Generic swap
You are encouraged to solve this task according to the task description, using any language you may know.
- Task
Write a generic swap function or operator which exchanges the values of two variables (or, more generally, any two storage places that can be assigned), regardless of their types.
If your solution language is statically typed please describe the way your language provides genericity.
If variables are typed in the given language, it is permissible that the two variables be constrained to having a mutually compatible type, such that each is permitted to hold the value previously stored in the other without a type violation. That is to say, solutions do not have to be capable of exchanging, say, a string and integer value, if the underlying storage locations are not attributed with types that permit such an exchange.
Generic swap is a task which brings together a few separate issues in programming language semantics.
Dynamically typed languages deal with values in a generic way quite readily, but do not necessarily make it easy to write a function to destructively swap two variables, because this requires indirection upon storage places or upon the syntax designating storage places.
Functional languages, whether static or dynamic, do not necessarily allow a destructive operation such as swapping two variables regardless of their generic capabilities.
Some static languages have difficulties with generic programming due to a lack of support for (Parametric Polymorphism).
Do your best!
11l
F swap(&a, &b)
(a, b) = (b, a)360 Assembly
Three consecutive exclusive OR's swap variable contents
SWAP CSECT , control section start
BAKR 14,0 stack caller's registers
LR 12,15 entry point address to reg.12
USING SWAP,12 use as base
MVC A,=C'5678____' init field A
MVC B,=C'____1234' init field B
LA 2,L address of length field in reg.2
WTO TEXT=(2) Write To Operator, results in:
* +5678________1234
XC A,B XOR A,B
XC B,A XOR B,A
XC A,B XOR A,B. A holds B, B holds A
WTO TEXT=(2) Write To Operator, results in:
* +____12345678____
PR , return to caller
LTORG , literals displacement
L DC H'16' halfword containg decimal 16
A DS CL8 field A, 8 bytes
B DS CL8 field B, 8 bytes
END SWAP program end6502 Assembly
There are no atomic swap operations in 6502 Assembly but there are a few slower ways of doing it, using the stack. The 65c02 and 65816 revisions added a few commands which make swapping easier. The need for atomic swapping isn't really an issue since most 6502-based computers didn't use parallel processing; the biggest concern for registers or memory getting clobbered was from interrupts, which can be easily avoided by restricting the registers and/or memory they use, and using the stack to back up and restore their values at the beginning and end.
NMOS 6502
The original 6502 can't push X or Y onto the stack directly, and must do so through the accumulator. The most common way of swapping values is by pushing them onto the stack and popping them in the "wrong" order on purpose.
;swap X with Y
pha ;push accumulator
txa
pha ;push x
tya
pha ;push y
pla
tax ;pop y into x
pla
tay ;pop x into y
pla ;pop accumulatorSwapping A with X or Y is easiest with using zero page memory as a temporary store, or with self-modifying code.
sta temp ;a label representing a zero page memory address
txa
pha ;push x
ldx temp
pla ;pop x into accumulatorSwapping memory locations is fairly straightforward on all versions of the 6502:
temp equ $00
temp2 equ $01 ;these values don't matter.
lda temp
pha ;push temp
lda temp2
pha ;push temp2
pla
sta temp ;pop temp2 into temp
pla
sta temp2 ;pop temp into temp2CMOS 65c02
This revision of the 6502 can push X and Y onto the stack directly. The above code can be condensed quite a bit. This is a different example that better illustrates how swapping can be done:
lda #$33
ldx #$44
ldy #$55
pha
phx
phy
pla ;a=#$55
ply ;y=#$44
plx ;x=#$3368000 Assembly
Two registers can be swapped with EXG:
EXG D0,D1 ;swap the contents of D0 and D1
EXG A0,A1 ;swap the contents of A0 and A1
However, these commands cannot be used with memory. If you want to do that you'll need to use the stack or a data register.
MOVE.L ($00FF0000),D0
MOVE.L ($00FF1000),D1
MOVE.L D0,($00FF1000)
MOVE.L D1,($00FF0000)
The SWAP switches the high and low words of a data register. It can't be used with memory either.
MOVE.L #$1234ABCD,D0
SWAP D0 ;now D0 = #$ABCD1234
8086 Assembly
The XCHG command swaps two registers' values. It can also work with memory that is pointed to by an index register such as SI, DI,or BP. Both registers must be the same size, which enforces type matching. You cannot XCHG AX,CL for example since AX is 16-bit and CL is only 8-bit.
Here are some examples of the XCHG command in action:
xchg ax,bx ;exchanges ax with bx
xchg ah,al ;swap the high and low bytes of ax
;XCHG a register with memory
mov dx,0FFFFh
mov word ptr [ds:userRam],dx
mov si,offset userRam
mov ax,1234h
xchg ax,[si] ;exchange ax with the value stored at userRam. Now, ax = 0FFFFh and the value stored at userRam = 1234h
;XCHG a register with a value on the stack.
mov ax,1234h
mov bx,4567h
push bx
push bp
mov bp,sp ;using [sp] as an operand for XCHG will not work. You need to use bp instead.
xchg ax,[2+bp] ;exchanges AX with the value that was pushed from BX onto the stack. Now, AX = 4567h,
;and the entry on the stack just underneath the top of the stack is 1234h.
8th
swap
Or to swap between the stack and a var:
dup @ -rot !
ACL2
(defun swap (pair)
(cons (cdr pair)
(car pair)))
(let ((p (cons 1 2)))
(cw "Before: ~x0~%After: ~x1~%" p (swap p)))
Action!
INCLUDE "D2:REAL.ACT" ;from the Action! Tool Kit
PROC Swap(BYTE POINTER ptr1,ptr2 INT size)
BYTE tmp
INT i
FOR i=0 TO size-1
DO
tmp=ptr1^ ptr1^=ptr2^ ptr2^=tmp
ptr1==+1 ptr2==+1
OD
RETURN
PROC Main()
BYTE b1=[13],b2=[56]
INT i1=[65234],i2=[534]
REAL r1,r2
CHAR ARRAY s1="abcde",s2="XYZ"
Put(125) PutE() ;clear the screen
ValR("32.5",r1) ValR("-0.63",r2)
Print("Swap bytes: ")
PrintB(b1) Put(32) PrintB(b2) Print(" -> ")
Swap(@b1,@b2,1)
PrintB(b1) Put(32) PrintBE(b2)
Print("Swap integers: ")
PrintI(i1) Put(32) PrintI(i2) Print(" -> ")
Swap(@i1,@i2,2)
PrintI(i1) Put(32) PrintIE(i2)
Print("Swap floats: ")
PrintR(r1) Put(32) PrintR(r2) Print(" -> ")
Swap(r1,r2,6)
PrintR(r1) Put(32) PrintRE(r2)
Print("Swap strings: ")
Print(s1) Put(32) Print(s2) Print(" -> ")
Swap(@s1,@s2,2)
Print(s1) Put(32) PrintE(s2)
RETURN- Output:
Screenshot from Atari 8-bit computer
Swap bytes: 13 56 -> 56 13 Swap integers: -302 534 -> 534 -302 Swap floats: 32.5 -0.63 -> -0.63 32.5 Swap strings: abcde XYZ -> XYZ abcde
Ada
The generic parameters for an Ada generic procedure are defined in a procedure specification, while the algorithm is defined in a procedure body. The first code snippet is the procedure specification. The second code snippet is the procedure body.
generic
type Swap_Type is private; -- Generic parameter
procedure Generic_Swap (Left, Right : in out Swap_Type);
procedure Generic_Swap (Left, Right : in out Swap_Type) is
Temp : constant Swap_Type := Left;
begin
Left := Right;
Right := Temp;
end Generic_Swap;
usage
To use the generic swap procedure, you need to instantiate the procedure for each type that you intend to use.
with Generic_Swap;
...
type T is ...
procedure T_Swap is new Generic_Swap (Swap_Type => T);
A, B : T;
...
T_Swap (A, B);
Aime
Aime is statically typed. A generic swap utility may nonetheless be defined in terms of parameters of unspecified type and pass by reference.
void
__swap(&, &,,)
{
set(0, $3);
set(1, $2);
}
void
swap(&, &)
{
xcall(xcall, __swap);
}ALGOL 68
A generic swap operator =:= was proposed in ALGOL Bulletin for standard ALGOL 68 so that the compiler could optimise the operation. However such an operator was not adopted and needs to be manually defined for each mode required.
MODE GENMODE = STRING;
GENMODE v1:="Francis Gary Powers", v2:="Vilyam Fisher";
PRIO =:= = 1;
OP =:= = (REF GENMODE v1, v2)VOID: (
GENMODE tmp:=v1; v1:=v2; v2:=tmp
);
v1 =:= v2;
print(("v1: ",v1, ", v2: ", v2, new line))- Output:
v1: Vilyam Fisher, v2: Francis Gary Powers
Special option
The B6700 Algol compiler offered access to a special machine operation via a function called ReadLock(a,b) that could be invoked on a variety of operands. By using the ability to #define this = that; one could define Swap(a,b) to be a:=ReadLock(a,b) to attain the appearance of a Swap operation. This all relied on the working of magnetic core memory, specifically that to read a word, the word is made zero and in the process the memory hardware notes which bits were thereby flipped. Thus it passes on the value in the word and meanwhile, rewrites that content back to the word so as to preserve its value on reading. Similarly, to write a value to a word, the word is first zeroed.
ReadLock(a,b) functioned by reading a and writing its value to b, but also, recovering the value that was in b which it returns as the result of the function - which is written to a by the assignment, completing the swap. The ReadLock part is "atomic" or not interruptable, so it is used in semaphores and the like, but was available for other use. It swapped a single word, so could swap types such as integers or floating-point numbers (single precision) thus being somewhat generic.
Amazing Hopper
#include <flow.h>
DEF-MAIN(argv,argc)
DIM(10) AS-INT-RAND( 10, random array )
SET( single var, 0.5768 )
PRNL( "SINGLE VAR: ", single var, "\nRANDOM ARRAY: ", random array )
single var <-> random array
PRNL( "SINGLE VAR: ", single var, "\nRANDOM ARRAY: ", random array )
END- Output:
$ hopper swap.flw SINGLE VAR: 0.5768 RANDOM ARRAY: 4,2,5,10,8,3,4,8,3,5 SINGLE VAR: 4,2,5,10,8,3,4,8,3,5 RANDOM ARRAY: 0.5768 $
AmigaE
The simpler way to write a swap is to use the Amiga E ability to return multiple values. All basic data type in Amiga E can be held by its LONG type, and complex data type (like lists) are indeed pointers (which fits into a LONG too); so, because of the fact that Amiga E is not strongly typed, this solution works for any type.
PROC swap(a,b) IS b,a
PROC main()
DEF v1, v2, x
v1 := 10
v2 := 20
v1, v2 := swap(v1,v2)
WriteF('\d \d\n', v1,v2) -> 20 10
v1 := [ 10, 20, 30, 40 ]
v2 := [ 50, 60, 70, 80 ]
v1, v2 := swap(v1,v2)
ForAll({x}, v1, `WriteF('\d ',x)) -> 50 60 70 80
WriteF('\n')
ForAll({x}, v2, `WriteF('\d ',x)) -> 10 20 30 40
WriteF('\n')
ENDPROCAppleScript
AppleScript has built-in support for swapping. This is generic and works for all combinations of data types.
set {x,y} to {y,x}
Arc
(mac myswap (a b)
(w/uniq gx
`(let ,gx a
(= a b)
(= b ,gx))))
(with (a 1
b 2)
(myswap a b)
(prn "a:" a #\Newline "b:" b))Arturo
Using multi-assignment
swap: function [a,b]-> @[b,a]
c: 1
d: 2
print [c d]
[c,d]: swap c d
print [c d]
- Output:
1 2 2 1
In-place modification
swap: function [a,b].exportable [
tmp: var b
let b var a
let a tmp
]
c: 1
d: 2
print [c d]
swap 'c 'd
print [c d]
- Output:
1 2 2 1
AutoHotkey
Swap(ByRef Left, ByRef Right)
{
temp := Left
Left := Right
Right := temp
}
AWK
# syntax: GAWK -f GENERIC_SWAP.AWK
BEGIN {
printf("%s version %s\n",ARGV[0],PROCINFO["version"])
foo = 1
bar = "a"
printf("\n%s %s\n",foo,bar)
rc = swap("foo","bar") # ok
printf("%s %s %s\n",foo,bar,rc?"ok":"ng")
printf("\n%s %s\n",foo,bar)
rc = swap("FOO","BAR") # ng
printf("%s %s %s\n",foo,bar,rc?"ok":"ng")
exit(0)
}
function swap(a1,a2, tmp) { # strings or numbers only; no arrays
if (a1 in SYMTAB && a2 in SYMTAB) {
if (isarray(SYMTAB[a1]) || isarray(SYMTAB[a2])) {
return(0)
}
tmp = SYMTAB[a1]
SYMTAB[a1] = SYMTAB[a2]
SYMTAB[a2] = tmp
return(1)
}
return(0)
}
- Output:
gawk version 4.1.0 1 a a 1 ok a 1 a 1 ng
Axe
The Exch() command can swap data of any size at any two addresses. This example swaps two 2-byte variables.
Exch(°A,°B,2)BASIC
Applesoft BASIC
A=43:B=47:H=A:A=B:B=H:?" A="A" B="B;A=47 B=43
BaCon
x = 1
y$ = "hello"
SWAP x, y$
PRINT y$
BASIC256
global a, b
a = "one"
b = "two"
print a, b
call swap(a, b)
print a, b
end
subroutine swap(a, b)
temp = a : a = b : b = temp
end subroutineBBC BASIC
Built-in function
a = 1.23 : b = 4.56
SWAP a,b
PRINT a,b
a$ = "Hello " : b$ = "world!"
SWAP a$,b$
PRINT a$,b$
Custom function
a = 1.23 : b = 4.56
PROCswap(^a,^b, 5)
PRINT a,b
a$ = "Hello " : b$ = "world!"
PROCswap(^a$,^b$, 6)
PRINT a$,b$
END
DEF PROCswap(a%, b%, s%)
LOCAL i%
FOR i% = 0 TO s%-1
SWAP a%?i%,b%?i%
NEXT
ENDPROC
- Output:
4.56 1.23 world! Hello
Chipmunk Basic
The GW-BASIC solution works without any changes.
FreeBASIC
FreeBASIC already has a built-in generic Swap procedure but a macro can be used to build another one:
' FB 1.05.0
#Macro Declare_Swap(T)
Sub Swap_##T(ByRef t1 As T, ByRef t2 As T)
Dim temp As T = t2
t2 = t1
t1 = temp
End Sub
#EndMacro
Dim As Integer i, j
i = 1 : j = 2
Declare_Swap(Integer) ' expands the macro
Swap_Integer(i, j)
Print i, j
Dim As String s, t
s = "Hello" : t = "World"
Declare_Swap(String)
Swap_String(s, t)
Print s, t
Print
Print "Press any key to exit"
Sleep
- Output:
2 1 World Hello
Gambas
Click this link to run this code
Public Sub Main()
Dim vA As Variant = " World"
Dim vB As Variant = 1
Swap vA, vB
Print vA; vB
End
Output:
1 World
GW-BASIC
10 CLS : REM 10 HOME for Applesoft BASIC
20 A = 1 : B = 2
30 PRINT " A=";A," B=";B
40 T = A : A = B : B = T
50 PRINT " A=";A," B=";B
60 END
- Output:
A=1 B=2 A=2 B=1
IS-BASIC
100 DEF SWAP(REF A,REF B)
110 LET T=A:LET A=B:LET B=T
120 END DEF
130 LET A=1:LET B=2
140 PRINT A,B
150 CALL SWAP(A,B)
160 PRINT A,BMinimal BASIC
10 LET A = 1
20 LET B = 2
30 PRINT " A=";A;" B=";B
40 LET T = A
50 LET A = B
60 LET B = T
70 PRINT " A=";A;" B=";B
80 END
MSX Basic
The GW-BASIC solution works without any changes.
OxygenBasic
macro Swap(a,b, c)
typeof(a) c
c=a
a=b
b=c
end macro
'demo with compound types:
'=========================
type point { float x,y}
point p={1,2}
point q={3,4}
swap p,q
print "p: " p.x "," p.y 'p: 3,4
print "q: " q.x "," q.y 'q: 1,2
PureBasic
Built in function:
Swap a, b
QBasic
QBasic already has a generic swap procedure built in, but a new subroutine can be defined:
SUB nswap (a, b)
temp = a: a = b: b = temp
END SUB
a = 1
b = 2
PRINT a, b
CALL nswap(a, b)
PRINT a, b
Quite BASIC
The Minimal BASIC solution works without any changes.
Run BASIC
Run BASIC does not have support for swapping built in:
a = 1
b = 2
'----- swap ----
tmp = a
a = b
b = tmp
endThinBASIC
Generic function, swap the content of two variables.
Swap Var1, Var2
TI-89 BASIC
TI-89 BASIC is dynamically typed, so the genericity is implicit. It has no pass by reference, so we must pass the variable names as strings. It is dynamically scoped, so we must choose hopefully distinct names for the variables.
Define swap(swapvar1, swapvar2) = Prgm
Local swaptmp
#swapvar1 → swaptmp
#swapvar2 → #swapvar1
swaptmp → #swapvar2
EndPrgm
1 → x
2 → y
swap("x", "y")
x
2
y
1Tiny BASIC
LET a = 11
LET b = 22
PRINT a, " ", b
GOSUB 100
PRINT a, " ", b
END
100 REM swap(a, b)
LET t = a
LET a = b
LET b = t
RETURNTrue BASIC
SUB swap(a, b)
LET temp = a
LET a = b
LET b = temp
END SUB
LET a = 1
LET b = 2
PRINT a, b
CALL swap(a, b)
PRINT a, b
END
uBasic/4tH
Since uBasic/4tH has a stack (just like Forth) and it is an integer BASIC only, this is quite trivial. However, making a function or procedure with the same functionality is impossible, because there is no way to pass variables by reference.
a = 5 : b = 7
Print a,b
Push a,b : a = Pop() : b = Pop()
Print a,b
VBScript
This works for everything: strings, dates, booleans ... The fact is, with everything being a Variant, it's always generic.
sub swap( byref x, byref y )
dim temp
temp = x
x = y
y = temp
end sub
Usage:
dim a
a = "woof"
dim b
b = now()
swap a,b
wscript.echo a
wscript.echo b
- Output:
5/02/2010 2:35:36 PM
woof
Visual Basic
Visual Basic can use the VBScript example above, with the caveat that it won't work if any DEFtype (except DefVar) has been used. (The default data type is Variant, which can be used as a stand-in for any variable type.)
Also, the sub will fail if one arg is a string containing non-numeric data and the other arg is numeric.
Visual Basic .NET
Semantically identical to C#
Sub Swap(Of T)(ByRef a As T, ByRef b As T)
Dim temp = a
a = b
b = temp
End Sub
Usage:
Dim a = 1, b = 2
Swap(a, b)
XBasic
XBasic already has a generic swap procedure built in, but a new subroutine can be defined:
PROGRAM "nswap"
VERSION "0.0000"
DECLARE FUNCTION Entry ()
FUNCTION Entry ()
a = 1
b = 2
PRINT a, b
GOSUB nswap
PRINT a, b
SUB nswap
temp = a: a = b: b = temp
END SUB
END FUNCTION
END PROGRAM
Yabasic
Yabasic already has a generic swap procedure built in.
a = 1
b = 2
print a, b
//----- swap ----
temp = a : a = b : b = temp
print a, bBatch File
Swap using pass-by-name
@echo off
setlocal enabledelayedexpansion
set a=1
set b=woof
echo %a%
echo %b%
call :swap a b
echo %a%
echo %b%
goto :eof
:swap
set temp1=!%1!
set temp2=!%2!
set %1=%temp2%
set %2=%temp1%
goto :eof
Beads
beads 1 program 'Generic swap'
var
a = [1 2 "Beads" 3 4]
b = [1 2 "Language" 4 5]
calc main_init
swap a[4] <=> b[3]- Output:
A = [1, 2, "Beads", 3, 4] B = [1, 2, "Language", 4, 5] ------------------------------------------------- A = [1, 2, "Beads", "Language", 4] B = [1, 2, 3, 4, 5]
Binary Lambda Calculus
From https://github.com/tromp/AIT/blob/master/rosetta/swap.lam we get the 25-bit swap function
0001100000000101101101110
BQN
Two variables can be exchanged with modified assignment:
a‿b ⌽↩
With no right-hand side, this expands to a one-argument call a‿b ↩ ⌽ a‿b. BQN doesn't make variable names or references first-class, so it wouldn't make sense to make this generic over variable names.
Bracmat
(!a.!b):(?b.?a)Burlesque
\/Stack-based swap.
C
This has a restriction that a and b must be the same size.
void swap(void *va, void *vb, size_t s)
{
char t, *a = (char*)va, *b = (char*)vb;
while(s--)
t = a[s], a[s] = b[s], b[s] = t;
}
If you have gcc, you can write a preprocessor macro with __typeof__.
- Caution: __typeof__ is a gcc extension, not part of standard C. __typeof__ does not conflict with C89 because the standard allows compilers to add keywords with underscores like __typeof__.
#define Swap(X,Y) do{ __typeof__ (X) _T = X; X = Y; Y = _T; }while(0)
Usage examples are:
#include <stdio.h>
#define Swap(X,Y) do{ __typeof__ (X) _T = X; X = Y; Y = _T; }while(0)
struct test
{
int a, b, c;
};
int main()
{
struct test t = { 1, 2, 3 };
struct test h = { 4, 5, 6 };
double alfa = 0.45, omega = 9.98;
struct test *pt = &t;
struct test *th = &h;
printf("%d %d %d\n", t.a, t.b, t.c );
Swap(t, h);
printf("%d %d %d\n", t.a, t.b, t.c );
printf("%d %d %d\n", h.a, h.b, h.c );
printf("%lf\n", alfa);
Swap(alfa, omega);
printf("%lf\n", alfa);
printf("%d\n", pt->a);
Swap(pt, th);
printf("%d\n", pt->a);
}
This is tested with GCC with -std=c89 option.
C#
C#: Using a generic method
C# 2.0 introduced the concept of generics to the language. Generics are outwardly similar to C++ templates, but are implemented quite differently: generics are maintained generically at runtime rather than being substitued with definite types by the compiler. Generics are intended to promote reusable, efficient, type-safe code, and are used widely throughout the .NET framework and 3rd party libraries, especially in collections. C# generics are less flexible than C++ templates, but are more strongly typed and arguably easier to work with.
static void Swap<T>(ref T a, ref T b)
{
T temp = a;
a = b;
b = temp;
}
Usage:
int a = 1;
int b = 2;
Swap(ref a, ref b); // Type parameter is inferred.
C#: Using tuple syntax
C# 7.0 introduced language support for tuples, which are implemented using the ValueTuple family of structs. The example below creates a tuple with the values of b and a and uses deconstructing assignment to assign the members of the tuple back to the variables.
int a = 1;
int b = 2;
(a, b) = (b, a);
C++
Generic programming in C++ is provided through templates. Templates in C++ are quite powerful: They form a Turing-complete compile-time sub-language. However, that power isn't needed for swap. Note that the C++ standard library already provides a swap function which contains optimized implementations for standard library types; thus it's advisable to use that instead of a self-written variant like the one below.
While the standard allows to separate declaration and definition of templates into different files using the export keyword, most compilers (including the most used ones) don't implement that. Therefore in practice, templates declared in header files also have to be defined there.
The implementation of the swap function template is straightforward:
template<typename T> void swap(T& left, T& right)
{
T tmp(left);
left = right;
right = tmp;
}
Note that this function requires that the type T has an accessible copy constructor and assignment operator.
The standard utility 'swap' can be used to swap two values:
std::swap(x,y);
It will work with any types.
C++11
C++11 adds move constructors which can be more efficient than copy constructors.
template<class T>
void swap(T &lhs, T &rhs){
T tmp = std::move(lhs);
lhs = std::move(rhs);
rhs = std::move(tmp);
}
Chapel
Chapel includes a swap operator:
a <=> b
and supports swapping directly via tuples and destructuring:
(a, b) = (b, a)
Both variables must be of the same type. The Fibonnacci implementation contains an example.
Clojure
(defn swap [pair] (reverse pair)) ; returns a list
(defn swap [[a b]] '(b a)) ; returns a list
(defn swap [[a b]] [b a]) ; returns a vector
The latter two implementations use destructured binding to define local names for the two elements.
CMake
CMake has only one data type: the string.
function(swap var1 var2)
set(_SWAP_TEMPORARY "${${var1}}")
set(${var1} "${${var2}}" PARENT_SCOPE)
set(${var2} "${_SWAP_TEMPORARY}" PARENT_SCOPE)
endfunction(swap)
set(x 42)
set(y "string")
swap(x y)
message(STATUS ${x}) # -- string
message(STATUS ${y}) # -- 42
Because of limitations in CMake, there are a few specific situations where swap() will fail to swap the variables.
- When _SWAP_TEMPORARY is the name of the second variable: Inside swap(), its local variable _SWAP_TEMPORARY shadows the original _SWAP_TEMPORARY from the parent scope, preventing access to the original value.
set(x 42) set(_SWAP_TEMPORARY "string") swap(x _SWAP_TEMPORARY) message(STATUS ${x}) # -- 42 message(STATUS ${_SWAP_TEMPORARY}) # -- 42
- When value of either variable is "CACHE" or "PARENT_SCOPE": swap() can never set a variable to "CACHE" or "PARENT_SCOPE", because these are keywords of set() command.
string(TOUPPER CACHE x) set(y "string") swap(x y) # CMake Error... set given invalid arguments for CACHE mode.
COBOL
PROGRAM-ID. SWAP-DEMO.
AUTHOR. Bill Gunshannon.
INSTALLATION. Home.
DATE-WRITTEN. 16 December 2021.
************************************************************
** Program Abstract:
** A simple program to demonstrate the SWAP subprogram.
**
************************************************************
DATA DIVISION.
WORKING-STORAGE SECTION.
01 Val1 PIC X(72).
01 Val2 PIC X(72).
PROCEDURE DIVISION.
Main-Program.
DISPLAY 'Enter a Value: ' WITH NO ADVANCING.
ACCEPT Val1.
DISPLAY 'Enter another Value: ' WITH NO ADVANCING.
ACCEPT Val2.
DISPLAY ' ' .
DISPLAY 'First value: ' FUNCTION TRIM(Val1) .
DISPLAY 'Second value: ' FUNCTION TRIM(Val2) .
CALL "SWAP" USING BY REFERENCE Val1, BY REFERENCE Val2.
DISPLAY ' '.
DISPLAY 'After SWAP '.
DISPLAY ' '.
DISPLAY 'First value: ' FUNCTION TRIM(Val1).
DISPLAY 'Second value: ' FUNCTION TRIM(Val2).
STOP RUN.
END PROGRAM SWAP-DEMO.
IDENTIFICATION DIVISION.
PROGRAM-ID. SWAP.
AUTHOR. Bill Gunshannon.
INSTALLATION. Home.
DATE-WRITTEN. 16 December 2021.
************************************************************
** Program Abstract:
** SWAP any Alphanumeric value. Only limit is 72
** character size. But that can be adjusted for
** whatever use one needs.
************************************************************
DATA DIVISION.
WORKING-STORAGE SECTION.
01 TEMP PIC X(72).
LINKAGE SECTION.
01 Field1 PIC X(72).
01 Field2 PIC X(72).
PROCEDURE DIVISION
USING BY REFERENCE Field1, BY REFERENCE Field2.
MOVE Field1 to TEMP.
MOVE Field2 to Field1.
MOVE TEMP to Field2.
GOBACK.
END PROGRAM SWAP.
- Output:
Enter a Value: 33 Enter another Value: 77 First value: 33 Second value: 77 After SWAP First value: 77 Second value: 33 -------------------------------- Enter a Value: Hello World Enter another Value: Good Bye First value: Hello World Second value: Good Bye After SWAP First value: Good Bye Second value: Hello World
ColdFusion
This is another standard swap.
<cfset temp = a />
<cfset a = b />
<cfset b = temp />
Common Lisp
(rotatef a b)
(psetq a b b a)
Computer/zero Assembly
LDA 29
STA 31
LDA 30
STA 29
LDA 31
STA 30
STP
---
org 29
byte $0F
byte $E0
byte $00
- Output:
Emulator with program loaded Hexdump of address 29-31 after running:
E0 0F 0F
Crystal
Crystal directly supports swapping:
a, b = b, a
D
import std.algorithm: swap; // from Phobos standard library
// The D solution uses templates and it's similar to the C++ one:
void mySwap(T)(ref T left, ref T right) {
auto temp = left;
left = right;
right = temp;
}
void main() {
import std.stdio;
int[] a = [10, 20];
writeln(a);
// The std.algorithm standard library module
// contains a generic swap:
swap(a[0], a[1]);
writeln(a);
// Using mySwap:
mySwap(a[0], a[1]);
writeln(a);
}
- Output:
[10, 20] [20, 10] [10, 20]
dc
We use two registers to swap in POSIX dc.
1 2 SaSbLaLb f
=2 1Reverse (r) is a built-in stack command available as a GNU extension for dc.
1 2 r f
=2 1DCL
symbols do not have to be declared, they can be integers or strings, they can change type on the fly
$ a1 = 123
$ a2 = "hello"
$ show symbol a*
$ gosub swap
$ show symbol a*
$ exit
$
$ swap:
$ t = a1
$ a1 = a2
$ a2 = t
$ return
- Output:
$ @generic_swap A1 = 123 Hex = 0000007B Octal = 00000000173 A2 = "hello" A1 = "hello" A2 = 123 Hex = 0000007B Octal = 00000000173
Delphi
Delphi does not have generics as such. The following code must be copied for each type that a swap is required. T should be changed to the required type.
procedure Swap_T(var a, b: T);
var
temp: T;
begin
temp := a;
a := b;
b := temp;
end;
Generics were introduced with Delphi 2009
program GenericSwap;
type
TSwap = class
class procedure Swap<T>(var left, right: T);
end;
class procedure TSwap.Swap<T>(var left, right: T);
var
temp : T;
begin
temp := left;
left := right;
right := temp;
end;
var
a, b : integer;
begin
a := 5;
b := 3;
writeln('Before swap: a=', a, ' b=', b);
TSwap.Swap<integer>(a, b);
writeln('After swap: a=', a, ' b=', b);
end.
Déjà Vu
To swap the two top-most items on the stack:
swapTo swap two variables without needing a third name, using the stack for temporary storage:
set :a set :b @a @bDuckDB
Columns in the same table
No special syntax is required to swap two columns in the same table, but the columns must have compatible types:
D select a as b, b as a from (select 1 as a, 2 as b); ┌───────┬───────┐ │ b │ a │ │ int32 │ int32 │ ├───────┼───────┤ │ 1 │ 2 │ └───────┴───────┘
Keys of a JSON object
create or replace function json_swap(j, a, b) as (
json_merge_patch(j, json_object(a, (j ->> b), b, (j ->> a) ))
);
select json_swap( {'a':1, 'b':2, 'c': 3}::JSON, 'a', 'b') as swapped;
- Output:
┌─────────────────────────┐
│ swapped │
│ json │
├─────────────────────────┤
│ {"c":3,"a":"2","b":"1"} │
└─────────────────────────┘
DuckDB variables
One can also swap two DuckDB named variables using an intermediate variable:
set variable a=1;
set variable b=2;
set variable tmp=getvariable('a');
set variable a=getvariable('b');
set variable b=getvariable('tmp');
E
(slots)
def swap(&left, &right) {
def t := left
left := right
right := t
}(functional)
def swap([left, right]) {
return [right, left]
}EchoLisp
;; 1)
;; a macro will do it, as shown in Racket (same syntax)
(define-syntax-rule (swap a b)
(let ([tmp a])
(set! a b)
(set! b tmp)))
(define A 666)
(define B "simon")
(swap A B)
A → "simon"
B → 666
;; 2)
;; The list-swap! function allows to swap two items inside a list, regardless of their types
;; This physically alters the list
(define L ' ( 1 2 3 4 🎩 ))
(list-swap! L 1 ' 🎩 )
→ (🎩 2 3 4 1)
Elena
ELENA 4.1 :
import extensions;
swap(ref object v1, ref object v2)
{
var tmp := v1;
v1 := v2;
v2 := tmp
}
public program()
{
var n := 2;
var s := "abc";
console.printLine(n," ",s);
swap(ref n, ref s);
console.printLine(n," ",s)
}2 abc abc 2
Elixir
Elixir provides a robust mechanism of pattern matching; the = operator is actually the match operator. Using the match operator, values can be assigned and variables can be bound or unbound, but only on the left (=:).
x = 4
y = 5
{y,x} = {x,y}
y # => 4
x # => 5
[x,y] = [y,x]
x # => 4
y # => 5
Data structures can be used both for matching and for generally destructuring complex data. We can use anonymous functions to create a generic swap in iex. Note: using multiple value requires a data construct on which to match (as opposed to, say, Ruby's a,b = 1,2), but we can use a list:
swap = fn x,y -> [y|x] end
[x|y] = swap.(1,2)
x # => 2
y # => 1
Variables can be bound and rebound regardless of type
swap_tuple = fn {x,y} -> {y,x} end
{a,b} = swap_tuple.({1,:ok})
a # => :ok
b # => 1
swap_list = fn [x,y] -> [y,x] end
[a,b] = swap_list.([1,"2"])
a # => "2"
b # => 1
Emacs Lisp
(defun swap (a-sym b-sym)
"Swap values of the variables given by A-SYM and B-SYM."
(let ((a-val (symbol-value a-sym)))
(set a-sym (symbol-value b-sym))
(set b-sym a-val)))
(swap 'a 'b)
A macro can take variable names unquoted. Here prog1 eliminates the temporary variable above so as to avoid any chance of a name clash between the two variables and the temporary.
(defmacro swap (a b)
`(setq ,b (prog1 ,a (setq ,a ,b))))
A macro could use the cl-lib psetf which can store to various kinds of expressions as locations, for example list elements. psetf evaluates all its values before storing (like the Common Lisp example).
(require 'cl-lib)
(defmacro swap (a b)
`(cl-psetf ,a ,b
,b ,a))
(setq lst (list 123 456))
(swap (car lst) (cadr lst))
;; now lst is '(456 123)
EMal
fun swap = Pair by var a, var b do return var%var(b => a) end
int a = 1
int b = 2
writeLine("before swap: a=" + a + ", b=" + b)
Pair pair = swap(a, b)
a = pair[0]
b = pair[1]
writeLine(" after swap: a=" + a + ", b=" + b)- Output:
before swap: a=1, b=2 after swap: a=2, b=1
Erlang
Erlang variables are single assignment and Erlang is dynamically typed, so this task doesn't really apply.
The closest thing would be to swap the items in a list (shown in the shell).
1> L = [a, 2].
[a,2]
2> lists:reverse(L).
[2,a]
Or swap the items in a tuple (also shown in the shell).
1> T = {2,a}.
{2,a}
2> list_to_tuple(lists:reverse(tuple_to_list(T))).
{a,2}
Euphoria
include std/console.e -- for display
object x = 3.14159
object y = "Rosettacode"
{y,x} = {x,y}
display("x is now []",{x})
display("y is now []",{y})x is now Rosettacode y is now 3.14159
objects are generic, and can accept any type.
More strongly-typed variables can be swapped, if their types match or if the value is "acceptable" to the declared type.
In the above example, if we declared y as an integer, the swap would fail, pi is not an integer. If we declare y as an atom, it would succeed, as atoms can contain any valid number. If we declare y as a sequence or string, it would fail, because x is atomic.
F#
let swap (a,b) = (b,a)
Factor
Depending on how you look at it: this task doesn't apply, or it's trivial:
swap
Falcon
a = 1
b = 2
a,b = arr = b,aReading right to left: Assign b & a into an array variable called arr, then assign into a & b
Fish
Swap the top two values on the stack:
$
Forth
Since the Forth stack can contain pointers to any data type all we need is...
swap
Fortran
MODULE Genericswap
IMPLICIT NONE
INTERFACE Swap
MODULE PROCEDURE Swapint, Swapreal, Swapstring
END INTERFACE
CONTAINS
SUBROUTINE Swapint(a, b)
INTEGER, INTENT(IN OUT) :: a, b
INTEGER :: temp
temp = a ; a = b ; b = temp
END SUBROUTINE Swapint
SUBROUTINE Swapreal(a, b)
REAL, INTENT(IN OUT) :: a, b
REAL :: temp
temp = a ; a = b ; b = temp
END SUBROUTINE Swapreal
SUBROUTINE Swapstring(a, b)
CHARACTER(*), INTENT(IN OUT) :: a, b
CHARACTER(len(a)) :: temp
temp = a ; a = b ; b = temp
END SUBROUTINE Swapstring
END MODULE Genericswap
PROGRAM EXAMPLE
USE Genericswap
IMPLICIT NONE
INTEGER :: i1 = 1, i2 = 2
REAL :: r1 = 1.0, r2 = 2.0
CHARACTER(3) :: s1="abc", s2="xyz"
CALL Swap(i1, i2)
CALL Swap(r1, r2)
CALL Swap(s1, s2)
WRITE(*,*) i1, i2 ! Prints 2 and 1
WRITE(*,*) r1, r2 ! Prints 2.0 and 1.0
WRITE(*,*) s1, s2 ! Prints xyz and abc
END PROGRAM EXAMPLE
Free Pascal
{$ifdef fpc}{$mode delphi}{$H+}{$endif}
{ note this is compiled with delphi mode but will only compile in Free Pascal }
{ Delphi doesn't support this syntax }
procedure swap<T>(var left,right:T);
var
temp:T;
begin
temp:=left;
left:=right;
right:=temp;
end;
var
a:string = 'Test';
b:string = 'me';
begin
writeln(a:6,b:6);
swap<string>(a,b);
writeln(a:6,b:6);
end.
Output:
Test me
me Test
Frink
The following example will work on all Frink data types:
[b,a] = [a,b]Fōrmulæ
Fōrmulæ programs are not textual, visualization/edition of programs is done showing/manipulating structures but not text. Moreover, there can be multiple visual representations of the same program. Even though it is possible to have textual representation —i.e. XML, JSON— they are intended for storage and transfer purposes more than visualization and edition.
Programs in Fōrmulæ are created/edited online in its website.
In this page you can see and run the program(s) related to this task and their results. You can also change either the programs or the parameters they are called with, for experimentation, but remember that these programs were created with the main purpose of showing a clear solution of the task, and they generally lack any kind of validation.
Solution
Fōrmulæ supports assignments of several symbols (provided as a list) from an expression that reduces to a list of the same cardinality (the expression is first reduced before the actual assignment).
This can be used to do a generic swap as follows:
Test case
FutureBasic
window 1, @"Generic Swap", (0,0,480,270)
text ,,,,, 60
long i, j
double x, y
CFStringRef a, b
i = 1059 : j = 62
print i, j
swap i, j
print i, j
print
x = 1.23 : y = 4.56
print x, y
swap x, y
print x, y
print
a = @"Hello" : b = @"World!"
print a, b
swap a, b
print a, b
HandleEvents- Output:
1059 62 62 1059 1.23 4.56 4.56 1.23 Hello World! World! Hello
Gecho
1 !0 2 !1Now tape[0] and tape[1] are set to 1 and 2, respectively.
&0 &1 !0 pop !1This pushes the value of tape[0] to the stack, tape[1] to the stack, sets tape[0] to the top element, and then pops it, then tape[1] to the top element.
Go
Built in
Not a valid solution, since the task requires writing a function or operator, but it is worth mentioning that Go's built in assignment operator does generic swap. The following swaps the values of a and b as long as they are of identical type.
a, b = b, a
Pass interfaces
A generic swap function can easily be written however, if you require the caller to use variables of the empty interface type. The empty interface can hold a value of any type.
package main
import "fmt"
func swap(a, b *interface{}) {
*a, *b = *b, *a
}
func main() {
var a, b interface{} = 3, "four"
fmt.Println(a, b)
swap(&a, &b)
fmt.Println(a, b)
}
- Output:
3 four four 3
Pass pointers
Somewhat less restrictive, this version allows pointers of any type to be passed, as long as they are the same type.
package main
import (
"fmt"
"reflect"
)
func swap(a, b interface{}) error {
ta := reflect.TypeOf(a)
tb := reflect.TypeOf(b)
if ta != tb {
return fmt.Errorf("swap args are different types: %v and %v", ta, tb)
}
if ta.Kind() != reflect.Ptr {
return fmt.Errorf("swap args must be pointers")
}
ea := reflect.ValueOf(a).Elem()
eb := reflect.ValueOf(b).Elem()
temp := reflect.New(ea.Type()).Elem()
temp.Set(ea)
ea.Set(eb)
eb.Set(temp)
return nil
}
func main() {
a, b := 3, "cats"
fmt.Println("a b:", a, b)
err := swap(a, b)
fmt.Println(err, "\n")
c, d := 3, 4
fmt.Println("c d:", c, d)
err = swap(c, d)
fmt.Println(err, "\n")
e, f := 3, 4
fmt.Println("e f:", e, f)
swap(&e, &f)
fmt.Println("e f:", e, f, "\n")
type mult struct {
int
string
}
g, h := mult{3, "cats"}, mult{4, "dogs"}
fmt.Println("g h:", g, h)
swap(&g, &h)
fmt.Println("g h:", g, h)
}
- Output:
a b: 3 cats
swap args are different types: int and string
c d: 3 4
swap args must be pointers
e f: 3 4
e f: 4 3
g h: {3 cats} {4 dogs}
g h: {4 dogs} {3 cats}
Gri
Putting & in a call makes the parameter "call by reference", giving a command the opportunity to modify a variable in the caller. So to swap variables,
`Swap Vars &.a. &.b.'
{
new .temp.
.temp. = \.word2.
\.word2. = \.word3.
\.word3. = .temp.
delete .temp.
}
.foo. = 123
.bar. = 456
Swap Vars &.foo. &.bar.
show .foo. " " .bar.
# prints "456 123"Or similar to swap synonyms (strings),
`Swap Syns &\a &\b'
{
new \temp
\temp = "\.word2."
\.word2. = "\.word3."
\.word3. = "\temp"
delete \temp
}
\quux = "one"
\xyzzy = "two"
Swap Syns &\quux &\xyzzy
show "\quux \xyzzy"
# prints "two one"Groovy
Groovy has support for swapping built in:
(a, b) = [b, a]
But the task calls for a "generic swap method" to be written, so here it is:
def swap(a, b) {
[b, a]
}
This function doesn't mutate anything, but simply returns a new list with the order of the elements switched. It can be used like shown below:
def (x, y) = swap(1, 3)
assert x == 3
assert y == 1
Some examples here show an in-place swap of indexed elements in an array or collection, so for completeness here is an in-place swap of arbitrary indexed elements in a list:
def listSwap = { a, i, j ->
assert (0..<(a.size())).containsAll([i,j]);
a[[j,i]] = a[[i,j]]
}
def list = [2,4,6,8]
listSwap(list, 1, 3)
assert list == [2,8,6,4]
Harbour
Harbour has no build-in swap function, however, implemention of a UDF is a fairly easy task. And since it's a dynamic typed lang, there won't be problem while swapping different value types.
Implementation: (We exploit "pass by reference" method on the parameters used)
PROCEDURE Swap( /*@*/v1, /*@*/v2 )
LOCAL xTmp
xTmp := v1
v1 := v2
v2 := xTmp
RETURN
Sample code:
FUNCTION Main()
LOCAL v1 := "World!", v2 := "Hello"
? v1, v2 // --> World! Hello
Swap( @v1, @v2 )
? v1, v2 // --> Hello World!
RETURN 0
Haskell
Pure swap
Usually Haskellers prefer to work with immutable data. The following function doesn't mutate anything, but simply returns a new pair with the order of the elements switched.
The type signature, the first line, is optional; it may be inferred.
swap :: (a, b) -> (b, a)
swap (x, y) = (y, x)
This swap function is available in the Data.Tuple standard library module in GHC 7.0+
Swap mutable variables
The following function swaps the contents of two mutable references. Again the type signature is optional.
import Control.Monad.Ref
swap :: MonadRef r m => r a -> r a -> m ()
swap xRef yRef = do
x<-readRef xRef
y<-readRef yRef
writeRef xRef y
writeRef yRef x
Icon and Unicon
Icon provides a :=: operator for this. Additionally, there is a reversible exchange operator <-> that reverses the exchange if resumed.
IDL
IDL is dynamically typed and array-centric, so swapping is quite easy for any data type. The TEMPORARY function sets its argument to "undefined", and allows us to swap without any large copying.
pro swap, a, b
c = temporary(a)
a = temporary(b)
b = temporary(c)
end
J
J is dynamically typed and J's cycle primitive (C.) will swap elements of an arbitrary list. See also J's reference documentation on C.
Shown here are a list of prime numbers and the result of J's parser on some random text (inverting the parsing process on the swapped result):
(<2 4) C. 2 3 5 7 11 13 17 19
2 3 11 7 5 13 17 19
(<0 3)&C.&.;:'Roses are red. Violets are blue.'
Violets are red. Roses are blue.
Also, if the argument list can be guaranteed to be a pair, J's reverse primitive will swap the pair.
|.2 3
3 2
|.&.;:'one two'
two one
A generic destructive swap of named values would instead require reference to the locations being destroyed. Here's an implementation of that:
destructiveSwap=: {{ EMPTY[ (m;n)=: n ,&<&do m }}
Example use:
V1=: 'cat'
V2=: 7
'V1' destructiveSwap 'V2'
V1
7
V2
cat
Jakt
fn swap<T>(anon a: &mut T, anon b: &mut T) {
let temporary = *a
*a = *b
*b = temporary
}
fn main() {
mut a = "Hello"
mut b = "World"
println("{} {}", a, b)
swap(&mut a, &mut b)
println("{} {}", a, b)
mut c = 1
mut d = 2
println("{} {}", c, d)
swap(&mut c, &mut d)
println("{} {}", c, d)
}Java
Java uses references, so it can't swap the values of two variables that don't belong to a class.
class Pair<T> {
T first;
T second;
}
public static <T> void swap(Pair<T> p) {
T temp = p.first;
p.first = p.second;
p.second = temp;
}
JavaScript
JavaScript uses references, but if a function reassigns a parametric reference, the new object only has a local reference. However, if we wrap the variables to be switched in some other structure, like an object or an array, we can easily swap the values.
There's no actual "generics", since all variables are just that, variables of some kind.
The below function expects an array of length 2 (or longer), and switches the first two values in place, in the same array. This is closely related to how the Java solution works.
function swap(arr) {
var tmp = arr[0];
arr[0] = arr[1];
arr[1] = tmp;
}
Also there is metaprogramming solution. It uses code generation and eval. To avoid naming conflicts(user can pass 'tmp', which causes var tmp = tmp) it uses buildin, per activation context (thats why it is enclosed into self executing lambda), var arguments for temp storage.
function swap(aName, bName) {
eval('(function(){ arguments[0] = aName; aName = bName; bName = arguments[0] })()'
.replace(/aName/g, aName)
.replace(/bName/g, bName)
)
}
var x = 1
var y = 2
swap('x', 'y')
Solution without eval(), assuming that the code is running in the browser (window is the global object)
function swap(a, b) {
var tmp = window[a];
window[a] = window[b];
window[b] = tmp;
}
var x = 1;
var y = 2;
swap('x', 'y');
Another solution for swapping array items using destructing assignment:
const arr = [1, 2, 3, 4, 5];
[arr[0], arr[1]] = [arr[1], arr[0]]
Joy
Provided that the stack contains at least two elements:
swapchanges the order of those elements.
jq
In practice, one is more likely to want to swap two values within a JSON object or array than to swap the values of two jq variables, but all are possible, it being understood however that jq is a purely functional language.
For swapping values within an object, no intermediate variable or special syntax is required:
{a: 1, b: 2} | {a: .b, b: a.}The built-in filter, reverse, can be used to o swap the elements of a two-element array:
reverseMore generally, here is a filter that, if presented with an array, will in effect copy it and then swap the i-th and j-th items, it being understood that if a is an array and k < 0 or k >= (a|length), then a[k] will evaluate to null:
def swap(i;j): .[i] as $t | .[i] = .[j] | .[j] = $t;jq does have variables and their values can be swapped, for example, using an intermediate variable, say $tmp, as illustrated here:
jq -n '1 as $a | 2 as $b | $a as $tmp | $b as $a | $tmp as $b | [$a,$b]'Julia
Similar to Python, Julia has built-in support for swapping:
a, b = b, a
Kotlin
As Kotlin does not support passing parameters by reference and tuples cannot be destructured automatically to pre-existing variables, it's just as easy to swap variable values 'inline' rather than using a function. However, here's one of way of doing it generically using the latter:
fun <T> swap(t1: T, t2: T) = Pair(t2, t1)
fun main() {
var a = 3
var b = 4
val c = swap(a, b) // infers that swap<Int> be used
a = c.first
b = c.second
println("a = $a")
println("b = $b")
var d = false
var e = true
val f = swap(d, e) // infers that swap<Boolean> be used
d = f.first
e = f.second
println("d = $d")
println("e = $e")
}
- Output:
a = 4 b = 3 d = true e = false
You can also explicitly create a container class and swap the value that it contains (but this is a bad idea in practice):
data class Ref<T>(var value: T) {
fun swap(other: Ref<T>) {
val tmp = this.value
this.value = other.value
other.value = tmp
}
override fun toString() = "$value"
}
fun main() {
val a = Ref(1)
val b = Ref(2)
a.swap(b)
println(a)
println(b)
}Lambdatalk
1) using an Immediately Invoked Function Expression:
{{lambda {:x :y} :y :x} hello world}
-> world hello
2) or user defined function
{def swap {lambda {:x :y} :y :x}}
-> swap
3) applied on words (which can be numbers)
{swap hello world}
-> world hello
{swap hello brave new world}
-> brave new hello world
{swap {cons hello brave} {cons new world}}
-> (new world) (hello brave)Lang
# Swap function with call-by-pointer
fp.swap = ($[aPtr], $[bPtr]) -> {
$tmp = $*aPtr
$*aPtr = $*bPtr
$*bPtr = $tmp
}
$a = 42
$b = A short text
fn.println($a, $b)
fp.swap($a, $b)
fn.println($a, $b)- Output:
42, A short text A short text, 42
Lang5
swap # stack
reverse # arraylangur
Values can be swapped using multi-variable assignment.
var abc = [1, 2, 3]
var def = [5, 6, 7]
abc[3], def[3] = def[3], abc[3]
writeln abc
writeln def- Output:
[1, 2, 7] [5, 6, 3]
Lasso
define swap(a, b) => (: #b, #a)
local(a) = 'foo'
local(b) = 42
local(a,b) = swap(#a, #b)
stdoutnl(#a)
stdoutnl(#b)- Output:
42 foo
Using Decompositional Assignment
local(a) = 'hair'
local(b) = 'moose'
local(a,b) = (: #b, #a)
stdoutnl(#a)
stdoutnl(#b)- Output:
moose hair
Lhogho
Lhogho is very similar except that it does not have a localmake opcode.
to swap :s1 :s2
local "t
make "t thing :s1
make :s1 thing :s2
make :s2 :t
end
make "a 4
make "b "dog
swap "a "b ; pass the names of the variables to swap
show list :a :b ; [dog 4]Lingo
A generic swap function is not possible in Lingo, since scalar values are passed by value. But the following solution shows how such generic swapping still can be achieved by executing a single line of code:
on swap (x, y)
return "tmp="&x&RETURN&x&"="&y&RETURN&y&"=tmp"
endUsage:
x = 1
y = 2
do(swap("x","y"))
put x, y
-- 2 1Lisaac
(a, b) := (b, a);LiveCode
put "first" into a1
put "last" into b2
swap a1,b2
put a1 && b2
command swap @p1, @p2
put p2 into p3
put p1 into p2
put p3 into p1
end swapLogo
to swap :s1 :s2
localmake "t thing :s1
make :s1 thing :s2
make :s2 :t
end
make "a 4
make "b "dog
swap "a "b ; pass the names of the variables to swap
show list :a :b ; [dog 4]Logtalk
:- object(paws).
:- public(swap/4).
swap(First, Second, Second, First).
:- end_object.Usage examples:
| ?- paws::swap(apples, oranges, X, Y).
X = oranges
Y = apples
yes
| ?- paws::swap(3.14, ext(lgt), X, Y).
X = ext(lgt)
Y = 3.14yes
LOLCODE
LOLCODE's dynamic typing makes generic swapping trivial. In addition, the special IT variable‒which contains the most recently evaluated expression‒permits doing so without explicitly creating a temporary variable.
HAI 1.3
I HAS A foo ITZ "kittehz"
I HAS A bar ITZ 42
foo, foo R bar, bar R IT
VISIBLE foo BTW, 42
VISIBLE bar BTW, kittehz
KTHXBYELua
Lua evaluates the values on the right-hand side before assigning them to the variables on the left-hand side. This behaviour allows the following notation to be used to swap two values:
x, y = y, x -- swap the values inside x and y
t[1], t[2] = t[2], t[1] -- swap the first and second values inside table tUsage example:
x, y = 3, 4
print(x, y) --> 3 4
x, y = y, x -- swap
print(x, y) --> 4 3M2000 Interpreter
Swap is a statement in M2000 which get two identifiers, variables or array items. Variables and Array items by default are all internal type of Variant (see exception later). Normally a numeric variable hold the first type we assign to it. Numeric types are: Double, Single, Decimal, Currency, Decimal, Long, Integer. Boolean is also a type but true and false are not boolean, they are double -1 and 0). When we use Swap internal only variant swap happen, without check of type of variant.
Here we make a local Swap and pass by reference, numbers and strings. References created without testing what type of variant we use. So calling swap we make a swap moving bytes, and for strings this means moving pointers to BSTR type of strings.
\\ programming again Swap (for local use)
Module Swap (&a, &b) {
\\ this call internal command - by default is by reference without using character &
Swap a, b
}
X=20
Y=100
Swap &x, &y
Print X, Y, Type$(X)="Double",Type$(Y)="Double"
A$="A$"
B$="B$"
Swap &A$, &B$
Print A$="B$", B$="A$"Using Swap (internal command), for variables, groups (only for variables inside groups), pointers to groups, pointers to containers, etc.
a=1000
b=50
Swap a,b
Print a, b
A$="Hello"
B$="There"
Swap A$, B$
Print A$, B$
Dim A(4)
A(0):=1,2,3,4
Swap A(3), A(2)
Print A(3), A(2)
\\ Groups are Values
Group alfa {
x=10, y=20
}
Group Beta {
x=40, y=50
}
\\ with List we show the public variables
\\ so among other variables there are:
\\ alfa[Group], alfa.x=10, alfa.y=20, beta[group], beta.x=40, beta.y=50
\\ So Alfa.x and Beta.x are simple variables, we can use swap
Swap Alfa.x, Beta.x
Print Alfa.x, Beta.x
Swap Alfa.x, Beta.x
List
\\ We have to use a third variable to hold value
For This {
\\ Local always make a new variable, and shadow any same local variable
Local M=alfa
alfa=beta
beta=m
}
\\ Now M erased (defined in For This block)
Print Alfa.x=40, Alfa.y=50
Print Beta.x=10, Beta.y=20
\\ Using -> we make pointers to Alfa, and Beta
\\ These pointers are valid until Alfa and Beta erased, or get Empty Group (->0)
pA->Alfa
pB->Beta
Print pA=>x=40, pA=>y=50
Print pB=>x=10, pB=>y=20
Swap pA,pB
Print pA=>x=10, pA=>y=20 ' pA point to beta
Print pB=>x=40, pB=>y=50 'pB point to alfa
Print type$(pA)="Group",Valid(pA=>X)=True
pA->0
pB->0
Print type$(pA)="Group",Valid(pA=>X)=False
\\ These pointers are valid until get Empty Group (->0), they point to a copy of Alfa and Beta
\\ both are in heap as "closed groups"
pA->(Alfa)
pB->(Beta)
Print pA=>x, pA=>y
\\ swap need variables or arrays
\\ pA=>x are closed to object so we have to open the object, and use the open one, where all public variables of group can be used
For pA, pB {
Swap .x, .y
}
Print pA=>x, pA=>y
For pA, pB {
Swap .x, .y
}
Print pA=>x, pA=>y
Print pB=>x, pB=>y
Swap pA,pB
Print pA=>x, pA=>y
Print pB=>x, pB=>y
L1=lambda x=1->{=x : x++}
L2=lambda x=100->{=x : x--}
Print L1()=1, L2()=100
Swap L1, L2
Print L1()=99, L2()=2
Swap L1, L2
Print L1()=3, L2()=98
\\ swap change pointers to containers (here pointers to arrays)
A=(1,2,3,4,5)
B=(6,7,8,9,10)
Swap A, B
Print A
Print B
\\ Arrays with () in names are values
Dim A(10)=1, B(10)=2
For This {
Dim C()
C()=A()
A()=B()
B()=C()
}
Print A()
Print B()We can make arrays with specific type. Here we make Byte type. The swap works fine.
dim a(10) as byte
a(3)=2
a(4)=100
Print a()
swap a(3), a(4)
Print a()
try ok {
a(0)=-1
}
if not ok then print error$ =" Overflow Byte"
try ok {
a(0)=256
}
if not ok then print error$ =" Overflow Byte"
a()=a()#rev()
Print a()
Print type$(a(3))
Print a(10-3-1)=100
Print a(10-4-1)=2M4
define(`def2', `define(`$1',`$2')define(`$3',`$4')')dnl
define(`swap', `def2(`$1',defn(`$2'),`$2',defn(`$1'))')dnl
dnl
define(`a',`x')dnl
define(`b',`y')dnl
a b
swap(`a',`b')
a b- Output:
x y y x
Maple
The assignment operator in Maple can swap values, since the right hand side is evaluated before the assignment occurs.
> a, b := 2, "foo":
> a;
2
> b;
"foo"
> a, b := b, a: # SWAP
> a;
"foo"
> b;
2Mathematica / Wolfram Language
Mathematica functions are generic by default; however, it has to be told not to evaluate the arguments before executing the function.
swap[a_, b_] := {a, b} = {b, a}
SetAttributes[swap, HoldAll]MATLAB / Octave
Numercial swaps are trivial operations. In fact, they are so natural to the language that multiple swaps can be performed simultaneously.
Example:
>> a = [30 40 50 60 70]
a =
30 40 50 60 70
>> a([1 3]) = a([3 1]) %Single swap
a =
50 40 30 60 70
>> a([1 2 4 3]) = a([2 3 1 4]) %Multiple swap, a.k.a permutation.
a =
40 30 60 50 70
A generic swap (compatible with any variable type) can be performed with the deal command:
>> a = 12
a = 12
>> b = 'foo'
b = foo
>> [b, a] = deal (a, b)
b = 12
a = fooMaxima
a: 10$
b: foo$
/* A simple way to swap values */
[a, b]: [b, a]$
a; /* foo */
b; /* 10 */
/* A macro to hide this */
swap(x, y) ::= buildq([x, y], ([x, y]: [y, x], 'done))$
swap(a, b)$
a; /* 10 */
b; /* foo */MAXScript
swap a bMetafont
In Metafont, only numeric declarations can be omitted; any other type, must be explicitly given. So our swap, in order to declare and use a proper temporary variable(? in this code), must check the type of the variable passed (we check only for a; if b is of another kind, an error will occur)
vardef swap(suffix a, b) =
save ?; string s_;
if boolean a: boolean ?
elseif numeric a: numeric ? % this one could be omitted
elseif pair a: pair ?
elseif path a: path ?
elseif pen a: pen ?
elseif picture a: picture ?
elseif string a: string ?
elseif transform a: transform ? fi;
? := a; a := b; b := ?
enddef;Examples:
j := 10;
i := 5;
show j, i;
swap(j,i);
show j, i;
boolean truth[];
truth1 := true;
truth2 := false;
show truth1, truth2;
swap(truth1,truth2);
show truth1, truth2;min
Like many other stack languages, this is trivial.
swapMiniScript
Like many other languages, MiniScript passes references by value, so a straightforward swap is impossible. However, there is a trick: given the map the variales are in (e.g. locals) and the names of the variables, we can swap them.
swap = function(map, a, b)
temp = map[a]
map[a] = map[b]
map[b] = temp
end function
x = 1
y = 2
print "BEFORE: x=" + x + ", y=" + y
swap(locals, "x", "y")
print "AFTER: x=" + x + ", y=" + y- Output:
BEFORE: x=1, y=2 AFTER: x=2, y=1
Modula-3
GENERIC INTERFACE GenericSwap(Elem);
PROCEDURE Swap(VAR left: Elem.T; VAR right: Elem.T);
END GenericSwap.GENERIC MODULE GenericSwap(Elem);
PROCEDURE Swap(VAR left: Elem.T; VAR right: Elem.T) =
VAR temp: Elem.T := left;
BEGIN
left := right;
right := temp;
END Swap;
BEGIN
END GenericSwap.Here is an example usage for integers:
INTERFACE IntSwap = GenericSwap(Integer) END IntSwap.MODULE IntSwap = GenericSwap(Integer) END IntSwap.MODULE Main;
IMPORT IntSwap, IO, Fmt;
VAR left := 10;
right := 20;
BEGIN
IO.Put("Left = " & Fmt.Int(left) & "\n");
IntSwap.Swap(left, right);
IO.Put("Left = " & Fmt.Int(left) & "\n");
END Main.- Output:
Left = 10 Left = 20
Nemerle
For pairs, namespace Nemerle.Utility.Pair contains Swap():
def coords = (1, -1);
def invcoords = Swap(coords);Or to swap two mutable variables of the same type:
a <-> b;But, enough about built in functionality, let's demonstrate using generics:
Swap[T, U] (a : T, b : U) : U * T
{
(b, a)
}NetRexx
Values stored in the default Rexx data type are treated as typeless data; context is based on the contents. Swapping the contents of variables stored in Rexx object can be achieved via the PARSE instruction.
/* NetRexx */
options replace format comments java crossref symbols nobinary
-- Simple values with no spaces can be swapped without the use of a parse template
lval = 27
rval = 5
say 'Before - <lval>'lval'</lval> <rval>'rval'</rval>'
parse (lval rval) rval lval
say 'After - <lval>'lval'</lval> <rval>'rval'</rval>'
say
-- More complex data needs to use some form of parsing template
lval = 'This value started on the left'
rval = 'This value started on the right'
dlm = 12x80facebead01 -- some delimiting value that is unlikely to occur in the LVAL to be swapped
say 'Before - <lval>'lval'</lval> <rval>'rval'</rval>'
parse (lval || dlm || rval) rval (dlm) lval
say 'After - <lval>'lval'</lval> <rval>'rval'</rval>'
say
return- Output:
Before - <lval>27</lval> <rval>5</rval> After - <lval>5</lval> <rval>27</rval> Before - <lval>This value started on the left</lval> <rval>This value started on the right</rval> After - <lval>This value started on the right</lval> <rval>This value started on the left </rval>
NewLISP
(swap a b)Nial
Like J
|reverse 1 2
=2 1Nim
Builtin procedure swap. Example usage:
swap(a, b)OASYS Assembler
You can swap variable %A# with %B# by writing:
%A#%B#<%B#%A#<>>A method which can be called to implement it can be written like:
[&SW,A^,B^],A^<,B^<<,B^<,A^<<>>To call such method:
+%A#%B#&SWOCaml
Tuples are immutable in OCaml. This function doesn't mutate anything, but simply returns a new pair with the order of the elements switched.
let swap (x, y) = (y, x)If the arguments are constrained to be reference values, a swap function is simple:
let swapref x y =
let temp = !x in
x := !y;
y := tempOforth
swapOz
Oz variables are dataflow variables and cannot be changed once a value has been assigned. So a swap operation on dataflow variables does not make sense.
We can write a swap procedure for cells, though. Cells are mutable references.
proc {SwapCells A B}
Tmp = @A
in
A := @B
B := Tmp
endOr shorter, if we exploit the fact that the assignment operator := returns the old value of the cells:
proc {SwapCells A B}
B := A := @B
endA functional swap, operating on pairs:
fun {SwapPair A#B}
B#A
endPARI/GP
Pari is near-typeless—everything is a GEN.
my(tmp=a);
a=b;
b=tmp;[a,b]=[b,a]Pascal
Standard Pascal does not have generics, but FreePascal has a start:
program generictest;
{$mode objfpc}
type
generic TSwap<T> = procedure (var a, b: T);
procedure Proc1(var a, b: integer);
var
temp: integer;
begin
temp := a;
a := b;
b := temp;
end;
var
S, T: integer;
SwapInt: specialize TSwap<integer>;
begin
S := 4;
T := 3;
SwapInt := @Proc1;
writeln(S, T:2);
SwapInt(S, T);
writeln(S, T:2);
end.- Output:
4 3 3 4
since FreePascal version 3.2.0:
program generic_test;
{$mode objfpc}{H+}
uses
SysUtils;
generic procedure GSwap<T>(var L, R: T);
var
Tmp: T;
begin
Tmp := L;
L := R;
R := Tmp;
end;
var
I, J: Integer;
begin
I := 100;
J := 11;
WriteLn('I = ', I, ', J = ', J);
specialize GSwap<Integer>(I, J);
WriteLn('I = ', I, ', J = ', J);
end.- Output:
I = 100, J = 11 I = 11, J = 100
PascalABC.NET
procedure Swap<T> (var a,b: T) := (a,b) := (b,a);
begin
var (i,j) := (1,2);
Swap(i,j);
Println(i,j);
var (s1,s2) := ('PascalABC.NET','Hello');
Swap(s1,s2);
Println(s1,s2);
end.- Output:
2 1 Hello PascalABC.NET
Perl
Perl has support for swapping built-in
($y, $x) = ($x, $y);Here's a generic swap routine:
sub swap {@_[0, 1] = @_[1, 0]}Phix
The following applies to any types. Subscripting and nesting may also be used freely on either side.
{a,b} = {b,a}
Phixmonti
Applies to any types.
a b var a var bPHP
function swap(&$a, &$b) {
list($a, $b) = array($b, $a);
}Picat
Picat supports re-assignment (:=), but it is not possible to swap two variables like this:
A = 1,
B = 2,
[A,B] := [B,A],
% ...(This throws an error: "invalid left-hand side of assignment").
The following works, but it's probably not general enough for this task:
A = 1,
B = 2,
T = A, % Assuming that T is not used elsewhere in the clause
A := B,
B := T,
% ...Instead swaps has to be done by some other means.
Inline swapping in a list
One way is to place the values in a list (or array) and use inline swapping of the list.
% Swapping positions in a list/array
swap2(L) =>
swap_list(L,1,2).
% Swap two elements in a list
swap_list(L,I,J) =>
T = L[I],
L[I] := L[J],
L[J] := T.Usage:
L = [1,2],
swap2(L),
% ...L is now [2,1]
Note: Placing the variables A, B in swap2
swap2([A,B])will NOT change the values of A and B.
Using a second list
Another way is to place the swapped values in a second list:
swap_pair([A,B],[B,A]).Usage:
A = 1,
B = 2,
swap_pair([A,B],X),
% ...X is now [2,1]. A is still 1 and B is still 2.
PicoLisp
xchg works with any data type
(let (A 1 B 2)
(xchg 'A 'B)
(println A B) )
(let (Lst1 '(a b c) Lst2 '(d e f))
(xchg (cdr Lst1) (cdr Lst2))
(println Lst1 Lst2) )- Output:
2 1 (a e c) (d b f)
Pike
mixed first = 5;
mixed second = "foo";
array pair = ({ 5, "foo" });
void swapvars(string a, string b)
{
[this[a], this[b]] = ({ this[b], this[a] });
}
void swaparray(array swapit)
{
[swapit[1], swapit[0]] = ({ swapit[0], swapit[1] });
}
void main()
{
write("swap variables:\n");
write("%O, %O\n", first, second);
// we could just use [first, second] = ({ second, first });
swapvars("first", "second");
write("%O, %O\n", first, second);
write("swap array:\n");
write("%{ %O %}\n", pair);
// we could just use [pair[1], pair[0]] = ({ pair[0], pair[1] });
// directly, but since arrays are called by reference,
// it also works through a function
swaparray(pair);
write("%{%O %}\n", pair);
}- Output:
swap variables: 5, "foo" "foo", 5 swap array: 5 "foo" "foo" 5
PL/I
Using the preprocessor
%swap: procedure (a, b);
declare (a, b) character;
return ( 't=' || a || ';' || a || '=' || b || ';' || b '=t;' );
%end swap;
%activate swap;The statement:-
swap (p, q);
is replaced, at compile time, by the three statements as in-line code:
t = p; p = q; q = t;
Using generic procedures
declare swap generic (
swapf when (float, float),
swapc when (char, char));
swapf: proc (a, b);
declare (a, b, t) float;
t = a; a = b; b = t;
end swapf;
swapc: proc (a, b);
declare (a, b) character(*);
declare t character (length(b));
t = a; a = b; b = t;
end swapc;
declare (r, s) character (5);
call swap (r, s);Both of the above are not completely generic, but depend on either the presence of
- a temporary variable with the same attributes of the variables to be swapped, OR
- data-attribute specific procedures for the swap
The following code is completely generic, but, in line with the usual safety offered by PL/I, swaps only the contents up to the storage occupied by the smallest of the two variables: Prino 01:24, 11 February 2011 (UTC)
Completely generic code using the pre-processor
%swap: proc(x,y);
dcl (x, y) char;
x = trim(x); /* Just for neatness sake */
y = trim(y);
ans('begin; ') skip;
ans(' dcl c char (1); ') skip;
ans(' dcl sx char (1) based(px); ') skip;
ans(' dcl sy char (1) based(py); ') skip;
ans(' dcl i fixed bin (31); ') skip;
ans(' dcl px ptr init (addr(' || x || ')); ') skip;
ans(' dcl py ptr init (addr(' || y || ')); ') skip;
ans(' do i = 1 to min(stg(' || x || '), stg(' || y || '));') skip;
ans(' c = sx; ') skip;
ans(' sx = sy; ') skip;
ans(' sy = c; ') skip;
ans(' px = px + 1; ') skip;
ans(' py = py + 1; ') skip;
ans(' end; ') skip;
ans('end; ') skip;
%end swap;
%act swap;
dcl c1 char (10) init ('1234567890');
dcl c2 char (10) init ('ABCDEFGHIJ');
dcl f1 fixed bin (31) init (12345);
dcl f2 fixed bin (31) init (98765);
put data(c1, c2, f1, f2);
swap(c1, c2);
swap(f1, f2);
put data(c1, c2, f1, f2);
f1 = -656877352; /* '5a5a5a5a'x, aka 'QQQQ' */
swapper(c1, f1);
put data(c1,f1);The code generated by 'swap(c1, c2);' looks like
begin;
dcl c char (1);
dcl sx char (1) based(px);
dcl sy char (1) based(py);
dcl i fixed bin (31);
dcl px ptr init (addr(C1));
dcl py ptr init (addr(C2));
do i = 1 to min(stg(C1), stg(C2));
c = sx;
sx = sy;
sy = c;
px = px + 1;
py = py + 1;
end;
end;and, because declarations in PL/I begin blocks are local to that block, generating several blocks with the same variables will not cause any problems.
The result of compiling, linking and executing the above code:
C1='1234567890' C2='ABCDEFGHIJ' F1= 12345 F2= 98765; C1='ABCDEFGHIJ' C2='1234567890' F1= 98765 F2= 12345; C1='QQQQEFGHIJ' F1= -1044200508;
Or, using "Like"
The key problem is that a temporary storage area is needed (there alas being no compiler-recognised "swap" statement), and the waystation variable must have the correct type, nor can there be reliance on a suitable "t" variable being available for use. Devising a different Swap for each type of parameter would be tedious and tiresome to use, however one could employ the "generic" facility as above demonstrated, and use the pre-processor to generate a collection of Swap routines by it employing a template and stepping through a list of accommodated types.
Instead, the first example can be generalised via two steps. Firstly, it is possible to declare a variable to be of a type "like" some named variable (otherwise a third parameter naming the type could be supplied), and secondly, placing the in-line code between Begin ... End; means that any declaration is local to within that block only. Further, this bracketing allows a Swap to be invoked via an if-statement, as in If ... then Swap(x,y); - otherwise there would be a mess. Thus:
%Swap:Procedure(a,b);
declare (a,b) character; /*These are proper strings of arbitrary length, pre-processor only.*/
return ('Begin; declare t like '|| a ||'; t='|| a ||';'|| a ||'='|| b ||';'|| b ||'=t; End;');
%End Swap;Whereupon a Swap(this,that); would generate a rather longer text of in-line source code. This and other text expansions caused odd difficulties, because the 1980s compiler replaced the invocation by the expansion and then reformatted the result into lines of 71 characters (not 72) as necessary, and then, since any additional lines were given the same source sequence number as the original line, added 100000 as needed to generate strictly increasing sequence numbers. If many lines overflowed, eventually the sequence field (eight digits) overflowed, and all following source lines thereby acquired the same source sequence number. For this and other reasons, one approach was two-stage compilation: the output from the pre-processor stage could be saved and further compilation cancelled. That file could then be resequenced and fed to the pl/i compiler afresh.
Such a file would have the expansion of Swap(this,that) as follows (but with added layout here):
Begin;
declare t like this;
t = this;
this = that;
that = t;
End;There would however be trouble if the type of this differed from the type of that, and a pl/i compiler may not generate a warning because it handles many type conversions in an assignment without complaint. There are no pre-processor enquiry functions to inspect the types at pre-processor time - if there were, a more accomplished Swap procedure could produce suitable error reports, which can be classed as "warning" or "severe", etc. The "storage" function produces its result at run time, but, each invocation of Swap being compiled would have its actual parameters known as the compiler dealt with the code produced by that invocation of Swap, and so for each invocation, the results of "storage" would be constants - except for items that were allocated at run time.
The bracketing could be via DO; ... END; instead of BEGIN; ... END; but in that case the declared temporary variable would be visible outside its fragment and there could be conflicts, either of differing type for the same name or of multiple declaration. This could be solved by adjusting Swap to generate a different name each time. One could try a prefix (or suffix) to the name of the first parameter (thus generating say SwapTemp_this or similar), but there would still be difficulty if there were multiple swaps involving the same variable. Instead, Swap could count its invocations and generate a name involving that. Temporary variables would then litter the storage area, and they could consume a lot of space. On the other hand, the BEGIN; ... END; arrangement, though typically involving temporary space on the data stack, could have its own constraints. In the 1980s, the IBM mainframe pl/i compiler had a limit of no more than 240 (or so) BEGIN; ... END; blocks, plus procedure blocks, plus a few other items, in any one compilation otherwise there would be a failure "in phase PI". Separate compilation and the linking of pieces introduced its own oddities, as when pieces had been compiled with different compiler options.
Plain English
Plain English includes a routine for swapping two values in its noodle.
Swap [a value] with [another value].Pop11
Swap is easily done via multiple assignment:
(a, b) -> (b, a);Pop11 is dynamically typed, so the code above is "generic".
PostScript
Works with anything you can put on the operand stack:
exchPowerShell
PowerShell allows swapping directly, through tuple assignment:
$b, $a = $a, $bBut one can also define a function which swaps the values of two references:
function swap ([ref] $a, [ref] $b) {
$a.Value, $b.Value = $b.Value, $a.Value
}When using this function the arguments have to be explicitly given as references:
swap ([ref] $a) ([ref] $b)Prolog
swap(A,B,B,A).
?- swap(1,2,X,Y).
X = 2,
Y = 1.Python
Python has support for swapping built in:
a, b = b, aBut the task calls for a "generic swap method" to be written, so here it is:
def swap(a, b):
return b, aNote that tuples are immutable in Python. This function doesn't mutate anything, but simply returns a new pair with the order of the elements switched.
Quackery
Quackery objects reside on the stack while in use. The word swap exchanges the top and second item on the stack. The closest thing Quackery has to variables are ancillary stacks. The word exchange defined here exchanges the top items on two ancillary stacks.
[ over take over take
2swap dip put put ] is exchange ( s s --> )
R
R function arguments are passed by value, not by reference. You can work around this, however, by using their names and environment:
swap <- function(name1, name2, envir = parent.env(environment()))
{
temp <- get(name1, pos = envir)
assign(name1, get(name2, pos = envir), pos = envir)
assign(name2, temp, pos = envir)
}Usage:
> x <- 1
> y <- 2
> swap('x', 'y')
> cat(x, y)
2 1
Racket
A swap operation can be easily written as a macro in Racket. The macro will even work as expected in Typed Racket.
#lang racket/load
(module swap racket
(provide swap)
;; a simple macro to swap two variables
(define-syntax-rule (swap a b)
(let ([tmp a])
(set! a b)
(set! b tmp))))
;; works fine in a statically typed setting
(module typed typed/racket
(require 'swap)
(: x Integer)
(define x 3)
(: y Integer)
(define y 4)
(swap x y)
(printf "x is ~a~n" x)
(printf "y is ~a~n" y))Raku
(formerly Perl 6)
Similar to Perl 5. Raku supports type constraints for variables and subroutines, unlike Perl 5, but the default is still to permit all values.
Alternatively, you can write it like this:
($x, $y) .= reverse;REBOL
REBOL [
Title: "Generic Swap"
URL: http://rosettacode.org/wiki/Generic_swap
Reference: [http://reboltutorial.com/blog/rebol-words/]
]
swap: func [
"Swap contents of variables."
a [word!] b [word!] /local x
][
x: get a
set a get b
set b x
]
answer: 42 ship: "Heart of Gold"
swap 'answer 'ship ; Note quoted variables.
print rejoin ["The answer is " answer ", the ship is " ship "."]- Output:
The answer is Heart of Gold, the ship is 42.
Retro
swapREXX
REXX has no primitive for swapping, but it can easily be performed using a temporary variable.
(This is the slowest of the three versions.)
using temp
a = 'I see you.'
b = -6
_temp_ = a /*swap ··· */
a = b /* A ··· */
b = _temp_ /* and B */using VALUE
This version will work with any values.
a = "bull feathers"
b = 10
a= value('b', a) /*swap A and B */using PARSE
If it's known that there are
- no blanks
- no null values
- (maybe) no whitespace (such as tabs)
in the values, the following method can be used:
(This is the fastest of the three versions.)
a = -199e-12
b = 12.
parse value a b with b a /*swap A and B */Note that some REXX interpreters handle whitespace differently, some honor whitespace other than blanks,
others don't (particularly the older versions).
Ring
a = 1
b = 2
temp = a
a = b
b = temp
see "a = " + a + nl
see "b = " + b + nlRLaB
RLaB does not have a built-in function for swapping the content of two variables. However, there is a workaround which comes from the fact that the global variable space $$ contains all the variables var1, var2 and so forth as $$.var1, ...
Let we want to swap the content of two variables, which names are a and b, then the following function would do the trick
swap = function(x,y)
{
if (!exist($$.[x]))
{ return 0; }
if (!exist($$.[y]))
{ return 0; }
local (t);
t = $$.[x];
$$.[x] = $$.[y];
$$.[y] = t;
return 1;
};
>> a=1
1
>> b = "fish"
fish
>> swap( "a" , "b" );
>> a
fish
>> b
1RPL
| Code | Comments |
|---|---|
|
≪ OVER TYPE 6 == OVER TYPE 6 == 2 * + 1 +
{ ≪ SWAP ≫
≪ OVER EVAL SWAP ROT STO ≫
≪ DUP EVAL ROT ROT STO ≫
≪ OVER EVAL OVER EVAL 4 ROLL STO SWAP STO ≫ }
SWAP GET EVAL
≫ 'GSWAP' STO
1 2 GSWAP
'A' 'B' GSWAP
3 'A' GSWAP
|
":#" means "stack level #" calculate case index upon types of data at :1 and :2 case 1: swap :2 with :1 case 2: swap content of variable named at :2 with :1 case 3: swap content of variable named at :1 with :2 case 4: swap content of variables named at :1 and :2 select program from case index and execute it |
Ruby
Ruby has support for swapping built in:
a, b = b, aBut the task calls for a "generic swap method", so here it is:
def swap(a, b)
return b, a
endThis method does not swap the original variables, because Ruby passes parameters by value. Instead, this method returns simply a new array with the order of the elements switched. The caller may assign the original variables with the return value:
x = 42
y = "string"
x, y = swap x, y
puts x # prints string
puts y # prints 42Rust
Rust does not allow for swapping the value of two variables with different types, but if the types are the same it can be done using generic types and lifetimes.
fn generic_swap<'a, T>(var1: &'a mut T, var2: &'a mut T) {
std::mem::swap(var1, var2)
}This function can be used in e.g. the following ways:
fn main() {
let mut a: String = "Alice".to_owned();
let mut b: String = "Bob".to_owned();
let mut c: i32 = 1;
let mut d: i32 = 2;
generic_swap(&mut a, &mut b);
generic_swap(&mut c, &mut d);
println!("a={}, b={}", a, b);
println!("c={}, d={}", c, d);
}- Output:
a=Bob, b=Alice c=2, d=1
Sather
A possible way that needs the type of the objects to be specified:
class SWAP{T} is
swap(inout a, inout b:T) is
t ::= a;
a := b;
b := t;
end;
end;class MAIN is
main is
x ::= 10;
y ::= 20;
SWAP{INT}::swap(inout x, inout y);
#OUT + x + ", " + y + "\n";
end;
end;Scala
Scala has type parameters and abstract types (not to be confused with abstract data types). The swap example is about as simple as such things can be, with no variance or high-order type parameters.
The return type need not be declared in the example below, but it is shown for clarity. However, as Scala does not pass parameters by reference, it cannot swap values in-place. To make up for that, it receives two values, and returns a tuple with the values inverted.
def swap[A,B](a: A, b: B): (B, A) = (b, a)Scheme
; swap elements of a vector
; vector-swap! is not part of r5rs, so we define it
(define (vector-swap! v i j)
(let ((a (vector-ref v i)) (b (vector-ref v j)))
(vector-set! v i b)
(vector-set! v j a)))
(let ((vec (vector 1 2 3 4 5)))
(vector-swap! vec 0 4)
vec)
; #(5 2 3 4 1)
; we can swap also in lists
(define (list-swap! v i j)
(let* ((x (list-tail v i))
(y (list-tail v j))
(a (car x))
(b (car y)))
(set-car! x b)
(set-car! y a)))
(let ((lis (list 1 2 3 4 5)))
(list-swap! lis 0 4)
lis)
; (5 2 3 4 1)
; using macros (will work on variables, not on vectors or lists)
(define-syntax swap!
(syntax-rules ()
((_ a b)
(let ((tmp a))
(set! a b)
(set! b tmp)))))
; try it
(let ((a 1) (b 2)) (swap! a b) (list a b))
; (2 1)sed
There are no variables in sed. Just a "pattern space" and a "hold space" — that's all. The x command exchanges the contents of both.
Seed7
A generic template to generate swap functions is defined with:
const proc: generate_swap (in type: aType) is func
begin
const proc: swap (inout aType: left, inout aType: right) is func
local
var aType: temp is aType.value;
begin
temp := left;
left := right;
right := temp;
end func;
end func;An instance of a swap function can be generated with:
generate_swap(integer);
generate_swap(string);A swap function can be called with:
swap(a, b);SenseTalk
set [x,y] to [13,"Hello"] -- assign values to two variables
put x,y
put
set [x,y] to [y,x] -- swap the variable values
put x,yOutput:
13
Hello
Hello
13Sidef
func swap(Ref a, Ref b) {
var tmp = *a;
*a = *b;
*b = tmp;
}or:
func swap(Ref a, Ref b) {
(*a, *b) = (*b, *a);
}or:
func swap(Ref a, Ref b) {
[*a, *b] » (b, a);
}The swap functions must be called with variable references.
var (x, y) = (1, 2);
swap(\x, \y);Slate
This must be done with a macro method in Slate, but is in the standard library:
x@(Syntax LoadVariable traits) swapWith: y@(Syntax LoadVariable traits) &environment: env
"A macro that expands into simple code swapping the values of two variables
in the current scope."
[
env ifNil: [error: 'Cannot swap variables outside of a method'].
tmpVar ::= env addVariable.
{tmpVar store: x variable load.
x variable store: y variable load.
y variable store: tmpVar load} parenthesize
].Usage:
a `swapWith: bSmalltalk
An OrderedCollection can collect any kind of objects; so this swap implementend extending the OrderedCollection class is really generic.
OrderedCollection extend [
swap: a and: b [
|t|
t := self at: a.
self at: a put: (self at: b).
self at: b put: t
]
]SNOBOL4
The "canonical" version from M. Emmers tutorial:
* SWAP(.V1, .V2) - Exchange the contents of two variables.
* The variables must be prefixed with the name operator
* when the function is called.
DEFINE('SWAP(X,Y)TEMP') :(SWAP_END)
SWAP TEMP = $X
$X = $Y
$Y = TEMP :(RETURN)
SWAP_ENDStandard ML
Tuples are immutable in Standard ML. This function doesn't mutate anything, but simply returns a new pair with the order of the elements switched.
fun swap (x, y) = (y, x)If the arguments are constrained to be reference values, a swap function is simple:
fun swapref (x, y) =
let temp = !x in x := !y; y := temp endStata
The Mata swap function is built-in.
mata
a=1,2,3
b="ars longa vita brevis"
swap(a, b)
endNotice that swap only works with variables, not with indexed arrays. For instance, swap(a[i],a[j]) does not work. One would instead write a[(i,j)]=a[(j,i)].
Swift
func swap<T>(inout a: T, inout b: T) {
(a, b) = (b, a)
}Note: The Swift standard library has already a swap function.
Tcl
proc swap {aName bName} {
upvar 1 $aName a $bName b
lassign [list $a $b] b a
}proc swap {aName bName} {
upvar 1 $aName a $bName b
foreach {b a} [list $a $b] break
}alternatively:
proc swap {aName bName} {
upvar 1 $aName a $bName b
set a $b[set b $a; list]
}set a 1
set b 2
puts "before\ta=$a\tb=$b"
swap a b
puts "after\ta=$a\tb=$b"- Output:
before a=1 b=2 after a=2 b=1
An idiomatic method:
set a 1
set b 2
puts "before\ta=$a\tb=$b"
set a $b[set b $a;lindex {}]
puts "after\ta=$a\tb=$b"- Output:
before a=1 b=2 after a=2 b=1
Trith
As with other stack-based languages (e.g. Factor and Joy), the solution to this task is a trivial matter of swapping the top two operands on the stack:
swapTXR
TXR Lisp has a swap macro operator. However, an operator just like it can be user-defined (let us call it swp). Moreover, the user-defined version can be just as robust, ensuring once-only evaluation for both expressions.
Swapping can be achieved with pset and rotate also. We won't use these in the following examples.
Naive macro
This allows multiple evaluation of the argument expressions.
(defmacro swp (left right)
(with-gensyms (tmp)
^(let ((,tmp ,left))
(set ,left ,right
,right ,tmp))))Using placelet
TXR Lisp's placelet macro allows the programmer to bind a lexically scoped alias for a syntactic place. The place can be accessed and stored through this alias. Yet, the place is evaluated only once. With placelet it is easy to write many kinds of place-manipulating macros very simply. We can write a robust swap which evaluates the left and right expressions just once:
(defmacro swp (left right)
(with-gensyms (tmp lpl rpl)
^(placelet ((,lpl ,left)
(,rpl ,right))
(let ((,tmp ,lpl))
(set ,lpl ,rpl
,rpl ,tmp)))))Using place expanders
Finally, the following is closely based on how swap is actually implemented in TXR Lisp's library. This explicitly uses the general mechanism for handling places, on which placelet is based also:
(defmacro swp (left right :env env)
(with-gensyms (tmp)
(with-update-expander (l-getter l-setter) left env
(with-update-expander (r-getter r-setter) right env
^(let ((,tmp (,l-getter)))
(,l-setter (,r-getter))
(,r-setter ,tmp))))))with-update-expander is a macro which writes code for accessing and updating a place, and makes that code available as local macros. The result is wrapped around the body of code passed to the macro; the body can access these functions, using a backquote to insert the symbols which refer to them. For instance the macro call (,l-getter) expands to code which accesses the prior value of the left place, and (,r-setter ,tmp) stores the value of the temporary variable into the right place.
UNIX Shell
$ swap() { typeset -n var1=$1 var2=$2; set -- "$var1" "$var2"; var1=$2; var2=$1; }
$ a=1 b=2
$ echo $a $b
1 2
$ swap a b
$ echo $a $b
2 1
$ swap a b
$ echo $a $b
1 2$ swap() { local var1=$1 var2=$2; set -- "${!var1}" "${!var2}"; declare -g "$var1"="$2" "$var2"="$1"; }
$ a=1 b=2
$ echo $a $b
1 2
$ swap a b
$ echo $a $b
2 1
$ swap a b
$ echo $a $b
1 2Ursala
Most functions are polymorphic without any special provision to that effect. Swapping a pair is a very inexpensive operation because no actual copying or overwriting is performed.
pmgs("x","y") = ("y","x") # the pattern matching way
ugs = ~&rlX # the idiosyncratic Ursala way
#cast %sWL
test = <pmgs ('a','b'),ugs ('x','y')>- Output:
<('b','a'),('y','x')>
V
Using the view to shuffle the stack.
[swap [a b : b a] view].
1 2 swap
= 2 1
'hello' 'hi' swap='hi' 'hello'
Verbexx
// user-defined swap verb -- parms are passed by alias, not value, so they can be updated:
'<==> [_a] @FN [_b] { _a _b = _b _a } by_alias: ;
// test out swap verb
@VAR a = 12345;
@VAR b = "*****";
@SAY "a=" a " b=" b;
\b <==> \a; // "\" verb prevents evaluation of a and b here,
// so they can be passed by alias to <==>
@SAY "a=" a " b=" b;
a b = b a; // swap them back, just using the usual = verb
@SAY "a=" a " b=" b;−
Visual FoxPro
Since Visual FoxPro is not strongly typed, this will work with any data types.
*!* Swap two variables
LOCAL a, b
a = 1
b = "Hallo"
? a, b
*!* Pass a and b by reference
Swap(@a, @b)
? a, b
PROCEDURE Swap(v1, v2)
LOCAL dum
dum = v1
v1 = v2
v2 = dum
ENDPROC- Output:
1 Hallo Hallo 1
Wart
There's a primitive for modifying bindings.
(swap! x y)New bindings can be created in parallel.
let (x y) (list y x)
...Wren
Wren is dynamically typed and, once a variable has been declared, a value of any type can be assigned to it. Generic programming is not therefore a problem here.
However, it is still not possible to write a generic swap function. This is because all variables are passed to functions or methods by value and prescribing that they should be passed by reference instead is not supported. Moreover, variables of simple types (numbers, bools and nulls) are never boxed and so a function cannot mutate the original variable.
Perhaps the nearest we can get to a generic swap function is to pass the variables in a list (the list's address is then passed by value under the hood), swap the list elements (lists do have a swap method) and then unpack the list to the original variables after the function returns.
Another approach would be to box simple variables (using a user defined class) so that they can be mutated. However, the problem with this is that they usually need to be 'unboxed' when the function returns.
Both approaches are illustrated below.
var swap = Fn.new { |l| l.swap(0, 1) }
var a = 6
var b = 3
var c = [a, b]
swap.call(c) // pass a list instead of individual variables
a = c[0] // unpack
b = c[1] // ditto
System.print("a is now %(a)")
System.print("b is now %(b)")
System.print()
// all user defined classes are reference types
class Box {
construct new(v) { _v = v }
v { _v }
v=(value) { _v = value }
}
// by passing boxed arguments we can mutate them
var boxSwap = Fn.new { |a, b|
var t = a.v
a.v = b.v
b.v = t
}
var d = Box.new(4)
var e = Box.new(8)
boxSwap.call(d, e)
d = d.v // unbox
e = e.v // ditto
System.print("d is now %(d)")
System.print("e is now %(e)")- Output:
a is now 3 b is now 6 d is now 8 e is now 4
XBS
There is a keyword for swapping variables. Here is an example of 2 ways to swap variables
Swap Keyword
set x=1;
set y=2;
log("Before Swap");
log(x);
log(y);
swap x y;
log("After Swap");
log(x);
log(y);- Output:
Before Swap 1 2 After Swap 2 1
Swap Function
set x = 1;
set y = 2;
func Swap(a,b){
send[b,a];
}
set z = Swap(x,y);
x=z[0];
y=z[1];
log(x)
log(y);- Output:
2 1
XPL0
The name Swap is normally used to call an intrinsic routine that swaps bytes in an integer. Thus Exch is used here instead. A and B must both be the same size.
include c:\cxpl\codes;
proc Exch(A, B, S);
char A, B, S;
int I, T;
for I:= 0 to S-1 do
[T:= A(I); A(I):= B(I); B(I):= T];
real X, Y;
[X:= 3.0; Y:= 4.0;
Exch(addr X, addr Y, 8);
RlOut(0, X); RlOut(0, Y); CrLf(0);
]- Output:
4.00000 3.00000
Yorick
Yorick has a built-in function swap for exchanging the contents of two variables without requiring a temporary copy. Example of use:
> a = 1 > b = "foo" > swap, a, b > a "foo" > b 1
Swapping elements in an array can be accomplished using index lists. Arbitrary permutations of swaps are also straightforward. Example:
> foo = [10,20,30,40,50] > foo([1,2]) = foo([2,1]) > foo [20,10,30,40,50] > foo([3,4,5]) = foo([4,5,3]) > foo [20,10,40,50,30]
Z80 Assembly
Zilog Z80
The Z80 has a few commands for swapping register contents:
EX DE,HLwill swap the contents ofDEwith the contents ofHL. This is helpful because many instructions are only compatible withHL.EXXwill swap outBC,DE, andHLwith their "shadow registers." To switch back to the standard registers useEXXagain. The accumulator and flags have their own shadow registers, which can be swapped withEX AF,AF'.EX (SP),HLwill exchange HL with the top two bytes of the stack.- Any two registers that you can
PUSH/POPcan be exchanged by pushing both registers and popping them in the "wrong" order on purpose. For example, to swap theIXandIYregisters:
push ix
push iy
pop ix ;the value that was once in IY is now in IX
pop iy ;the value that was once in IX is now in IYSwapping two memory locations takes a bit more work. The Z80 can do an 8-bit swap or a 16-bit swap on memory. There are several ways to do this, but the methods differ slightly depending on which addressing modes you use. Only the accumulator can load/store a single byte directly from/to memory, but the register pairs BC, DE, HL, SP, IX, and IY can all load a 16-bit word directly from memory, or vice versa.
;8-bit swap using the stack.
ld a,(&C000)
push af
ld a,(&D000)
ld (&C000),a ;store the byte at &D000 into &C000
pop af ;now a = the byte at &C000
ld (&D000),a ;now the byte at &D000 equals the byte that was originally at &C000;16-bit swap:
ld hl,(&C000) ;load the byte at &C000 into L and the byte at &C001 into H.
ld de,(&D000) ;load the byte at &D000 into E and the byte at &D001 into D.
ld (&D000),hl ;store the contents of L into &D000 and H into &D001.
ld (&C000),de ;store the contents of E into &C000 and D into &C001.Game Boy
The Game Boy is missing all the exchange commands that the Z80 and 8080 both have. There are no shadow registers, EX, or EXX, and you cannot load/store 16-bit register pairs to/from immediate memory addresses. The only register that can store directly to memory is the stack pointer (SP), and it can only store, not load! But not all is lost. EX DE,HL can be mimicked with the following sequence; although it's much slower than the Zilog's EX DE,HL, it's as quick as it possibly can be.
push hl
ld h,d
ld L,e ;store de into HL. This is much faster than "push de pop hl."
pop de ;put old HL into DESwapping from memory must be done one byte at a time on the Game Boy. The Game Boy exclusive LDI/LDD commands can help us in this task.
ld hl,&C000
push bc
ldi a,(hl) ;equivalent of "ld a,(hl) inc hl" but is faster than the two separately. Some assemblers call this "ld a,(hl+)"
ld c,a
ld a,(hl) ;we don't need to increment hl this time. There's no wasted time or bytecode if we did however.
ld b,a ;on Zilog Z80 we would have just done "LD BC,(&C000)" but Game Boy can't do that.
; now we do the swap.
ld hl,&D000
ld a,(hl) ;get the byte at &D000
ld (&C000),a ;store it into &C000
ld (hl),c ;store the old byte at &C000 into &D000
inc hl ;inc HL to &D001
ld a,(hl) ;get the byte at &D001
ld (&C001),a ;store it at &C001
ld (hl),b ;store the old byte at &C001 into &D001
pop bczkl
As a general case, no. Here a fake around for two known classes (which are the same for ease of example):
class C{var v; fcn init(n){v=n}}
var c1=C(1), c2=C(2);
println(c1.v," : ",c2.v);
fcn swap(ca,cb,name){
tmp:=ca.resove(name);
ca.setVar(name,cb.resolve(name)); cb.setVar(name,tmp)
}
swap(c1,c2,"v");
println(c1.v," : ",c2.v);- Output:
1 : 2 2 : 1
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