 3, 4, 5] Size of queue = 5 Popping: 1 2 3 Remaining in queue: [4, 5] Top element is now 4 After clearing, queue is empty Can't pop elements from an empty queue
LabVIEW
This image is a VI Snippet, an executable image of LabVIEW code. The LabVIEW version is shown on the top-right hand corner. You can download it, then drag-and-drop it onto the LabVIEW block diagram from a file browser, and it will appear as runnable, editable code.
Lambdatalk
The APIs of queues are built on lambdatalk array primitives, [A.new, A.disp, A.join, A.split, A.array?, A.null?, A.empty?, A.in?, A.equal?, A.length, A.get, A.first, A.last, A.rest, A.slice, A.duplicate, A.reverse, A.concat, A.map, A.set!, A.addlast!, A.sublast!, A.addfirst!, A.subfirst!, A.reverse!, A.sort!, A.swap!, A.lib]. Note that the [A.addlast!, A.sublast!, A.addfirst!, A.subfirst!] primitives are the standard [push!, shift!, pop!, unshift!] ones.
{def queue.add
{lambda {:v :q}
{let { {_ {A.addlast! :v :q}}}
} ok}}
-> queue.add
{def queue.get
{lambda {:q}
{let { {:v {A.first :q}}
{_ {A.subfirst! :q}}
} :v}}}
-> queue.get
{def queue.empty?
{lambda {:q}
{A.empty? :q}}}
-> queue.empty?
{def Q {A.new}} -> Q []
{queue.add 1 {Q}} -> ok [1]
{queue.add 2 {Q}} -> ok [1,2]
{queue.add 3 {Q}} -> ok [1,2,3]
{queue.get {Q}} -> 1 [2,3]
{queue.add 4 {Q}} -> ok [2,3,4]
{queue.empty? {Q}} -> false
{queue.get {Q}} -> 2 [3,4]
{queue.get {Q}} -> 3 [4]
{queue.get {Q}} -> 4 []
{queue.get {Q}} -> undefined
{queue.empty? {Q}} -> true
Lasso
Definition:
define myqueue => type {
data store = list
public onCreate(...) => {
if(void != #rest) => {
with item in #rest do .`store`->insert(#item)
}
}
public push(value) => .`store`->insertLast(#value)
public pop => {
handle => {
.`store`->removefirst
}
return .`store`->first
}
public isEmpty => (.`store`->size == 0)
}
Usage:
local(q) = myqueue('a')
#q->isEmpty
// => false
#q->push('b')
#q->pop
// => a
#q->pop
// => b
#q->isEmpty
// => true
#q->pop
// => void
Lua
Queue = {}
function Queue.new()
return { first = 0, last = -1 }
end
function Queue.push( queue, value )
queue.last = queue.last + 1
queue[queue.last] = value
end
function Queue.pop( queue )
if queue.first > queue.last then
return nil
end
local val = queue[queue.first]
queue[queue.first] = nil
queue.first = queue.first + 1
return val
end
function Queue.empty( queue )
return queue.first > queue.last
end
M2000 Interpreter
A Stack object can be used as LIFO or FIFO. Data push to bottom of stack. Read pop a value to a variable from top of stack.
Module Checkit {
a=Stack
Stack a {
Data 100,200, 300
}
Stack a {
While not empty {
Read N
Print N
}
}
}
CheckitMathematica /Wolfram Language
EmptyQ[a_] := Length[a] == 0
SetAttributes[Push, HoldAll]; Push[a_, elem_] := AppendTo[a, elem]
SetAttributes[Pop, HoldAllComplete]; Pop[a_] := If[EmptyQ[a], False, b = First[a]; Set[a, Most[a]]; b]
MATLAB / Octave
Here is a simple implementation of a queue, that works in Matlab and Octave.
myfifo = {};
% push
myfifo{end+1} = x;
% pop
x = myfifo{1}; myfifo{1} = [];
% empty
isempty(myfifo)
Below is another solution, that encapsulates the fifo within the object-orientated "class" elements supported by Matlab. For this to work it must be saved in a file named "FIFOQueue.m" in a folder named "@FIFOQueue" in your current Matlab directory.
%This class impliments a standard FIFO queue.
classdef FIFOQueue
properties
queue
end
methods
%Class constructor
function theQueue = FIFOQueue(varargin)
if isempty(varargin) %No input arguments
%Initialize the queue state as empty
theQueue.queue = {};
elseif (numel(varargin) > 1) %More than 1 input arg
%Make the queue the list of input args
theQueue.queue = varargin;
elseif iscell(varargin{:}) %If the only input is a cell array
%Make the contents of the cell array the elements in the queue
theQueue.queue = varargin{:};
else %There is one input argument that is not a cell
%Make that one arg the only element in the queue
theQueue.queue = varargin;
end
end
%push() - pushes a new element to the end of the queue
function push(theQueue,varargin)
if isempty(varargin)
theQueue.queue(end+1) = {[]};
elseif (numel(varargin) > 1) %More than 1 input arg
%Make the queue the list of input args
theQueue.queue( end+1:end+numel(varargin) ) = varargin;
elseif iscell(varargin{:}) %If the only input is a cell array
%Make the contents of the cell array the elements in the queue
theQueue.queue( end+1:end+numel(varargin{:}) ) = varargin{:};
else %There is one input argument that is not a cell
%Make that one arg the only element in the queue
theQueue.queue{end+1} = varargin{:};
end
%Makes changes to the queue permanent
assignin('caller',inputname(1),theQueue);
end
%pop() - pops the first element off the queue
function element = pop(theQueue)
if empty(theQueue)
error 'The queue is empty'
else
%Returns the first element in the queue
element = theQueue.queue{1};
%Removes the first element from the queue
theQueue.queue(1) = [];
%Makes changes to the queue permanent
assignin('caller',inputname(1),theQueue);
end
end
%empty() - Returns true if the queue is empty
function trueFalse = empty(theQueue)
trueFalse = isempty(theQueue.queue);
end
end %methods
end
Sample usage:
>> myQueue = FIFOQueue({'hello'})
myQueue =
FIFOQueue
>> push(myQueue,'jello')
>> pop(myQueue)
ans =
hello
>> pop(myQueue)
ans =
jello
>> pop(myQueue)
??? Error using ==> FIFOQueue.FIFOQueue>FIFOQueue.pop at 61
The queue is empty
Maxima
defstruct(queue(in=[], out=[]))$
enqueue(x, q) := (q@in: cons(x, q@in), done)$
dequeue(q) := (if not emptyp(q@out) then first([first(q@out), q@out: rest(q@out)])
elseif emptyp(q@in) then 'fail
else (q@out: reverse(q@in), q@in: [], first([first(q@out), q@out: rest(q@out)])))$
q:new(queue); /* queue([], []) */
enqueue(1, q)$
enqueue(2, q)$
enqueue(3, q)$
dequeue(q); /* 1 */
enqueue(4, q)$
dequeue(q); /* 2 */
dequeue(q); /* 3 */
dequeue(q); /* 4 */
dequeue(q); /* fail */
Nanoquery
This is a fully-featured FIFO queue class definition. In addition to the functions required by the task, it also demonstrates redefining operators for Nanoquery classes by redefining +, *, and =.
class FIFO
declare contents
// define constructors for FIFO objects
def FIFO()
this.contents = {}
end
def FIFO(contents)
this.contents = contents
end
// define methods for this class
def push(value)
contents.append(value)
end
def pop()
if !this.empty()
value = contents[len(contents) - 1]
contents.remove(len(contents) - 1)
return value
else
// we could throw our own exception here but
// we'll return null instead
return null
end
end
def length()
return len(contents)
end
def extend(itemlist)
contents += itemlist
end
def empty()
return len(contents) = 0
end
// define operators for this class
def toString()
return str(contents)
end
def operator+(other)
return this.contents + other.contents
end
def operator*(n)
return this.contents * n
end
def operator=(other)
return this.contents = other.contents
end
endNetRexx
Unlike Rexx, NetRexx does not include built–in support for queues but the language's ability to access the Java SDK permits use of any number of Java's "Collection" classes.
The following sample implements a stack via the ArrayDeque double–ended queue.
/* NetRexx */
options replace format comments java crossref savelog symbols nobinary
mqueue = ArrayDeque()
viewQueue(mqueue)
a = "Fred"
mqueue.push('') /* Puts an empty line onto the queue */
mqueue.push(a 2) /* Puts "Fred 2" onto the queue */
viewQueue(mqueue)
a = "Toft"
mqueue.add(a 2) /* Enqueues "Toft 2" */
mqueue.add('') /* Enqueues an empty line behind the last */
viewQueue(mqueue)
loop q_ = 1 while mqueue.size > 0
parse mqueue.pop.toString line
say q_.right(3)':' line
end q_
viewQueue(mqueue)
return
method viewQueue(mqueue = Deque) private static
If mqueue.size = 0 then do
Say 'Queue is empty'
end
else do
Say 'There are' mqueue.size 'elements in the queue'
end
return
Queue is empty There are 2 elements in the queue There are 4 elements in the queue 1: Fred 2 2: 3: Toft 2 4: Queue is empty
Nim
type
Node[T] = ref object
value: T
next: Node[T]
Queue*[T] = object
head, tail: Node[T]
length: Natural
func initQueue*[T](): Queue[T] = Queue[T]()
func len*(queue: Queue): Natural =
queue.length
func isEmpty*(queue: Queue): bool {.inline.} =
queue.len == 0
func push*[T](queue: var Queue[T]; value: T) =
let node = Node[T](value: value, next: nil)
if queue.isEmpty: queue.head = node
else: queue.tail.next = node
queue.tail = node
inc queue.length
func pop*[T](queue: var Queue[T]): T =
if queue.isEmpty:
raise newException(ValueError, "popping from empty queue.")
result = queue.head.value
queue.head = queue.head.next
dec queue.length
if queue.isEmpty: queue.tail = nil
when isMainModule:
var fifo = initQueue[int]()
fifo.push(26)
fifo.push(99)
fifo.push(2)
echo "Fifo size: ", fifo.len()
try:
echo "Popping: ", fifo.pop()
echo "Popping: ", fifo.pop()
echo "Popping: ", fifo.pop()
echo "Popping: ", fifo.pop()
except ValueError:
echo "Exception catched: ", getCurrentExceptionMsg()
- Output:
Fifo size: 3 Popping: 26 Popping: 99 Popping: 2 Exception catched: popping from empty queue.
Objeck
class Test {
function : Main(args : String[]) ~ Nil {
queue := Queue->New();
queue->Push(3); queue->Push(6); queue->Push(9);
queue->Pop();
queue->Push(12);
queue->Push(15);
while(<>queue->Empty()) {
queue->Top()->GetValue()->PrintLine();
queue->Pop()->PrintLine();
};
queue->Pop()->PrintLine();
}
}
class Queue {
@head, @tail : Node;
New() {
}
method : public : Push(value : Int) ~ Nil {
if(@head = Nil) {
@head := @tail := Node->New(value);
}
else {
tail := Node->New(@tail, value);
@tail->SetNext(tail);
@tail := tail;
};
}
method : public : Pop() ~ Bool {
if(@head <> Nil) {
if(@head = @tail) {
@head := @tail := Nil;
}
else {
prev := @tail->GetPrev();
prev->SetNext(Nil);
@tail := prev;
};
return true;
};
return false;
}
method : public : Top() ~ Node {
return @tail;
}
method : public : Empty() ~ Bool {
return @head = Nil;
}
}
class Node {
@value : Int;
@next : Node;
@prev : Node;
New(value : Int) {
@value := value;
}
New(prev : Node, value : Int) {
@prev := prev;
@value := value;
}
method : public : GetValue() ~ Int {
return @value;
}
method : public : SetNext(next : Node) ~ Nil {
@next := next;
}
method : public : GetPrev() ~ Node {
return @prev;
}
method : public : ToString() ~ String {
return @value->ToString();
}
}15 12 6 3 false
OCaml
The standard way to manage fifo in functional programming is to use a pair of list for the fifo queue, one is the input, the other is the output. When the output is empty just take the input list and reverse it.
module FIFO : sig
type 'a fifo
val empty: 'a fifo
val push: fifo:'a fifo -> item:'a -> 'a fifo
val pop: fifo:'a fifo -> 'a * 'a fifo
val is_empty: fifo:'a fifo -> bool
end = struct
type 'a fifo = 'a list * 'a list
let empty = [], []
let push ~fifo:(input,output) ~item = (item::input,output)
let is_empty ~fifo =
match fifo with
| [], [] -> true
| _ -> false
let rec pop ~fifo =
match fifo with
| input, item :: output -> item, (input,output)
| [], [] -> failwith "empty fifo"
| input, [] -> pop ([], List.rev input)
end
and a session in the top-level:
# open FIFO;;
# let q = empty ;;
val q : '_a FIFO.fifo = <abstr>
# is_empty q ;;
- : bool = true
# let q = push q 1 ;;
val q : int FIFO.fifo = <abstr>
# is_empty q ;;
- : bool = false
# let q =
List.fold_left push q [2;3;4] ;;
val q : int FIFO.fifo = <abstr>
# let v, q = pop q ;;
val v : int = 1
val q : int FIFO.fifo = <abstr>
# let v, q = pop q ;;
val v : int = 2
val q : int FIFO.fifo = <abstr>
# let v, q = pop q ;;
val v : int = 3
val q : int FIFO.fifo = <abstr>
# let v, q = pop q ;;
val v : int = 4
val q : int FIFO.fifo = <abstr>
# let v, q = pop q ;;
Exception: Failure "empty fifo".
The standard ocaml library also provides a FIFO module, but it is imperative, unlike the implementation above which is functional.
Oforth
If queue is empty, null is returned.
Object Class new: Queue(mutable l)
Queue method: initialize ListBuffer new := l ;
Queue method: empty @l isEmpty ;
Queue method: push @l add ;
Queue method: pop @l removeFirst ;OxygenBasic
This buffer pushes any primitive data (auto converted to strings), and pops strings. The buffer can expand or contract according to usage.
'==========
Class Queue
'==========
'FIRST IN FIRST OUT
bstring buf 'buffer to hold queue content
int bg 'buffer base offset
int i 'indexer
int le 'length of buffer
method constructor()
====================
buf=""
le=0
bg=0
i=0
end method
method destructor()
===================
del buf
le=0
bg=0
i=0
end method
method Encodelength(int ls)
===========================
int p at (i+strptr buf)
p=ls
i+=sizeof int
end method
method push(string s)
=====================
int ls=len s
if i+ls+8>le then
buf+=nuls 8000+ls*2 'extend buf
le=len buf
end if
EncodeLength ls 'length of input s
mid buf,i+1,s 'append input s
i+=ls
end method
method popLength() as int
=========================
if bg>=i then return -1 'buffer empty
int p at (bg+strptr buf)
bg+=sizeof int
return p
end method
method pop(string *s) as int
============================
int ls=popLength
if ls<0 then s="" : return ls 'empty buffer
s=mid buf,bg+1,ls
bg+=ls
'cleanup buffer
if bg>1e6 then
buf=mid buf,bg+1
le=len buf
i-=bg 'shrink buf
bg=0
end if
end method
method clear()
==============
buf=""
le=0
bg=0
i=0
end method
end class 'Queue
'====
'DEMO
'====
new Queue fifo
string s
'
fifo.push "HumptyDumpty"
fifo.push "Sat on a wall"
'
int er
do
er=fifo.pop s
if er then print "(buffer empty)" : exit do
print s
loop
del fifo
Oz
The semantics of the built-in "Port" datatype is essentially that of a thread-safe queue. We can implement the specified queue type as operations on a pair of a port and a mutable reference to the current read position of the associated stream.
It seems natural to make "Pop" a blocking operation (i.e. it waits for a new value if the queue is currently empty).
The implementation is thread-safe if there is only one reader thread. When multiple reader threads exist, it is possible that a value is popped more than once.
declare
fun {NewQueue}
Stream
WritePort = {Port.new Stream}
ReadPos = {NewCell Stream}
in
WritePort#ReadPos
end
proc {Push WritePort#_ Value}
{Port.send WritePort Value}
end
fun {Empty _#ReadPos}
%% the queue is empty if the value at the current
%% read position is not determined
{Not {IsDet @ReadPos}}
end
fun {Pop _#ReadPos}
%% blocks if empty
case @ReadPos of X|Xr then
ReadPos := Xr
X
end
end
Q = {NewQueue}
in
{Show {Empty Q}}
{Push Q 42}
{Show {Empty Q}}
{Show {Pop Q}}
{Show {Empty Q}}There is also a queue datatype in the Mozart standard library.
Pascal
This program should be Standard Pascal compliant (i.e. it doesn't make use of the advanced/non-standard features of FreePascal or GNU Pascal).
program fifo(input, output);
type
pNode = ^tNode;
tNode = record
value: integer;
next: pNode;
end;
tFifo = record
first, last: pNode;
end;
procedure initFifo(var fifo: tFifo);
begin
fifo.first := nil;
fifo.last := nil
end;
procedure pushFifo(var fifo: tFifo; value: integer);
var
node: pNode;
begin
new(node);
node^.value := value;
node^.next := nil;
if fifo.first = nil
then
fifo.first := node
else
fifo.last^.next := node;
fifo.last := node
end;
function popFifo(var fifo: tFifo; var value: integer): boolean;
var
node: pNode;
begin
if fifo.first = nil
then
popFifo := false
else
begin
node := fifo.first;
fifo.first := fifo.first^.next;
value := node^.value;
dispose(node);
popFifo := true
end
end;
procedure testFifo;
var
fifo: tFifo;
procedure testpop(expectEmpty: boolean; expectedValue: integer);
var
i: integer;
begin
if popFifo(fifo, i)
then
if expectEmpty
then
writeln('Error! Expected empty, got ', i, '.')
else
if i = expectedValue
then
writeln('Ok, got ', i, '.')
else
writeln('Error! Expected ', expectedValue, ', got ', i, '.')
else
if expectEmpty
then
writeln('Ok, fifo is empty.')
else
writeln('Error! Expected ', expectedValue, ', found fifo empty.')
end;
begin
initFifo(fifo);
pushFifo(fifo, 2);
pushFifo(fifo, 3);
pushFifo(fifo, 5);
testpop(false, 2);
pushFifo(fifo, 7);
testpop(false, 3);
testpop(false, 5);
pushFifo(fifo, 11);
testpop(false, 7);
testpop(false, 11);
pushFifo(fifo, 13);
testpop(false, 13);
testpop(true, 0);
pushFifo(fifo, 17);
testpop(false, 17);
testpop(true, 0)
end;
begin
writeln('Testing fifo implementation ...');
testFifo;
writeln('Testing finished.')
end.
PascalABC.NET
type
Node<T> = auto class
data: T;
next: Node<T>;
end;
MyQueue<T> = class
head,tail: Node<T>;
public
procedure Enqueue(x: T);
begin
if tail = nil then
begin
tail := new Node<T>(x,nil);
head := tail;
end
else
begin
tail.next := new Node<T>(x,nil);
tail := tail.next;
end;
end;
function Dequeue(): T;
begin
Result := head.data;
head := head.Next;
if head = nil then
tail := nil;
end;
function IsEmpty: boolean := head = nil;
end;
begin
var q := new MyQueue<integer>;
for var i:=1 to 5 do
q.Enqueue(i);
while not q.IsEmpty do
Print(q.Dequeue);
Print(q.IsEmpty);
end.
- Output:
1 2 3 4 5 True
Perl
Lists are a central part of Perl. To implement a FIFO using OO will to many Perl programmers seem a bit awkward.
use Carp;
sub my push :prototype(\@@) {my($list,@things)=@_; push @$list, @things}
sub maypop :prototype(\@) {my($list)=@_; @$list or croak "Empty"; shift @$list }
sub empty :prototype(@) {not @_}
Example:
my @fifo=qw(1 2 3 a b c);
mypush @fifo, 44, 55, 66;
mypop @fifo for 1 .. 6+3;
mypop @fifo; #empty now
Phix
with javascript_semantics sequence queue = {} procedure push_item(object what) queue = append(queue,what) end procedure function pop_item() object what = queue[1] queue = queue[2..$] return what end function function empty() return length(queue)=0 end function
As of 1.0.2 there are standard builtins for the above, named new_queue(), push(), and queue_empty() respectively, see docs.
Phixmonti
include ..\Utilitys.pmt
def push /# l i -- l&i #/
0 put
enddef
def empty? /# l -- flag #/
len 0 ==
enddef
def pop /# l -- l-1 #/
empty? if
"Empty"
else
head swap tail nip swap
endif
enddef
( ) /# empty queue #/
1 push 2 push 3 push
pop ? pop ? pop ? pop ?PHP
class Fifo {
private $data = array();
public function push($element){
array_push($this->data, $element);
}
public function pop(){
if ($this->isEmpty()){
throw new Exception('Attempt to pop from an empty queue');
}
return array_shift($this->data);
}
//Alias functions
public function enqueue($element) { $this->push($element); }
public function dequeue() { return $this->pop(); }
//Note: PHP prevents a method name of 'empty'
public function isEmpty(){
return empty($this->data);
}
}
Example:
$foo = new Fifo();
$foo->push('One');
$foo->enqueue('Two');
$foo->push('Three');
echo $foo->pop(); //Prints 'One'
echo $foo->dequeue(); //Prints 'Two'
echo $foo->pop(); //Prints 'Three'
echo $foo->pop(); //Throws an exception
Picat
First variant
go =>
println("Test 1"),
queue_test1,
nl.
empty(Q) => Q = [].
push(Queue, Value) = Q2 =>
Q2 = [Value] ++ Queue.
pop(Q,_) = _, Q==[] ; var(Q) =>
throw $error(empty_queue,pop,'Q'=Q).
pop(Queue,Q2) = Queue.last() =>
Q2 = [Queue[I] : I in 1..Queue.len-1].
queue_test1 =>
% create an empty queue
println("Start test 2"),
empty(Q0),
printf("Create queue %w%n%n", Q0),
% add numbers 1 and 2
println("Add numbers 1 and 2 : "),
Q1 = Q0.push(1),
Q2 = Q1.push(2),
% display queue
printf("Q2: %w\n\n", Q2),
% pop element
V = Q2.pop(Q3),
% display results
printf("Pop : Value: %w Queue: %w\n\n", V, Q3),
% test the queue
print("Test of the queue: "),
( Q3.empty() -> println("Queue empty"); println("Queue not empty") ),
nl,
% pop the elements
print("Pop the queue : "),
V1 = Q3.pop(Q4),
printf("Value %w Queue : %w%n%n", V1, Q4),
println("Pop empty queue:"),
catch(_V = Q4.pop(_Q5),Exception,println(Exception)),
nl,
println("\nEnd of tests.").- Output:
Test 1 Create queue [] Add numbers 1 and 2 : Q2: [2,1] Pop : Value: 1 Queue: [2] Test of the queue: Queue not empty Pop the queue : Value 2 Queue : [] Pop empty queue: error(empty_queue,pop,Q = []) End of tests.
Always returning the queue
Another approach is to always returns the queue which makes command chaining possible, e,g,
Q := Q.push2(1).push2(2), Q := Q.pop2(V1).pop2(V2)
go2 =>
println("Test 2"),
queue_test2,
nl.
empty2() = [].
push2(Queue, Value) = Q2 =>
Q2 = [Value] ++ Queue.
pop2(Q,_) = _, Q==[] ; var(Q) =>
throw $error(empty_queue,pop,'Q'=Q).
pop2(Queue,V) = [Queue[I] : I in 1..Queue.len-1] =>
V = Queue.last().
queue_test2 =>
% create an empty queue
Q = empty2(),
printf("Create queue %w%n%n", Q),
% add numbers 1 and 2
println("Add numbers 1 and 2 : "),
Q := Q.push2(1).push2(2),
% display queue
printf("Q: %w\n\n", Q),
% pop element
Q := Q.pop2(V),
% display results
printf("Pop : Value: %w Queue: %w\n\n", V, Q),
% test the queue
print("Test of the queue: "),
( Q.empty() -> println("Queue empty"); println("Queue not empty") ),
nl,
% pop the elements
print("Pop the queue : "),
Q := Q.pop2(V2),
printf("Value %w Queue : %w%n%n", V2, Q),
println("Pop empty queue:"),
catch(_ = Q.pop2(_V),Exception,println(Exception)),
% command chaining
println("\nCommand chaining: "),
Q := Q.push2(3).push2(4),
Q := Q.pop2(V3).pop2(V4),
printf("V3: %w V4: %w\n", V3, V4),
nl,
println(q=Q).- Output:
Test 2 Create queue [] Add numbers 1 and 2 : Q: [2,1] Pop : Value: 1 Queue: [2] Test of the queue: Queue not empty Pop the queue : Value 2 Queue : [] Pop empty queue: error(empty_queue,pop,Q = []) Command chaining: V3: 3 V4: 4 q = []
PicoLisp
The built-in function 'fifo' maintains a queue in a circular list, with direct access to the first and the last cell
(off Queue) # Clear Queue
(fifo 'Queue 1) # Store number '1'
(fifo 'Queue 'abc) # an internal symbol 'abc'
(fifo 'Queue "abc") # a transient symbol "abc"
(fifo 'Queue '(a b c)) # and a list (a b c)
Queue # Show the queue- Output:
->((a b c) 1 abc "abc" .)
PL/I
/* To push a node onto the end of the queue. */
push: procedure (tail);
declare tail handle (node), t handle (node);
t = new(:node:);
get (t => value);
if tail ^= bind(:null, node:) then
tail => link = t;
/* If the queue was non-empty, points the tail of the queue */
/* to the new node. */
tail = t; /* Point "tail" at the end of the queue. */
tail => link = bind(:node, null:);
end push;
/* To pop a node from the head of the queue. */
pop: procedure (head, val);
declare head handle (node), val fixed binary;
if head = bind(:node, null:) then signal error;
val = head => value;
head = head => pointer; /* pops the top node. */
if head = bind(:node, null:) then tail = head;
/* (If the queue is now empty, make tail null also.) */
end pop;
/* Queue status: the EMPTY function, returns true for empty queue. */
empty: procedure (h) returns (bit(1));
declare h handle (Node);
return (h = bind(:Node, null:) );
end empty;PostScript
% our queue is just [] and empty? is already defined.
/push {exch tadd}.
/pop {uncons exch}.
PowerShell
PowerShell can natively use the .Net Queue class.
$Q = New-Object System.Collections.Queue
$Q.Enqueue( 1 )
$Q.Enqueue( 2 )
$Q.Enqueue( 3 )
$Q.Dequeue()
$Q.Dequeue()
$Q.Count -eq 0
$Q.Dequeue()
$Q.Count -eq 0
try
{ $Q.Dequeue() }
catch [System.InvalidOperationException]
{ If ( $_.Exception.Message -eq 'Queue empty.' ) { 'Caught error' } }
- Output:
1 2 False 3 True Caught error
Prolog
Works with SWI-Prolog. One can push any data in queue.
empty(U-V) :-
unify_with_occurs_check(U, V).
push(Queue, Value, NewQueue) :-
append_dl(Queue, [Value|X]-X, NewQueue).
% when queue is empty pop fails.
pop([X|V]-U, X, V-U) :-
\+empty([X|V]-U).
append_dl(X-Y, Y-Z, X-Z).
PureBasic
For FIFO function PureBasic normally uses linked lists. Usage as described above could look like;
NewList MyStack()
Procedure Push(n)
Shared MyStack()
LastElement(MyStack())
AddElement(MyStack())
MyStack()=n
EndProcedure
Procedure Pop()
Shared MyStack()
Protected n
If FirstElement(MyStack()) ; e.g. Stack not empty
n=MyStack()
DeleteElement(MyStack(),1)
Else
Debug "Pop(), out of range. Error at line "+str(#PB_Compiler_Line)
EndIf
ProcedureReturn n
EndProcedure
Procedure Empty()
Shared MyStack()
If ListSize(MyStack())=0
ProcedureReturn #True
EndIf
ProcedureReturn #False
EndProcedure
;---- Example of implementation ----
Push(3)
Push(1)
Push(4)
While Not Empty()
Debug Pop()
Wend
;---- Now an extra Pop(), e.g. one to many ----
Debug Pop()
- Output:
3 1 4 Pop(), out of range. Error at line 17 0
Python
A python list can be used as a simple FIFO by simply using only it's .append() and .pop() methods and only using .pop(0) to consistently pull the head off the list. (The default .pop() pulls off the tail, and using that would treat the list as a stack.
To encapsulate this behavior into a class and provide the task's specific API we can simply use:
class FIFO(object):
def __init__(self, *args):
self.contents = list(args)
def __call__(self):
return self.pop()
def __len__(self):
return len(self.contents)
def pop(self):
return self.contents.pop(0)
def push(self, item):
self.contents.append(item)
def extend(self,*itemlist):
self.contents += itemlist
def empty(self):
return bool(self.contents)
def __iter__(self):
return self
def next(self):
if self.empty():
raise StopIteration
return self.pop()
if __name__ == "__main__":
# Sample usage:
f = FIFO()
f.push(3)
f.push(2)
f.push(1)
while not f.empty():
print f.pop(),
# >>> 3 2 1
# Another simple example gives the same results:
f = FIFO(3,2,1)
while not f.empty():
print f(),
# Another using the default "truth" value of the object
# (implicitly calls on the length() of the object after
# checking for a __nonzero__ method
f = FIFO(3,2,1)
while f:
print f(),
# Yet another, using more Pythonic iteration:
f = FIFO(3,2,1)
for i in f:
print i,
This example does add to a couple of features which are easy in Python and allow this FIFO class to be used in ways that Python programmers might find more natural. Our __init__ accepts and optional list of initial values, we add __len__ and extend methods which simply wrap the corresponding list methods; we define a __call__ method to show how one can make objects "callable" as functions, and we define __iter__ and next() methods to facilitate using these FIFO objects with Python's prevalent iteration syntax (the for loop). The empty method could be implemented as simply an alias for __len__ --- but we've chosen to have it more strictly conform to the task specification. Implementing the __len__ method allows code using this object to test of emptiness using normal Python idioms for "truth" (any non-empty container is considered to be "true" and any empty container evaluates as "false").
These additional methods could be omitted and some could have been dispatched to the "contents" object by defining a __getattr__ method. (All methods that are note defined could be relayed to the contained list). This would allow us to skip our definitions of extend, __iter__, and __len__, and would allow contents of these objects to be access by indexes and slices as well as supporting all other list methods.
That sort of wrapper looks like:
class FIFO: ## NOT a new-style class, must not derive from "object"
def __init__(self,*args):
self.contents = list(args)
def __call__(self):
return self.pop()
def empty(self):
return bool(self.contents)
def pop(self):
return self.contents.pop(0)
def __getattr__(self, attr):
return getattr(self.contents,attr)
def next(self):
if not self:
raise StopIteration
return self.pop()
As noted in the contents this must NOT be a new-style class, it must NOT but sub-classed from object nor any of its descendents. (A new-style implementation using __getattribute__ would be possible)
Python 2.4 and later includes a deque class, supporting thread-safe, memory efficient appends and pops from either side of the deque with approximately the same O(1) performance in either direction. For other options see Python Cookbook.
from collections import deque
fifo = deque()
fifo. appendleft(value) # push
value = fifo.pop()
not fifo # empty
fifo.pop() # raises IndexError when empty
Quackery
[ [] ] is queue ( --> [ )
[ [] = ] is empty? ( [ --> b )
[ nested join ] is push ( [ x --> [ )
[ dup empty? if
[ $ "Queue unexpectedly empty."
fail ]
behead ] is pop ( [ --> [ x )- Output:
Testing in the Quackery shell.
/O> queue
... 1111 push
... 2222 push
... 3333 push
... pop echo cr
... pop echo cr
... pop echo cr
... dup empty? if [ say "queue is enpty" cr ]
... pop echo cr
...
1111
2222
3333
queue is enpty
Problem: Queue unexpectedly empty.
Quackery Stack: [ ]
Return stack: {[...] 0} {quackery 1} {[...] 11} {shell 5} {quackery 1} {[...] 20} {pop 3}
R
Simple functional implementation
This simple implementation provides three functions that act on a variable in the global environment (user workspace) named l. the push and pop functions display the new status of l, but return NULL silently.
empty <- function() length(l) == 0
push <- function(x)
{
l <<- c(l, list(x))
print(l)
invisible()
}
pop <- function()
{
if(empty()) stop("can't pop from an empty list")
l[[1]] <<- NULL
print(l)
invisible()
}
l <- list()
empty()
# [1] TRUE
push(3)
# [[1]]
# [1] 3
push("abc")
# [[1]]
# [1] 3
# [[2]]
# [1] "abc"
push(matrix(1:6, nrow=2))
# [[1]]
# [1] 3
# [[2]]
# [1] "abc"
# [[3]]
# [,1] [,2] [,3]
# [1,] 1 3 5
# [2,] 2 4 6
empty()
# [1] FALSE
pop()
# [[1]]
# [1] 3
# [[2]]
# [1] "abc"
pop()
# [[1]]
# [1] 3
pop()
# list()
pop()
# Error in pop() : can't pop from an empty list
The problem with this is that the functions aren't related to the FIFO object (the list l), and they require the list to exist in the global environment. (This second problem is possible to get round by passing l into the function and then returning it, but that is extra work.)
Message passing
# The usual Scheme way : build a function that takes commands as parameters (it's like message passing oriented programming)
queue <- function() {
v <- list()
f <- function(cmd, val=NULL) {
if(cmd == "push") {
v <<- c(v, val)
invisible()
} else if(cmd == "pop") {
if(length(v) == 0) {
stop("empty queue")
} else {
x <- v[[1]]
v[[1]] <<- NULL
x
}
} else if(cmd == "length") {
length(v)
} else if(cmd == "empty") {
length(v) == 0
} else {
stop("unknown command")
}
}
f
}
# Create two queues
a <- queue()
b <- queue()
a("push", 1)
a("push", 2)
b("push", 3)
a("push", 4)
b("push", 5)
a("pop")
# [1] 1
b("pop")
# [1] 3
Object oriented implementation
A better solution is to use the object oriented facility in the proto package. (R does have it's own native object oriented code, though the proto package is often nicer to use.)
library(proto)
fifo <- proto(expr = {
l <- list()
empty <- function(.) length(.$l) == 0
push <- function(., x)
{
.$l <- c(.$l, list(x))
print(.$l)
invisible()
}
pop <- function(.)
{
if(.$empty()) stop("can't pop from an empty list")
.$l[[1]] <- NULL
print(.$l)
invisible()
}
})
#The following code provides output that is the same as the previous example.
fifo$empty()
fifo$push(3)
fifo$push("abc")
fifo$push(matrix(1:6, nrow=2))
fifo$empty()
fifo$pop()
fifo$pop()
fifo$pop()
fifo$pop()
Racket
Racket comes with a queue implementation in the data/queue library. Here's an explicit implementation:
#lang racket
(define (make-queue) (mcons #f #f))
(define (push! q x)
(define new (mcons x #f))
(if (mcar q) (set-mcdr! (mcdr q) new) (set-mcar! q new))
(set-mcdr! q new))
(define (pop! q)
(define old (mcar q))
(cond [(eq? old (mcdr q)) (set-mcar! q #f) (set-mcdr! q #f)]
[else (set-mcar! q (mcdr old))])
(mcar old))
(define (empty? q)
(not (mcar q)))
(define Q (make-queue))
(empty? Q) ; -> #t
(push! Q 'x)
(empty? Q) ; -> #f
(for ([x 3]) (push! Q x))
(pop! Q) ; -> 'x
(list (pop! Q) (pop! Q) (pop! Q)) ; -> '(0 1 2)
And this is an implementation of a functional queue.
#lang racket
;; Invariants:
;; The elements in the queue are (append front (reverse back)).
;; Front is always non-empty (except for the empty queue).
(struct queue (front back))
(define empty (queue '() '()))
(define (push x q)
(if (null? (queue-front q))
(queue (reverse (cons x (queue-back q))) '())
(queue (queue-front q) (cons x (queue-back q)))))
(define (empty? q)
(null? (queue-front q)))
(define (pop q)
(cond [(empty? q) (error 'pop "the queue is empty")]
[(not (null? (queue-front q)))
(if (null? (rest (queue-front q)))
(queue (reverse (queue-back q)) '())
(queue (rest (queue-front q)) (queue-back q)))]
[else (queue (reverse (queue-back q)) '())]))
(define (first q)
(cond [(empty? q) (error 'first "the queue is empty")]
[(car (queue-front q))]))
;; Example:
(first (pop (pop (for/fold ([q empty]) ([x '(1 2 3 4)])
(push x q)))))
;; => 3
Raku
(formerly Perl 6)
We could build a new container class to do FIFO pretty easily, but Arrays already do everything needed by a FIFO queue, so it is easier to just compose a Role on the existing Array class.
role FIFO {
method enqueue ( *@values ) { # Add values to queue, returns the number of values added.
self.push: @values;
return @values.elems;
}
method dequeue ( ) { # Remove and return the first value from the queue.
# Return Nil if queue is empty.
return self.elems ?? self.shift !! Nil;
}
method is-empty ( ) { # Check to see if queue is empty. Returns Boolean value.
return self.elems == 0;
}
}
# Example usage:
my @queue does FIFO;
say @queue.is-empty; # -> Bool::True
for <A B C> -> $i { say @queue.enqueue: $i } # 1 \n 1 \n 1
say @queue.enqueue: Any; # -> 1
say @queue.enqueue: 7, 8; # -> 2
say @queue.is-empty; # -> Bool::False
say @queue.dequeue; # -> A
say @queue.elems; # -> 4
say @queue.dequeue; # -> B
say @queue.is-empty; # -> Bool::False
say @queue.enqueue('OHAI!'); # -> 1
say @queue.dequeue until @queue.is-empty; # -> C \n Any() \n [7 8] \n OHAI!
say @queue.is-empty; # -> Bool::True
say @queue.dequeue; # ->
REBOL
rebol [
Title: "FIFO"
URL: http://rosettacode.org/wiki/FIFO
]
; Define fifo class:
fifo: make object! [
queue: copy []
push: func [x][append queue x]
pop: func [/local x][ ; Make 'x' local so it won't pollute global namespace.
if empty [return none]
x: first queue remove queue x]
empty: does [empty? queue]
]
; Create and populate a FIFO:
q: make fifo []
q/push 'a
q/push 2
q/push USD$12.34 ; Did I mention that REBOL has 'money!' datatype?
q/push [Athos Porthos Aramis] ; List elements pushed on one by one.
q/push [[Huey Dewey Lewey]] ; This list is preserved as a list.
; Dump it out, with narrative:
print rejoin ["Queue is " either q/empty [""]["not "] "empty."]
while [not q/empty][print [" " q/pop]]
print rejoin ["Queue is " either q/empty [""]["not "] "empty."]
print ["Trying to pop an empty queue yields:" q/pop]
- Output:
Queue is not empty. a 2 USD$12.34 Athos Porthos Aramis Huey Dewey Lewey Queue is empty. Trying to pop an empty queue yields: none
REXX
Support for LIFO & FIFO queues is built into the Rexx language.
The following are supported in REXX:
- PUSH (lifo)
- QUEUE (fifo)
- PULL --- which is a short version of:
- PARSE UPPER PULL
- PARSE LOWER PULL --- supported by some newer REXXes
- PARSE PULL
- QUEUED() [a BIF which returns the number of queued entries.]
/*REXX program to demonstrate FIFO queue usage by some simple operations*/
call viewQueue
a="Fred"
push /*puts a "null" on top of queue.*/
push a 2 /*puts "Fred 2" on top of queue.*/
call viewQueue
queue "Toft 2" /*put "Toft 2" on queue bottom.*/
queue /*put a "null" on queue bottom.*/
call viewQueue
do n=1 while queued()\==0
parse pull xxx
say "queue entry" n': ' xxx
end /*n*/
call viewQueue
exit /*stick a fork in it, we're done.*/
/*──────────────────────────────────viewQueue subroutine────────────────*/
viewQueue: if queued()==0 then say 'Queue is empty'
else say 'There are' queued() 'elements in the queue'
return
output
Queue is empty There are 2 elements in the queue There are 4 elements in the queue queue entry 1: Fred 2 queue entry 2: queue entry 3: Toft 2 queue entry 4: Queue is empty
Ring
# Project : Queue/Definition
load "stdlib.ring"
oQueue = new Queue
for n = 5 to 7
see "Push: " + n + nl
oQueue.add(n)
next
see "Pop: " + oQueue.remove() + nl
see "Push: 8" + nl
oQueue.add(8)
see "Pop: " + oQueue.remove() + nl
see "Pop: " + oQueue.remove() + nl
see "Pop: " + oQueue.remove() + nl
if len(oQueue) != 0
oQueue.print()
else
see "Error: queue is empty" + nl
okOutput:
Push: 5 Push: 6 Push: 7 Pop: 5 Push: 8 Pop: 6 Pop: 7 Pop: 8 Error: queue is empty
RPL
It is rather easy to create queues in RPL, thanks to the list data structure. For mysterious reasons, it is very simple to add an item to a list, but quite complex to remove one: the size difference between PUSH and POP highlights it.
| RPL code | Comment |
|---|---|
≪
QUEUE SIZE NOT ≫ 'EMPTY' STO
≪
QUEUE + 'QUEUE' STO ≫ 'PUSH' STO
≪
IF QUEUE SIZE THEN
LAST { }
1 LAST 1 - FOR j
QUEUE j GET + NEXT
QUEUE ROT GET SWAP 'QUEUE' STO
ELSE "ERR_Empty" END
≫ 'POP' STO
|
EMPTY ( -- ) Test the global variable QUEUE PUSH ( item -- ) Add the item at the beginning of the list POP ( -- item ) Initialize stack Copy all items except the last in a new list Get last item, update queue with new list Handles the case of an empty queue |
- Input:
{ } 'QUEUE' STO
EMPTY
"The" PUSH
7 PUSH
{ Wonders } PUSH
QUEUE
EMPTY
POP
- Output:
4: 1
3: { 'Wonders' 7 "The" }
2: 0
1: "The"
Shorter POP version
This approach might be suitable for small queue sizes only, since it uses the stack to temporarily store the whole queue.
≪ IF QUEUE SIZE THEN
QUEUE LIST→ ROLLD LAST 1 - →LIST 'QUEUE' STO
ELSE "ERR_Empty" END
≫ 'POP' STO
Ruby
The core class Array already implements all queue operations, so this class FIFO delegates everything to methods of Array.
require 'forwardable'
# A FIFO queue contains elements in first-in, first-out order.
# FIFO#push adds new elements to the end of the queue;
# FIFO#pop or FIFO#shift removes elements from the front.
class FIFO
extend Forwardable
# Creates a FIFO containing _objects_.
def self.[](*objects)
new.push(*objects)
end
# Creates an empty FIFO.
def initialize; @ary = []; end
# Appends _objects_ to the end of this FIFO. Returns self.
def push(*objects)
@ary.push(*objects)
self
end
alias << push
alias enqueue push
##
# :method: pop
# :call-seq:
# pop -> obj or nil
# pop(n) -> ary
#
# Removes an element from the front of this FIFO, and returns it.
# Returns nil if the FIFO is empty.
#
# If passing a number _n_, removes the first _n_ elements, and returns
# an Array of them. If this FIFO contains fewer than _n_ elements,
# returns them all. If this FIFO is empty, returns an empty Array.
def_delegator :@ary, :shift, :pop
alias shift pop
alias dequeue shift
##
# :method: empty?
# Returns true if this FIFO contains no elements.
def_delegator :@ary, :empty?
##
# :method: size
# Returns the number of elements in this FIFO.
def_delegator :@ary, :size
alias length size
# Converts this FIFO to a String.
def to_s
"FIFO#{@ary.inspect}"
end
alias inspect to_s
end
f = FIFO.new
f.empty? # => true
f.pop # => nil
f.pop(2) # => []
f.push(14) # => FIFO[14]
f << "foo" << [1,2,3] # => FIFO[14, "foo", [1, 2, 3]]
f.enqueue("bar", Hash.new, "baz")
# => FIFO[14, "foo", [1, 2, 3], "bar", {}, "baz"]
f.size # => 6
f.pop(3) # => [14, "foo", [1, 2, 3]]
f.dequeue # => "bar"
f.empty? # => false
g = FIFO[:a, :b, :c]
g.pop(2) # => [:a, :b]
g.pop(2) # => [:c]
g.pop(2) # => []
Rust
Using the standard library
The standard library has a double-ended queue implementation (VecDeque<T>) which will work here.
use std::collections::VecDeque;
fn main() {
let mut stack = VecDeque::new();
stack.push_back("Element1");
stack.push_back("Element2");
stack.push_back("Element3");
assert_eq!(Some(&"Element1"), stack.front());
assert_eq!(Some("Element1"), stack.pop_front());
assert_eq!(Some("Element2"), stack.pop_front());
assert_eq!(Some("Element3"), stack.pop_front());
assert_eq!(None, stack.pop_front());
}
A simple implementation
This shows the implementation of a singly-linked queue with dequeue and enqueue. There are two peek implementations, one returns an immutable reference, the other returns a mutable one. This implementation also shows iteration over the Queue by value (consumes queue), immutable reference, and mutable reference.
use std::ptr;
pub struct Queue<T> {
head: Link<T>,
tail: *mut Item<T>, // Raw, C-like pointer. Cannot be guaranteed safe
}
type Link<T> = Option<Box<Item<T>>>;
struct Item<T> {
elem: T,
next: Link<T>,
}
pub struct IntoIter<T>(Queue<T>);
pub struct Iter<'a, T:'a> {
next: Option<&'a Item<T>>,
}
pub struct IterMut<'a, T: 'a> {
next: Option<&'a mut Item<T>>,
}
impl<T> Queue<T> {
pub fn new() -> Self {
Queue { head: None, tail: ptr::null_mut() }
}
pub fn enqueue(&mut self, elem: T) {
let mut new_tail = Box::new(Item {
elem: elem,
next: None,
});
let raw_tail: *mut _ = &mut *new_tail;
if !self.tail.is_null() {
unsafe {
(*self.tail).next = Some(new_tail);
}
} else {
self.head = Some(new_tail);
}
self.tail = raw_tail;
}
pub fn dequeue(&mut self) -> Option<T> {
self.head.take().map(|head| {
let head = *head;
self.head = head.next;
if self.head.is_none() {
self.tail = ptr::null_mut();
}
head.elem
})
}
pub fn peek(&self) -> Option<&T> {
self.head.as_ref().map(|item| {
&item.elem
})
}
pub fn peek_mut(&mut self) -> Option<&mut T> {
self.head.as_mut().map(|item| {
&mut item.elem
})
}
pub fn into_iter(self) -> IntoIter<T> {
IntoIter(self)
}
pub fn iter(&self) -> Iter<T> {
Iter { next: self.head.as_ref().map(|item| &**item) }
}
pub fn iter_mut(&mut self) -> IterMut<T> {
IterMut { next: self.head.as_mut().map(|item| &mut **item) }
}
}
impl<T> Drop for Queue<T> {
fn drop(&mut self) {
let mut cur_link = self.head.take();
while let Some(mut boxed_item) = cur_link {
cur_link = boxed_item.next.take();
}
}
}
impl<T> Iterator for IntoIter<T> {
type Item = T;
fn next(&mut self) -> Option<Self::Item> {
self.0.dequeue()
}
}
impl<'a, T> Iterator for Iter<'a, T> {
type Item = &'a T;
fn next(&mut self) -> Option<Self::Item> {
self.next.map(|item| {
self.next = item.next.as_ref().map(|item| &**item);
&item.elem
})
}
}
impl<'a, T> Iterator for IterMut<'a, T> {
type Item = &'a mut T;
fn next(&mut self) -> Option<Self::Item> {
self.next.take().map(|item| {
self.next = item.next.as_mut().map(|item| &mut **item);
&mut item.elem
})
}
}
Scala
class Queue[T] {
private[this] class Node[T](val value:T) {
var next:Option[Node[T]]=None
def append(n:Node[T])=next=Some(n)
}
private[this] var head:Option[Node[T]]=None
private[this] var tail:Option[Node[T]]=None
def isEmpty=head.isEmpty
def enqueue(item:T)={
val n=new Node(item)
if(isEmpty) head=Some(n) else tail.get.append(n)
tail=Some(n)
}
def dequeue:T=head match {
case Some(item) => head=item.next; item.value
case None => throw new java.util.NoSuchElementException()
}
def front:T=head match {
case Some(item) => item.value
case None => throw new java.util.NoSuchElementException()
}
def iterator: Iterator[T]=new Iterator[T]{
private[this] var it=head;
override def hasNext=it.isDefined
override def next:T={val n=it.get; it=n.next; n.value}
}
override def toString()=iterator.mkString("Queue(", ", ", ")")
}
Usage:
val q=new Queue[Int]()
println("isEmpty = " + q.isEmpty)
try{q dequeue} catch{case _:java.util.NoSuchElementException => println("dequeue(empty) failed.")}
q enqueue 1
q enqueue 2
q enqueue 3
println("queue = " + q)
println("front = " + q.front)
println("dequeue = " + q.dequeue)
println("dequeue = " + q.dequeue)
println("isEmpty = " + q.isEmpty)
- Output:
isEmpty = true dequeue(empty) failed. queue = Queue(1, 2, 3) front = 1 dequeue = 1 dequeue = 2 isEmpty = false
Scheme
Using a vector for mutable data. Can be optimized by using an extra slot in the vector to hold tail pointer to avoid the append call.
(define (make-queue)
(make-vector 1 '()))
(define (push a queue)
(vector-set! queue 0 (append (vector-ref queue 0) (list a))))
(define (empty? queue)
(null? (vector-ref queue 0)))
(define (pop queue)
(if (empty? queue)
(error "can not pop an empty queue")
(let ((ret (car (vector-ref queue 0))))
(vector-set! queue 0 (cdr (vector-ref queue 0)))
ret)))
Message passing
(define (make-queue)
(let ((q (cons '() '())))
(lambda (cmd . arg)
(case cmd
((empty?) (null? (car q)))
((put) (let ((a (cons (car arg) '())))
(if (null? (car q))
(begin (set-car! q a) (set-cdr! q a))
(begin (set-cdr! (cdr q) a) (set-cdr! q a)))))
((get) (if (null? (car q)) 'empty
(let ((x (caar q)))
(set-car! q (cdar q))
(if (null? (car q)) (set-cdr! q '()))
x)))
))))
(define q (make-queue))
(q 'put 1)
(q 'put 6)
(q 'get)
; 1
(q 'get)
; 6
(q 'get)
; emptySenseTalk
A queue in SenseTalk is implemented using push and pull operations on a list.
set myFoods to be an empty list
push "grapes" into myFoods
push "orange" into myFoods
push "apricot" into myFoods
put "The foods in my queue are: " & myFoods
pull from myFoods into firstThingToEat
put "The first thing to eat is: " & firstThingToEat
if myFoods is empty then
put "The foods list is empty!"
else
put "The remaining foods are: " & myFoods
end ifOutput:
The foods in my queue are: (grapes,orange,apricot)
The first thing to eat is: grapes
The remaining foods are: (orange,apricot)Sidef
Implemented as a class:
class FIFO(*array) {
method pop {
array.is_empty && die "underflow";
array.shift;
}
method push(*items) {
array += items;
self;
}
method empty {
array.len == 0;
}
}Slate
Toy code based on Slate's Queue standard library (which is optimized for FIFO access):
collections define: #Queue &parents: {ExtensibleArray}.
q@(Queue traits) isEmpty [resend].
q@(Queue traits) push: obj [q addLast: obj].
q@(Queue traits) pop [q removeFirst].
q@(Queue traits) pushAll: c [q addAllLast: c].
q@(Queue traits) pop: n [q removeFirst: n].Smalltalk
An OrderedCollection can be easily used as a FIFO queue.
OrderedCollection extend [
push: obj [ ^(self add: obj) ]
pop [
(self isEmpty) ifTrue: [
SystemExceptions.NotFound signalOn: self
reason: 'queue empty'
] ifFalse: [
^(self removeFirst)
]
]
]
|f|
f := OrderedCollection new.
f push: 'example'; push: 'another'; push: 'last'.
f pop printNl.
f pop printNl.
f pop printNl.
f isEmpty printNl.
f pop. "queue empty error"Standard ML
Here is the signature for a basic queue:
signature QUEUE =
sig
type 'a queue
val empty_queue: 'a queue
exception Empty
val enq: 'a queue -> 'a -> 'a queue
val deq: 'a queue -> ('a * 'a queue)
val empty: 'a queue -> bool
end;A very basic implementation of this signature backed by a list is as follows:
structure Queue:> QUEUE =
struct
type 'a queue = 'a list
val empty_queue = nil
exception Empty
fun enq q x = q @ [x]
fun deq nil = raise Empty
| deq (x::q) = (x, q)
fun empty nil = true
| empty _ = false
end;Stata
Tcl
Here's a simple implementation using a list:
proc push {stackvar value} {
upvar 1 $stackvar stack
lappend stack $value
}
proc pop {stackvar} {
upvar 1 $stackvar stack
set value [lindex $stack 0]
set stack [lrange $stack 1 end]
return $value
}
proc size {stackvar} {
upvar 1 $stackvar stack
llength $stack
}
proc empty {stackvar} {
upvar 1 $stackvar stack
expr {[size stack] == 0}
}
proc peek {stackvar} {
upvar 1 $stackvar stack
lindex $stack 0
}
set Q [list]
empty Q ;# ==> 1 (true)
push Q foo
empty Q ;# ==> 0 (false)
push Q bar
peek Q ;# ==> foo
pop Q ;# ==> foo
peek Q ;# ==> barpackage require struct::queue
struct::queue Q
Q size ;# ==> 0
Q put a b c d e
Q size ;# ==> 5
Q peek ;# ==> a
Q get ;# ==> a
Q peek ;# ==> b
Q pop 4 ;# ==> b c d e
Q size ;# ==> 0UNIX Shell
queue_push() {
typeset -n q=$1
shift
q+=("$@")
}
queue_pop() {
if queue_empty $1; then
print -u2 "queue $1 is empty"
return 1
fi
typeset -n q=$1
print "${q[0]}" # emit the value of the popped element
q=( "${q[@]:1}" ) # and remove the first element from the queue
}
queue_empty() {
typeset -n q=$1
(( ${#q[@]} == 0 ))
}
queue_peek() {
typeset -n q=$1
print "${q[0]}"
}Usage:
# any valid variable name can be used as a queue without initialization
queue_empty foo && echo foo is empty || echo foo is not empty
queue_push foo bar
queue_push foo baz
queue_push foo "element with spaces"
queue_empty foo && echo foo is empty || echo foo is not empty
print "peek: $(queue_peek foo)"; queue_pop foo
print "peek: $(queue_peek foo)"; queue_pop foo
print "peek: $(queue_peek foo)"; queue_pop foo
print "peek: $(queue_peek foo)"; queue_pop foo- Output:
foo is empty foo is not empty peek: bar peek: baz peek: element with spaces peek: queue foo is empty
UnixPipes
Uses moreutils
init() {echo > fifo}
push() {echo $1 >> fifo }
pop() {head -1 fifo ; (cat fifo | tail -n +2)|sponge fifo}
empty() {cat fifo | wc -l}Usage:
push me; push you; push us; push them
|pop;pop;pop;pop
me
you
us
themV
V doesn't have mutable data. Below is an function interface for a fifo.
[fifo_create []].
[fifo_push swap cons].
[fifo_pop [[*rest a] : [*rest] a] view].
[fifo_empty? dup empty?].Using it
|fifo_create 3 fifo_push 4 fifo_push 5 fifo_push ??
=[5 4 3]
|fifo_empty? puts
=false
|fifo_pop put fifo_pop put fifo_pop put
=3 4 5
|fifo_empty? puts
=trueVBA
Public queue As New Collection
Private Sub push(what As Variant)
queue.Add what
End Sub
Private Function pop() As Variant
If queue.Count > 0 Then
what = queue(1)
queue.Remove 1
Else
what = CVErr(461)
End If
pop = what
End Function
Private Function empty_()
empty_ = queue.Count = 0
End FunctionVBScript
Using an ArrayList.
' Queue Definition - VBScript
Option Explicit
Dim queue, i, x
Set queue = CreateObject("System.Collections.ArrayList")
If Not empty_(queue) Then Wscript.Echo queue.Count
push queue, "Banana"
push queue, "Apple"
push queue, "Pear"
push queue, "Strawberry"
Wscript.Echo "Count=" & queue.Count
Wscript.Echo pull(queue) & " - Count=" & queue.Count '
Wscript.Echo "Head=" & queue.Item(0)
Wscript.Echo "Tail=" & queue.Item(queue.Count-1)
Wscript.Echo queue.IndexOf("Pear", 0)
For i=1 To queue.Count
Wscript.Echo join(queue.ToArray(), ", ")
x = pull(queue)
Next 'i
Sub push(q, what)
q.Add what
End Sub 'push
Function pull(q)
Dim what
If q.Count > 0 Then
what = q(0)
q.RemoveAt 0
Else
what = ""
End If
pull = what
End Function 'pull
Function empty_(q)
empty_ = q.Count = 0
End Function 'empty_- Output:
Count=4 Banana - Count=3 Head=Apple Tail=Strawberry 1 Apple, Pear, Strawberry Pear, Strawberry Strawberry
V (Vlang)
Updated to V (Vlang) version 0.2.2
const max_tail = 256
struct Queue<T> {
mut:
data []T
tail int
head int
}
fn (mut queue Queue<T>) push(value T) {
if queue.tail >= max_tail || queue.tail < queue.head {
return
}
println('push: $value')
queue.data << value
queue.tail++
}
fn (mut queue Queue<T>) pop() !T {
if queue.tail > 0 && queue.head < queue.tail {
result := queue.data[queue.head]
queue.head++
println('Dequeue: top of Queue was $result')
return result
}
return error('Queue Underflow!!')
}
fn (queue Queue<T>) peek() !T {
if queue.tail > 0 && queue.head < queue.tail {
result := queue.data[queue.head]
println('Peek: top of Queue is $result')
return result
}
return error('Out of Bounds...')
}
fn (queue Queue<T>) empty() bool {
return queue.tail == 0
}
fn main() {
mut queue := Queue<f64>{}
println('Queue is empty? ' + if queue.empty() { 'Yes' } else { 'No' })
queue.push(5.0)
queue.push(4.2)
println('Queue is empty? ' + if queue.empty() { 'Yes' } else { 'No' })
queue.peek() or { return }
queue.pop() or { return }
queue.pop() or { return }
queue.push(1.2)
}- Output:
Queue is empty? Yes Enqueue: 5.00 Enqueue: 4.20 Queue is empty? No Peek: top of Queue is 5.00 Dequeue: top of Queue was 5.00 Dequeue: top of Queue was 4.20 Enqueue: 1.20
Wart
Wart defines queues as lists with a pointer to the last element saved for constant-time enqueuing:
def (queue seq)
(tag queue (list seq lastcons.seq len.seq))
def (enq x q)
do1 x
let (l last len) rep.q
rep.q.2 <- (len + 1)
if no.l
rep.q.1 <- (rep.q.0 <- list.x)
rep.q.1 <- (cdr.last <- list.x)
def (deq q)
let (l last len) rep.q
ret ans car.l
unless zero?.len
rep.q.2 <- (len - 1)
rep.q.0 <- cdr.l
def (len q) :case (isa queue q)
rep.q.2empty? relies on len by default, so there's no need to separately override it.
Wren
The above module contains a suitable Queue class.
import "./queue" for Queue
var q = Queue.new()
var item = q.pop()
if (item == null) {
System.print("ERROR: attempted to pop from an empty queue")
} else {
System.print("'%(item)' was popped")
}- Output:
ERROR: attempted to pop from an empty queue
XLISP
A queue is similar to a stack, except that values are pushed onto and popped from different "ends" of it (whereas in a stack it is the same end for both operations). This implementation is based on the XLISP implementation of a stack, therefore, but with a push method that appends a new value to the end rather than sticking it onto the front. Attempting to pop from an empty queue will return the empty list, equivalent to Boolean "false".
(define-class queue
(instance-variables vals))
(define-method (queue 'initialize)
(setq vals '())
self)
(define-method (queue 'push x)
(setq vals (nconc vals (cons x nil))))
(define-method (queue 'pop)
(define val (car vals))
(setq vals (cdr vals))
val)
(define-method (queue 'emptyp)
(null vals))A sample REPL session:
[1] (define my-queue (queue 'new))
MY-QUEUE
[2] (my-queue 'push 1)
(1)
[3] (my-queue 'push 2)
(1 2)
[4] (my-queue 'emptyp)
()
[5] (my-queue 'pop)
1
[6] (my-queue 'pop)
2
[7] (my-queue 'emptyp)
#T
[8] (my-queue 'pop)
()XPL0
include c:\cxpl\codes;
def Size=8;
int Fifo(Size);
int In, Out; \fill and empty indexes into Fifo
proc Push(A); \Add integer A to queue
int A; \(overflow not detected)
[Fifo(In):= A;
In:= In+1;
if In >= Size then In:= 0;
];
func Pop; \Return first integer in queue
int A;
[if Out=In then \if popping empty queue
[Text(0, "Error"); exit 1]; \ then exit program with error code 1
A:= Fifo(Out);
Out:= Out+1;
if Out >= Size then Out:= 0;
return A;
];
func Empty; \Return 'true' if queue is empty
return In = Out;
[In:= 0; Out:= 0;
Push(0);
Text(0, if Empty then "true" else "false"); CrLf(0);
IntOut(0, Pop); CrLf(0);
Push(1);
Push(2);
Push(3);
IntOut(0, Pop); CrLf(0);
IntOut(0, Pop); CrLf(0);
IntOut(0, Pop); CrLf(0);
Text(0, if Empty then "true" else "false"); CrLf(0);
\A 256-byte queue is built in as device 8:
OpenI(8); OpenO(8);
ChOut(8, ^0); \push
ChOut(0, ChIn(8)); CrLf(0); \pop
ChOut(8, ^1); \push
ChOut(8, ^2); \push
ChOut(8, ^3); \push
ChOut(0, ChIn(8)); CrLf(0); \pop
ChOut(0, ChIn(8)); CrLf(0); \pop
ChOut(0, ChIn(8)); CrLf(0); \pop
]Output:
false 0 1 2 3 true 0 1 2 3
zkl
class Queue{
var [const] q=List();
fcn push { q.append(vm.pasteArgs()) }
fcn pop { q.pop(0) }
fcn empty { q.len()==0 }
}q:=Queue();
q.push(1,2,3);
q.pop(); //-->1
q.empty(); //-->False
q.pop();q.pop();q.pop() //-->IndexError thrown