Generic swap

The task is to 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!

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 instantiation. The second code snippet is the procedure body.

<lang ada>generic

  type Swap_Type is private; -- Generic parameter

procedure Generic_Swap(Left : in out Swap_Type; Right : in out Swap_Type);

procedure Generic_Swap(Left : in out Swap_Type; Right : in out Swap_Type) is

  Temp : Swap_Type := Left;

begin

  Left := Right;
  Right := Temp;

end Generic_Swap;</lang>

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.

Works with: ALGOL 68 version Revision 1 - no extensions to language used

Works with: ALGOL 68G version Any - tested with release 1.18.0-9h.tiny

Works with: ELLA ALGOL 68 version Any (with appropriate job cards) - tested with release 1.8-8d

<lang algol68>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))</lang> Output:

v1: Vilyam Fisher, v2: Francis Gary Powers

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. <lang amigae>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')

ENDPROC</lang>

AppleScript

AppleScript has built-in support for swapping. This is generic and works for all combinations of data types. <lang AppleScript>set {x,y} to {y,x}</lang>

AutoHotkey

<lang autohotkey>Swap(ByRef Left, ByRef Right) {

   temp := Left
   Left := Right
   Right := temp

}</lang>

Batch File

Swap using pass-by-name

<lang dos>@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</lang>

Bracmat

C

This has a restriction that a and b must be the same size.

<lang c>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;

}</lang>

Works with: gcc

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__.

<lang c>#define Swap(X,Y) do{ __typeof__ (X) _T = X; X = Y; Y = _T; }while(0)</lang>

Usage examples are:

<lang c>#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);

}</lang>

This is tested with GCC with -std=c89 option.

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:

<lang cpp>template<typename T> void swap(T& left, T& right) {

 T tmp(left);
 left = right;
 right = tmp;

}</lang> 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:

It will work with any types.

C#

Works with: C# version 2.0+

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 deal with.

<lang csharp>static void Swap<T>(ref T a, ref T b) {

   T temp = a;
   a = b;
   b = temp;

}</lang>

Clojure

<lang lisp> (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 </lang>

The latter two implementations use destructured binding to define local names for the two elements.

CMake

CMake has only one data type: the string.

<lang cmake>function(swap var1 var2)

 set(_SWAP_TEMPORARY "${${var1}}")
 set(${var1} "${${var2}}" PARENT_SCOPE)
 set(${var2} "${_SWAP_TEMPORARY}" PARENT_SCOPE)

endfunction(swap)</lang>

<lang cmake>set(x 42) set(y "string") swap(x y) message(STATUS ${x}) # -- string message(STATUS ${y}) # -- 42</lang>

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: <lang cmake>set(x 42)

set(_SWAP_TEMPORARY "string") swap(x _SWAP_TEMPORARY) message(STATUS ${x}) # -- 42 message(STATUS ${_SWAP_TEMPORARY}) # -- 42</lang> Inside swap(), its local variable _SWAP_TEMPORARY shadows the original _SWAP_TEMPORARY from the parent scope, preventing access to the original value.

When value of either variable is "CACHE" or "PARENT_SCOPE": <lang cmake>string(TOUPPER CACHE x)

set(y "string") swap(x y) # CMake Error... set given invalid arguments for CACHE mode.</lang> swap() can never set a variable to "CACHE" or "PARENT_SCOPE", because these are keywords of set() command.

Common Lisp

<lang lisp>(rotatef a b)

(psetq a b b a)</lang>

ColdFusion

This is another standard swap.

D

The solution for D is quite similar to that for C++:

<lang d>void swap(T)(ref T left, ref T right) {

 auto temp = left;
 left = right;
 right = temp;

}</lang> The std.algorithm standard library module contains a generic swap.

dc

We use two registers to swap in POSIX dc. <lang dc>1 2 SaSbLaLb f =2 1</lang> Reverse (r) is a built-in stack command available as a GNU extension for dc. <lang dc>1 2 r f =2 1</lang>

Delphi

This example is incorrect. Please fix the code and remove this message.

Details: Option 1 demonstrates a dubious arithmetic trick for swapping integers that works on two's complement hardware thanks to wrapping behavior of arithmetic overflow. Neither option demonstrates generic programming: swapping two variables using the same function or operator, regardless of their type.

Option 1: <lang Delphi> procedure Swap(var a, b: Integer); begin

 a := a + b;
 b := a - b;
 a := a - b;

end; </lang> Option 2: <lang Delphi> procedure Swap(var a, b: Integer); var

 temp: Integer;

begin

 temp := a;
 a := b;
 b := temp;

end; </lang>

E

(slots)

<lang e>def swap(&left, &right) {

 def t := left
 left := right
 right := t

}</lang>

(functional)

<lang e>def swap([left, right]) {

 return [right, left]

}</lang>

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).

<lang Erlang> 1> L = [a, 2]. [a,2] 2> lists:reverse(L). [2,a] </lang>

Or swap the items in a tuple (also shown in the shell).

<lang Erlang> 1> T = {2,a}. {2,a} 2> list_to_tuple(lists:reverse(tuple_to_list(T))). {a,2} </lang>

F#

Factor

Depending on how you look at it: this task doesn't apply, or it's trivial: <lang factor>swap</lang>

Falcon

<lang falcon> a = 1 b = 2 a,b = arr = b,a </lang> Reading right to left: Assign b & a into an array variable called arr, then assign into a & b

Fortran

Works with: Fortran version 90 and later

<lang 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</lang>

Forth

Frink

The following example will work on all Frink data types:

Gecho

<lang gecho> 1 !0 2 !1 </lang> Now tape[0] and tape[1] are set to 1 and 2, respectively. <lang gecho> &0 &1 !0 pop !1 </lang> This 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. <lang go>a, b = b, a</lang>

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. <lang go>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)

}</lang> 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. <lang go>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)

}</lang> 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}

Groovy

Groovy has support for swapping built in:

But the task calls for a "generic swap method" to be written, so here it is:

<lang groovy>def swap(a, b) {

   [b, a]

}</lang> 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: <lang groovy>def (x, y) = swap(1, 3) assert x == 3 assert y == 1</lang>

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: <lang groovy>def listSwap = { a, i, j ->

   assert (0..<(a.size())).containsAll([i,j]);
   aj,i = ai,j

}

def list = [2,4,6,8] listSwap(list, 1, 3) assert list == [2,8,6,4]</lang>

Haskell

Like everything else in Haskell, tuples are immutable. This 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.

<lang haskell>swap :: (a, b) -> (b, a) swap (x, y) = (y, x)</lang> This swap function is available in the Data.Tuple standard library module in GHC 7.0+

Icon and Unicon

Icon provides a :=: operator for this. Additionally, there is a reversible exchange operator <-> that reverses the exchange if resumed. <lang icon>procedure main()

  x := 1
  y := 2
  x :=: y
  write(x," ",y)
  # swap that will reverse if surrounding expression fails
  if x <-> y & x < y then write(x, " ", y)

end </lang>

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.

<lang IDL>pro swap, a, b

 c = temporary(a)
 a = temporary(b)
 b = temporary(c)

end</lang>

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):

<lang J> (<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.</lang>

Also, if the argument list can be guaranteed to be a pair, J's reverse primitive will swap the pair.

<lang J> |.2 3 3 2

  |.&.;:'one two'

two one</lang>

Java

Works with: Java version 1.5+

Java uses references, so it can't swap the values of two variables that don't belong to a class.

<lang java>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;

}</lang>

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.

<lang javascript>function swap(arr) {

 var tmp = arr[0];
 arr[0] = arr[1];
 arr[1] = tmp;

}</lang>

Joy

Provided that the stack contains at least two elements and/or aggregates: <lang joy>swap</lang> changes the order of those elements and/or aggregates.

Lisaac

Logo

<lang logo> 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] </lang>

Lhogho

Lhogho is very similar except that it does not have a localmake opcode.

<lang logo> 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] </lang>

Logtalk

<lang logtalk>:- object(paws).

   :- public(swap/4).
   swap(First, Second, Second, First).

- end_object.</lang>

Usage examples: <lang logtalk>| ?- paws::swap(apples, oranges, X, Y). X = oranges Y = apples yes

| ?- paws::swap(3.14, ext(lgt), X, Y). X = ext(lgt) Y = 3.14</lang> yes

Lua

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: <lang lua> 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 t </lang>

Usage example: <lang lua> x, y = 3, 4 print(x, y) --> 3 4 x, y = y, x -- swap print(x, y) --> 4 3 </lang>

M4

<lang m4>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</lang>

Output:

x y

y x

Mathematica

Mathematica functions are generic by default; however, it has to be told not to evaluate the arguments before executing the function. <lang mathematica>swap[a_, b_] := {a, b} = {b, a} SetAttributes[swap, HoldAll]</lang>

MATLAB / Octave

It is unnecessary to define a "swap" function in MATLAB because swaps are trivial operations. In fact, they are so natural to the language that multiple swaps can be performed simultaneously.

Example: <lang MATLAB>>> 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</lang>

MAXScript

Metafont

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)

<lang metafont>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;</lang>

Examples:

<lang metafont>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;</lang>

Modula-3

<lang modula3>GENERIC INTERFACE GenericSwap(Elem);

PROCEDURE Swap(VAR left: Elem.T; VAR right: Elem.T);

END GenericSwap.</lang> <lang modula3>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.</lang>

Here is an example usage for integers: <lang modula3>INTERFACE IntSwap = GenericSwap(Integer) END IntSwap.</lang> <lang modula3>MODULE IntSwap = GenericSwap(Integer) END IntSwap.</lang> <lang modula3>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.</lang>

Output:

Left = 10
Left = 20

Nemerle

For pairs, namespace Nemerle.Utility.Pair contains Swap(): <lang Nemerle>def coords = (1, -1); def invcoords = Swap(coords);</lang> Or to swap two mutable variables of the same type:<lang Nemerle>a <-> b;</lang> But, enough about built in functionality, let's demonstrate using generics: <lang Nemerle>Swap[T, U] (a : T, b : U) : U * T {

   (b, a)

}</lang>

Nial

Like J <lang nial>|reverse 1 2 =2 1</lang>

OCaml

Tuples are immutable in OCaml. This function doesn't mutate anything, but simply returns a new pair with the order of the elements switched. <lang ocaml>let swap (x, y) = (y, x)</lang> If the arguments are constrained to be reference values, a swap function is simple: <lang ocaml>let swapref x y =

 let temp = !x in
   x := !y;
   y := temp</lang>

Oz

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. <lang oz> proc {SwapCells A B}

    Tmp = @A
 in
    A := @B
    B := Tmp
 end</lang>

Or shorter, if we exploit the fact that the assignment operator := returns the old value of the cells: <lang oz> proc {SwapCells A B}

    B := A := @B
 end</lang>

A functional swap, operating on pairs: <lang oz> fun {SwapPair A#B}

    B#A
 end</lang>

PARI/GP

Pari is near-typeless—everything is a GEN. <lang parigp>my(tmp=a); a=b; b=tmp;</lang>

Perl

Perl has support for swapping built-in

Here's a generic swap routine:

Perl 6

As Perl 5. Perl 6 supports type constraints for variables and subroutines, unlike Perl 5, but the default is still to permit all values.

PHP

<lang php>function swap(&$a, &$b) {

   list($a, $b) = array($b, $a);

}</lang>

PicoLisp

xchg works with any data type <lang PicoLisp>(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) )</lang>

Output:

2 1
(a e c) (d b f)

PL/I

Using the preprocessor

<lang PL/I>%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:

  t = p; p = q; q = t;</lang>

Using generic procedures

<lang pli>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);</lang>

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

<lang pli>%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);</lang>

The code generated by 'swap(c1, c2);' looks like

<lang pli> 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;</lang>

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;

Pop11

Swap is easily done via multiple assignment:

Pop11 is dynamically typed, so the code above is "generic".

PostScript

Works with anything you can put on the operand stack:<lang PostScript>exch</lang>

PowerShell

PowerShell allows swapping directly, through tuple assignment: <lang powershell>$b, $a = $a, $b</lang> But one can also define a function which swaps the values of two references: <lang powershell>function swap ([ref] $a, [ref] $b) {

   $a.Value, $b.Value = $b.Value, $a.Value

}</lang> When using this function the arguments have to be explicitly given as references: <lang powershell>swap ([ref] $a) ([ref] $b)</lang>

Prolog

<lang prolog> swap(A,B,B,A).

?- swap(1,2,X,Y). X = 2, Y = 1. </lang>

PureBasic

Built in function: <lang PureBasic>Swap a, b</lang>

Python

Python has support for swapping built in:

But the task calls for a "generic swap method" to be written, so here it is:

<lang python>def swap(a, b):

   return b, a</lang>

Note 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.

R

R function arguments are passed by value, not by reference. You can work around this, however, by using their names and environment:

<lang R>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)

}</lang>

Usage:

> x <- 1
> y <- 2
> swap('x', 'y')
> cat(x, y)
2 1

REBOL

<lang REBOL>REBOL [ Title: "Generic Swap" Author: oofoe Date: 2009-12-06 URL: http://rosettacode.org/wiki/Generic_swap Reference: [1] ]

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 "."]</lang>

Output:

The answer is Heart of Gold, the ship is 42.

Retro

REXX

REXX has no primitive for swapping, but it can easily be performed using a termporary variable. <lang rexx> a='some value' b=6

_temp_=a

    a=b
    b=_temp_

</lang> If it is known that there are no leading, trailing, or imbedded blanks in the values, the following method can be used: <lang rexx> a=-199e-12 b=12.

parse value a b with b a /*swaps A and B */ </lang>

RLaB

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 <lang RLaB> 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 1 </lang>

Ruby

Ruby has support for swapping built in:

But the task calls for a "generic swap method", so here it is:

<lang ruby>def swap(a, b)

   return b, a

end</lang>

This 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:

<lang ruby>x = 42 y = "string" x, y = swap x, y puts x # prints string puts y # prints 42</lang>

Sather

A possible way that needs the type of the objects to be specified:

<lang sather>class SWAP{T} is

 swap(inout a, inout b:T) is
   t ::= a;
   a := b;
   b := t;
 end;

end;</lang>

<lang sather>class MAIN is

 main is
   x ::= 10;
   y ::= 20;
   SWAP{INT}::swap(inout x, inout y);
   #OUT + x + ", " + y + "\n";
 end;

end;</lang>

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.

Scheme

<lang 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)</lang>

Seed7

A generic template to generate swap functions is defined with: <lang seed7>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;</lang>

An instance of a swap function can be generated with: <lang seed7>generate_swap(integer); generate_swap(string);</lang> A swap function can be called with: <lang seed7>swap(a, b);</lang>

Slate

This must be done with a macro method in Slate, but is in the standard library: <lang slate>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

].</lang>

Usage: <lang slate>a `swapWith: b</lang>

Smalltalk

Works with: GNU Smalltalk

An OrderedCollection can collect any kind of objects; so this swap implementend extending the OrderedCollection class is really generic. <lang smalltalk>OrderedCollection extend [

   swap: a and: b [

|t| t := self at: a. self at: a put: (self at: b). self at: b put: t

]</lang>

SNOBOL4

The "canonical" version from M. Emmers tutorial:

<lang snobol4>* 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_END</lang>

Standard 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. <lang sml>fun swap (x, y) = (y, x)</lang> If the arguments are constrained to be reference values, a swap function is simple: <lang sml>fun swapref (x, y) =

   let temp = !x in x := !y; y := temp end</lang>

Tcl

Works with: Tcl version 8.5

<lang tcl>proc swap {aName bName} {

   upvar 1 $aName a $bName b
   lassign [list $a $b] b a

}</lang>

Works with: Tcl version 8.4

<lang tcl>proc swap {aName bName} {

   upvar 1 $aName a $bName b
   foreach {b a} [list $a $b] break

}</lang>

<lang tcl>set a 1 set b 2 puts "before\ta=$a\tb=$b" swap a b puts "after\ta=$a\tb=$b"</lang>

Outputs:

before	a=1	b=2
after	a=2	b=1

ThinBASIC

Generic function, swap the content of two variables. <lang ThinBASIC>Swap Var1, Var2</lang>

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.

<lang ti89b>Define swap(swapvar1, swapvar2) = Prgm

 Local swaptmp
 #swapvar1 → swaptmp
 #swapvar2 → #swapvar1
 swaptmp → #swapvar2

EndPrgm

1 → x 2 → y swap("x", "y") x

y

   1</lang>

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: <lang trith>swap</lang>

Ursala

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. <lang Ursala>pmgs("x","y") = ("y","x") # the pattern matching way

ugs = ~&rlX # the idiosyncratic Ursala way

cast %sWL

test = <pmgs ('a','b'),ugs ('x','y')></lang>

output:

<('b','a'),('y','x')>

V

Using the view to shuffle the stack.

<lang v>[swap [a b : b a] view].

1 2 swap = 2 1 'hello' 'hi' swap</lang>

='hi' 'hello'

VBScript

This works for everything: strings, dates, booleans ... The fact is, with everything being a Variant, it's always generic.

<lang vb>sub swap( byref x, byref y ) dim temp temp = x x = y y = temp end sub</lang>

Usage: <lang vb>dim a a = "woof" dim b b = now() swap a,b wscript.echo a wscript.echo b</lang>

Output: <lang vb>5/02/2010 2:35:36 PM woof</lang>

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.

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]