Swap the two given indices [duplicate]

Given an array of positive integers and two distinct valid indices, return the array with the two elements corresponding to the two indices swapped.

You may choose to use 0-indexing or 1-indexing, but the testcases below will be 0-indexed.

array        m n output
[1,2,3,4]    0 1 [2,1,3,4]
[5,8,9]      0 2 [9,8,5]
[11,13,15,3] 1 2 [11,15,13,3]
[11,13,15,3] 2 1 [11,15,13,3]
[11,15,15,3] 2 1 [11,15,15,3]

This is . Shortest answer in bytes wins. Standard loopholes apply.

• Jun 22, 2017 at 10:28
• Huh, this may well be a task that many golfing languages have a hard time with but most practical languages find easy. (Lists with mutable elements aren't a common thing for golfing languages to have.) If that is the case, it'll be quite interesting. (The golfing languages will probably still win, though, because they're so much terser they can get away with a more complex algorithm.)
– user62131
Jun 22, 2017 at 10:36
• Surprised this probably isn't a dupe, but this challenge is actually creative, since it's a real challenge for many golfing languages out there. Jun 22, 2017 at 10:45
• @LeakyNun I've got downvotes (and even delete votes) like that in the past, don't worry too much about it... Jun 22, 2017 at 10:46
• Can m and n be taken as an array?
– Okx
Jun 22, 2017 at 11:23

C/C++, 5350 39 bytes

f(a,m,n)int*a;{a[m]^=a[n]^=a[m]^=a[n];}

Try it online

Saved 11 bytes thanks to @Dennis

Operation Flashpoint scripting language, 98 95 bytes

f={t=_this;a=t select 0;b=+a;m=t select 1;n=t select 2;a set[m,b select n];a set[n,b select m]}

Modifies the array directly.

Explanation:

t=_this;                   // Give a shorter name for the array of arguments.

a=t select 0;              // Let 'a' be a pointer to the array that we modify.
// (The language doesn't have a concept of pointers really,
// yet its array variables are pointers to the actual array.)

b=+a;                      // Make a copy of the original array and save a pointer to it
// in the variable 'b'. This saves a few bytes later.

m=t select 1;              // Read the index arguments from the input array and save them
n=t select 2;              // to their respective variables.

a set[m,b select n];       // Do the swapping by reading the values from the copy and
a set[n,b select m]        // writing them to the original array. The last semicolon can
// be omitted because there are no more statements following
// the last statement.

Call with:

array = [1,2,3,4];
str = format["%1", array];
[array, 0, 1] call f;
hint format["%1\n%2", str, array];

Output:

JavaScript ES6, 36 32 bytes

Look, Ma, no temporary variable!

(a,m,n)=>[a[m],a[n]]=[a[n],a[m]]

Try it

Enter a comma separated list of elements for a and 2 integers for m & n.

f=
(a,m,n)=>[a[m],a[n]]=[a[n],a[m]]
oninput=_=>o.innerText=(f(b=i.value.split,,+j.value,+k.value),b);o.innerText=(f(b=(i.value="5,8,9").split,,j.value=0,k.value=2),b)
*{font-family:sans-serif}input{margin:0 5px 0 0;width:100px;}#j,#k{width:50px;}
<label for=i>a: </label><input id=i><label for=j>m: </label><input id=j type=number><label for=k>n: </label><input id=k type=number><pre id=o>

• Those instructions modify the array, which means you're allowed to not return the array, which will save you a few bytes.
– Neil
Jun 22, 2017 at 12:14
• @Neil: You're saying to just use (a,m,n)=>[a[m],a[n]]=[a[n],a[m]]? That would only output the 2 swapped elements without the rest of the array (e.g., [5,8,9],0,2 -> [9,5]). Jun 22, 2017 at 13:33
• @Neil: Right, which is why we need the a at the end to give us the complete, modified array. Or am I completely missing what you're trying to say? Jun 22, 2017 at 14:16
• @Neil: Hmm ... OK, I think I see what you'e getting at now (Sorry, trying to do too many things at the same time today). Thanks for the tip. Is there a consensus on that and, if so, would you have a link handy before I go searching for it myself? Jun 22, 2017 at 14:50
• codegolf.meta.stackexchange.com/a/4942/48934
– Neil
Jun 22, 2017 at 14:59

Jelly, 7 bytes

Ṛ,ḷyJ}ị

Try it online!

How it works

Ṛ,ḷyJ}ị  Main link. Left argument: [i, j]. Right argument: A (array)

Ṛ        Reverse; yield [j, i].
ḷ      Left; yield [i, j].
,       Pair; yield [[j, i], [i, j]].
J}   Indices right; yield all indices of A.
y     Transliterate; replace j with i and i with j.
ị  Index into A.
• tfw the wrapper is almost as long as the program... Jun 22, 2017 at 13:20
• I never knew of the existence of y Jun 22, 2017 at 13:23
• I knew of y, but didn't think of using it here. That's a pretty clever answer.
– user62131
Jun 22, 2017 at 22:40
• This got me thinking... is Jelly valid Jelly code? Jun 23, 2017 at 14:16
• @M.Herzkamp It is. I doubt its exceptionally useful though. Jun 23, 2017 at 15:21

MATL, 7 6 bytes

yyP)w(

Indices are 1-based.

Try it online!

Explanation

Consider inputs [11 13 15 3], [2 3].

yy   % Take two inputs implicitly. Duplicate them
% STACK: [11 13 15 3], [2 3], [11 13 15 3], [2 3]
P    % Flip
% STACK: [11 13 15 3], [2 3], [11 13 15 3], [3 2]
)    % Reference indexing (pick indexed entries)
% STACK: [11 13 15 3], [2 3], [15 13]
w    % Swap
% STACK: [11 13 15 3], [15 13], [2 3]
(    % Assignment indexing (write values into indexed entries). Implicitly display
% STACK: [11 15 13 3]

Python 3, 41 32 bytes

-9 bytes thanks to @notjagan

def f(a,m,n):a[m],a[n]=a[n],a[m]

Try it online!

Modifies its argument, which is a valid output format.

• It's funny how it's not even golfed that much comparing to idiomatic python code. Jun 22, 2017 at 13:59

{e\}

Try it online!

Japt, 17 16 bytes

hV(A=UgV UgW¹hWA

Try it online!

Saved a byte thanks to ETHproductions

• Nice. Don't think you need the comma though. Jun 22, 2017 at 11:23
• @ETHproductions Thanks, you're right.
– Tom
Jun 22, 2017 at 11:24
• Alternative 16 byte implementation but I'm still convinced there's a shorter solution. Jun 22, 2017 at 12:02
• 15 bytes Jun 22, 2017 at 16:10

C# (.NET Core), 48 43 31 bytes

(a,m,n)=>a[m]+=a[n]-(a[n]=a[m])

Try it online!

Swaps the numbers in the original array, no temporary variables used. Nonetheless I can't take credit for this answer as it has been Neil's idea.

• @LeakyNun it does not seem to work, as that leaves a[m] with a value of 0. Try it yourself! Jun 22, 2017 at 11:14
• (a,m,n)=>a[m]+=a[n]-(a[n]=a[m]) seems to work though.
– Neil
Jun 22, 2017 at 11:38
• (These answers are all also valid in ES6 JavaScript, no?)
– Neil
Jun 22, 2017 at 11:41

Common Lisp, 42 bytes

-2 bytes thanks to @coredump.

(lambda(a i j)(rotatef(elt a i)(elt a j)))

Try it online!

Quite straight forward, since there is a Common Lisp macro to swap: rotatef.

• You could use ELT instead of AREF Jun 23, 2017 at 11:10
• @coredump Right, thanks!
Jun 23, 2017 at 11:30

Javascript ES6, 36 34 bytes

(a,m,n)=>(x=a[m],a[m]=a[n],a[n]=x)
• -2 Bytes because the function is altering the array. No need to return the array. Thanks to @Neil

Demo

f=

(a,m,n)=>(x=a[m],a[m]=a[n],a[n]=x)

let a1=[1,2,3,4], a2=[5,8,9], a3=[11,13,15,3]

f(a1,0,1);
f(a2,0,2);
f(a3,1,2);

console.log(JSON.stringify(a1)); //[2,1,3,4]
console.log(JSON.stringify(a2)); //[9,8,5]
console.log(JSON.stringify(a3)); //[11,15,13,3]

• Those instructions modify the array, which means you're allowed to not return the array, which will save you a few bytes.
– Neil
Jun 22, 2017 at 12:14

Shenzhen I/O, 735 Bytes

23¥, 810 Power, 48 Lines of Code

[traces]
......................
......................
......................
......................
......................
......................
.14.14.14.............
.94.14.14.............
.A........1C..........
.3554..95556..........
.9554.16..............
.A....................
.2....................
......................

[chip]
[type] UC6
[x] 4
[y] 2
[code]
slx x0
mov x1 acc
mov x1 dat
mov acc x3
mov dat x3
mov acc x3
mov dat x3

[chip]
[type] UC6
[x] 8
[y] 5
[code]
slx x2
mov x2 x1
mov x0 dat
mov x2 x1
mov x0 acc
mov x2 x1
mov dat

[chip]
[type] UC4X
[x] 2
[y] 6
[code]
slx x0
mov 0 x3
j:  mov x0 acc
mov acc x2
teq acc 0
- jmp j
mov -999 x1

[chip]
[type] RAM
[x] 5
[y] 6

DISCLAIMER: Arrays are 0-terminated in this. Arrays are a pain in the ass to work with in Shenzhen I/O otherwise.

I actually made a steam level for this game. You can play it here.

EDIT: Aaand I just realized I said that the array in was ordered. Heck.

• Welcome to the site This is really cool! do you think that you might be able to remove some of the whitespace in the file and still have Shenzhen IO accept the file? I don't know how much you have fiddled with it, but you should try and see how flexible the format is. Jun 25, 2017 at 2:26
• I haven't played around with it! On the other hand, I am axing the header for the puzzle which contains the puzzle name and solution name, so I dunno if I should bother. Jun 25, 2017 at 2:33

Brachylog, 19 bytes

iʰ{pᵗz=ᵗ!|}htI&h∋↙I

Try it online!

The process of writing this generator was fairly clumsy, so there's probably some further golfing to do. I may not have thought to use i if I hadn't initially thought I had some use for the value half of the pair it outputs! Takes input as [array, [m, n]].

Java 8, 48 bytes

(a,b,c)->{int t=a[b];a[b]=a[c];a[c]=t;return a;}

Input:

int[] a
int b
int c
• How do you do lambdas with three arguments in Java? Jun 22, 2017 at 10:48
• Those instructions modify the array, which means you're allowed to not return the array, which will save you a few bytes.
– Neil
Jun 22, 2017 at 12:13
• @LeakyNun I'm not Okx, but here is a Try it now example with Okx's current answer and custom interface. Jun 22, 2017 at 13:25
• And based on Carlos Alejo's amazing C# answer (with @Neil's help), you can make it even shorter by getting rid of the temporary variable: (a,b,c)->a[b]+=a[c]-(a[c]=a[b]) (31 bytes) Jun 22, 2017 at 13:32
• cough cough Collections::swap is 17 bytes... at least assuming this holds for this challenge... Jun 23, 2017 at 0:57

D²è³ǝ¹³è²ǝ

Try it online!

Octave, 28 bytes

@(a,n){a(n)=a(flip(n)),a}{2}

Try it online!

Quite satisfied with this one actually :)

Takes input on the form: f([1,2,3,4],[1,2]), 1-indexed.

Explanation:

@(a,n)                         % Anonymous function that takes two 1-dimensional
% arrays as input
{               , }      % Create a cell with two elements
a(n)=a(flip(n))         % One element are the two number at indices given by
% the second input array. This will be a 1x2 array
{a(n)=a(flip(n)),a}      % Place those two in a cell together with the entire array a
% a is now updated, thanks to Octave's inline assignment
{a(n)=a(flip(n)),a}{2}   % Return the second element

Jellyfish, 7 bytes

p
ZRi
i

Takes a list and a pair of indices. Try it online!

Explanation

Jellyfish happens to have a "modify items at indices" function, Z, which does exactly what we need. The two is grab the inputs from STDIN. Z takes as arguments the second input, the reversal function R, and the list. Then Z performs the modification, and p prints the result.

R, 38 bytes

function(x,a,b){x[c(a,b)]=x[c(b,a)];x}

Feels rather long, but I can't get it much shorter. Sadly it requires the explicit returning through x, requiring {} around the function body. pryr::f() doesn't recognise the need for x as function argument so doesn't work :/.

• I think function(x,i)replace(x,i,rev(i)) would work, even with pryr syntax. Jun 22, 2017 at 17:09
• @Giuseppe Ah, I was looking for a convenient function to do the swap, but was searching with the wrong terms. Feel free to post that as an own answer.
Jun 22, 2017 at 19:41
• @Giuseppe I think you need to do replace(x,i,x[rev(i)]), else you'll place the indices instead of their values.
Jun 22, 2017 at 20:18

Java 8 + InverseY, 27 bytes

java.util.Collections::swap

Just calls the swap function... this is a method reference of the type Consumer3<List, Integer, Integer>.

Try it online! (header and footer for boilerplate & copy of Consumer3 interface)

• You don't need to add " + InverseY". It's valid in vanilla Java 8. Jun 23, 2017 at 15:57
• @OlivierGrégoire Agreed. Mar 30, 2021 at 21:43

Swift, 111 65 bytes (0-indexed)

Swift is already notorious for being one of the worst code-golf languages, but here is a function that makes use of ternary expressions:

func t(l:[Int],m:Int,n:Int){var r=l;r[m]=l[n];r[n]=l[m];print(r)}

Check it out! - Usage: t(l:[1,2,3],m:0,n:1).

• Using a default param for r would save you bytes and you can also just mutate the passed array (AFAIK swift array are pass by value) Jun 22, 2017 at 16:55
• Default parameter in Swift? How can I do that? Jun 22, 2017 at 17:40
• And parameters are constants in Swift @Downgoat Jun 22, 2017 at 17:41

k (kona), 13 bytes

{x[y]:x@|y;x}

Pretty basic, but it works. Ex:

k){x[y]:x@|y;x}[1 2 3 4; 0 1]
2 1 3 4

Perl 5, 32 bytes

-3 bytes thanks to @Dom Hastings!

30 bytes of code + -pa flags.

@F[pop@p,@p]=@F[@p=<>];$_="@F" Try it online! Quite straight forward, using array slices. • Hey hey, tinkered with this a little and managed to save 3 bytes! @F[pop@p,@p]=@F[@p=<>];$_="@F". Jun 22, 2017 at 13:27
• @DomHastings Hmm, nice, as always! Thanks :)
Jun 22, 2017 at 13:35

Mathematica, 32 bytes

(a=#;a[[{##2}]]=a[[{#3,#2}]];a)&
• a[[{##2}]]==a[[{#3,#2}]] should be a[[{##2}]]=a[[{#3,#2}]] (using Set, not Equals) Jun 22, 2017 at 13:12

C, 42 bytes

Modify the array in place with a temp value.

f(r,m,n){int*a=r;r=a[m];a[m]=a[n];a[n]=r;}

C, 60 58 bytes

A little more interesting, not using any temp value...

f(a,m,n)int*a;{a[m]+=a[n];a[n]-=a[m];a[n]*=-1;a[m]-=a[n];}

C, 49 bytes

Using XOR

f(a,m,n)int*a;{a[m]^=a[n];a[n]^=a[m];a[m]^=a[n];}
• Heh, I was just about to post f(x,i,j,t)int*x;{t=x[i];x[i]=x[j];x[j]=t;}. Jun 22, 2017 at 13:47
• @Dennis you saved me two bytes on the other solution, thanks! Jun 22, 2017 at 14:00
• Wouldn't the second solution be shorter (and safer) with ^? Jun 22, 2017 at 14:10
• -1 for the XOR version using a define instead of a function #define X(x,y,z)x[y]^=x[z],x[z]^=x[y],x[y]^=x[z] Jun 22, 2017 at 14:55
• f(r,m,n){int*a=r;r=a[m];a[m]=a[n];a[n]=r;} is broken: SIGSEGV. Jun 22, 2017 at 20:25

Julia 0.5, 20 bytes

x*t=x[t]=x[t[[2,1]]]

Try it online!

Pyth, 17 8 bytes

Saved 9 bytes thanks to Leaky Num.

@LQ.rUQE

Test it online!

This is 0-indexed, and the indices are provided as a tuple: (n, m).

Explanations

@LQ.rUQE

UQ     # Generate [0, 1, 2, ..., len(input)]
E    # Get the indices as the tuple (1, 2)
.r       # Translate each element of UQ to its cyclic successor in E
# Now the indices are permuted (e.g. [0, 2, 1, ..., len(input)]
@LQ         # For each index, get it's value. Implicit print
• 8 bytes: @LQ.rUQE Jun 22, 2017 at 16:49
• @LeakyNun It's so different I think you can post it for yourself!
– Jim
Jun 22, 2017 at 16:50
• I'm the OP; I don't post on my own challenge. Jun 22, 2017 at 16:50

Mathematica, 20 bytes

#~Permute~Cycles@#2&

Pure function taking two arguments in the following 1-indexed (and possibly abusive) format: the second test case [5,8,9]; 0 2; [9,8,5] would be called as

#~Permute~Cycles@#2& [ {5,8,9} , {{1,3}} ]

(spaces are extraneous and just for visible parsing). Permute is the builtin function that applies a permutation to a list, and Cycles[{{a,b}}] represents the permutation that exchanges the ath and bth elements of a list and ignores the rest.

• What do the ~ do? Jun 23, 2017 at 3:54
• ~ is Mathematica's infix notation for a binary function: x~f~y means the same thing as f[x,y]. Jun 23, 2017 at 7:35

x86 Machine Code, 10 bytes

8B 04 8B 87 04 93 89 04 8B C3

This is a function written in 32-bit x86 machine code that swaps the values at the specified indices in a given array. The array is modified in-place, and the function does not return a value.

A custom calling convention is used, requiring the function's parameters to be passed in registers:

• The address of the array (pointer to its first element) is passed in the EBX register.
• The zero-based index of element A is passed in the ECX register.
(Assumed to be a valid index.)
• The zero-based index of element B is passed in the EDX register.
(Assumed to be a valid index.)

This keeps the size down and complies with all formal requirements, but does mean that the function cannot be easily called from other languages like C. You'd need to call it from another assembly-language program. (You could rewrite it to use any input registers, though, without affecting the byte count; there's nothing magical about the ones I chose.)

Ungolfed:

8B 04 8B     mov  eax, DWORD PTR [ebx+ecx*4]   ; get value of element A
87 04 93     xchg eax, DWORD PTR [ebx+edx*4]   ; swap element A and element B
89 04 8B     mov  DWORD PTR [ebx+ecx*4], eax   ; store new value for element A
C3           ret                               ; return, with array modified in-place

R, 34 bytes

pryr::f([<-(a,c(m,n),a[c(n,m)]))

JavaScript (ES2015), 6657 49 bytes

A different (alas, longer) approach than previous JavaScript answers

(s,h,o,w=s.splice.bind(s))=>w(h,1,...w(o,1,s[h]))

Source

const swap = (arr, a, b, splice) => {
splice(a, 1, ...splice(arr[b], 1, arr[a]))
}
• (s,h,o,w=s.splice.bind(s))=>w(h,1,...w(o,1,s[h])) 49 bytes Jun 24, 2017 at 3:05
• Forgot about them default args. Thanks! Jun 24, 2017 at 23:58