Generic swap

From Rosetta Code
Generic swap
You are encouraged to solve this task according to the task description, using any language you may know.

Many statically typed languages provide a generic programming capability. In C++ this capability is called templates. In Ada it is called generics. Such generic capabilities are simply the natural approach to programming for dynamically typed languages.

This task asks you to create a generic swap method that can be used for a wide variety of data types. If your solution language is statically typed please describe the way your language provides genericity. (This is actually a simple example of Parametric Polymorphism)


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;


  Left := Right;
  Right := Temp;

end Generic_Swap;</lang>


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


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
 ForAll({x}, v2, `WriteF('\d ',x))  -> 10 20 30 40



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>


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

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


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


set temp1=!%1! set temp2=!%2! set %1=%temp2% set %2=%temp1% goto :eof</lang>


This has a restriction that a and b must be the same size. I think you could do better with preprocessor voodoo, but I am not enlightened enough to use it. I'm being cute here, so please don't hate me too much, and if a C guru could check it, I'd appreciate it (I tried it and it worked for me). It won't work for something like a linked list if you want all references to middle nodes to be translated, but it'll swap the heads just fine.

<lang c>void swap(void *a, void *b, size_t size) {

 char *ca, *cb;
 int i;
 ca = (char *)a;
 cb = (char *)b;


You could also do it with a third buffer but then it wouldn't work for extremely large void pointers. It'd be faster to use a larger pointer than char *, but I wanted to keep it simple.

Another maybe better way is to use preprocessor macros and __typeof__ (supported by C89 at least)

<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>

  1. 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.


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:

<lang cpp>std::swap(x,y);</lang>

It will work with any types.


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 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.

Common Lisp

<lang lisp>(rotatef a b)

(psetq a b b a)</lang>


This is another standard swap.

<lang cfm><cfset temp = a /> <cfset a = b /> <cfset b = temp /></lang>


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.


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>



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

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



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

 return [right, left]



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>


<lang fsharp>let swap (a,b) = (b,a)</lang>


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


<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


Works with: Fortran version 90 and later

<lang fortran>MODULE Genericswap

   MODULE PROCEDURE Swapint, Swapreal, Swapstring


 SUBROUTINE Swapint(a, b)
   INTEGER :: temp
   temp = a ; a = b ; b = temp
 SUBROUTINE Swapreal(a, b)
   REAL, INTENT(IN OUT) :: a, b
   REAL :: temp
   temp = a ; a = b ; b = temp
 SUBROUTINE Swapstring(a, b)
   CHARACTER(len(a)) :: temp
   temp = a ; a = b ; b = temp

END MODULE Genericswap


 USE Genericswap
 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



The method requested in the task description cannot be written easily in Go. Go does not provide a way to write this type of generic method, not without bypassing the language type safety and manipulating arbitrary memory. (Doing that would be tantamount to reimplementing the assignment operator logic from the compiler, which is about 400 lines.)

Moving on the second request of the task description, Go is statically typed, and does have several features for solving problems that are often solved with generics in other languages.

  • A number of features work with multiple, or even all, types in the language. The assignment operator, for example works with all types. If a and b are of identical type, then either can be assigned to the other. This is true for all types.
  • "Duck typing" allows the type to be inferred in many situations. Given variables a and b for example,

<lang go>c := a a = b b = c</lang> is valid code that does not require the type of c to be specified. The type is inferred statically, but nevertheless does not have to be typed by the programmer.

Go has multiple assignment. While it is no more generic than single assignment, it does make swapping a little more elegant: <lang go>a, b = b, a</lang> Other features useful in contexts of multiple types:

  • Interfaces allow a very useful sort of polymorphism. They allow methods with the same signature to be written for multiple concrete types and then called through an abstract interface.
  • Type assertions and type switches allow code to conditionally handle multiple types.
  • A type reflection package has multiple features for inspecting and working with types at run time.


Groovy has support for swapping built in:

<lang groovy>(a, b) = [b, a]</lang>

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>


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

  local x, y, v
  v := create(1 to 2)
  every (x | y) := @v
  write(x," ",y)
  x :=: y
  write(x," ",y)



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)



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>


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;



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;



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.


<lang Lisaac>(a, b) := (b, a);</lang>

<lang logo> to swap :s1 :s2

 localmake "t thing :s1
 make :s1 thing :s2
 make :s2 :t


make "a 4 make "b "dog swap "a "b  ; pass the names of the variables to swap show list :a :b  ; [dog 4] </lang>


<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 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>


<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>


x y

y x


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>


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> 


<lang maxscript>swap a b</lang>


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 := ?



<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>


<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;
   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;


 IO.Put("Left = " & Fmt.Int(left) & "\n");
 IntSwap.Swap(left, right);
 IO.Put("Left = " & Fmt.Int(left) & "\n");

END Main.</lang>


Left = 10
Left = 20


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


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>


GNU Octave has no way to pass a value by reference to a function; so to define a swap we must do as follow:

<lang octave>function [a, b] = swap(ia, ib)

 a = ib; b = ia;


% testing va = 2; vb = 5; printf("%d %d\n", va, vb); [va, vb] = swap(va, vb); printf("%d %d\n", va, vb);</lang>


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
    A := @B
    B := Tmp

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

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



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


Perl has support for swapping built-in

<lang perl>($y, $x) = ($x, $y);</lang>

Here's a generic swap routine:

<lang perl>sub swap {@_[0, 1] = @_[1, 0]}</lang>

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.


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

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



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>


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


<lang PL/I> /* FIRST WAY, USING THE PREPROCESSOR: */ /* 16 August 2010 */

%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;


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>


Swap is easily done via multiple assignment:

<lang pop11>(a, b) -> (b, a);</lang>

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


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>


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

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


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


Python has support for swapping built in:

<lang python>a, b = b, a</lang>

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.


<lang REBOL>REBOL [ Title: "Generic Swap" Author: oofoe Date: 2009-12-06 URL: 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>


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


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



</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 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 has support for swapping built in:

<lang ruby>a, b = b, a</lang>

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

<lang ruby>def swap(a, b)

   return b, a

end</lang> This function doesn't mutate anything, but simply returns a new array with the order of the elements switched.


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;


<lang sather>class MAIN is

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



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 needed not be declared in the example below. 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 is receiving two values, and returning a tuple with the values inverted.

<lang scala>def swap[A,B](a: A, b: B): (B, A) = (b, a)</lang>


This must be done with macros, since the parameters inside a procedure are copies of the actual parameters. <lang scheme>(define-syntax swap!

 (syntax-rules ()
   ((_ a b)
    (let ((tmp a))
      (set! a b)
      (set! b tmp)))))</lang>


A generic template to generate swap functions is defined with: <lang seed7>const proc: generate_swap (in type: aType) is func

   const proc: swap (inout aType: left, inout aType: right) is func
       var aType: temp is aType.value;
       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>


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." [| tmpVar |

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


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




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)


       $X = $Y
       $Y = TEMP                            :(RETURN)


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>


Works with: Tcl version 8.5

<lang tcl>proc swap {aName bName} {

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


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


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


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


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


1 → x 2 → y swap("x", "y") x





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>


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

  1. cast %sWL

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




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'


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 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
> b

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
> foo([3,4,5]) = foo([4,5,3])
> foo