Swap the two given indices

Question

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 code-golf. Shortest answer in bytes wins. Standard loopholes apply.

Answer 1

C/C++, 53 50 39 bytes

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

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Saved 11 bytes thanks to @Dennis

Answer 2

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:

Answer 3

JavaScript ES6, 36 32 bytes

Look, Ma, no temporary variable!

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

• 4 bytes saved thanks to Neil bringing this consensus to my attention.

---

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>

Answer 4

Python 3, 41 32 bytes

-9 bytes thanks to @notjagan

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

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Modifies its argument, which is a valid output format.

Answer 5

MATL, 7 6 bytes

yyP)w(

Indices are 1-based.

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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]

Answer 6

Jelly, 7 bytes

Ṛ,ḷyJ}ị

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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.

Answer 7

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

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

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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.

Answer 8

Japt, 17 16 bytes

hV(A=UgV UgW¹hWA

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Saved a byte thanks to ETHproductions

Answer 9

Common Lisp, 42 bytes

-2 bytes thanks to @coredump.

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

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

Answer 10

CJam, 4 bytes

{e\}

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Answer 11

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]

Answer 12

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

Answer 13

05AB1E, 10 bytes

D²è³ǝ¹³è²ǝ

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Answer 14

Octave, 28 bytes

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

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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

Answer 15

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.

Answer 16

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 :/.

Answer 17

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.

Answer 18

Mathematica, 32 bytes

(a=#;a[[{##2}]]=a[[{#3,#2}]];a)&

Answer 19

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).

Answer 20

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

Answer 21

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.

Answer 22

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];}

Answer 23

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

Answer 24

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.

Answer 25

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

Answer 26

R, 34 bytes

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

Answer 27

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)

Answer 28

JavaScript (ES2015), 66 57 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]))
}

Answer 29

awk, 31 bytes

{c=$a;$a=$b;$b=c;a=$1;b=$2}NR>1

Try it online!

Takes input in the format

1 2
1 2 3 4

and outputs as

2 1 3 4

(1-indexed).

Explanation

The entire program is a missing pattern with an action followed by a pattern with a missing action.

Since a missing pattern runs on each line, the code inside the braces runs for both input lines. The c=$a;$a=$b;$b=c; part swaps the two values at indices a and b (through the temporary variable c). This only has an effect on the second line, since on the first line a and b are not yet defined. The a=$1;b=$2 part defines a to be the first field and b to be the second field, which sets the appropriate values for the first part to run on the second line.

Since a missing action is equivalent to {print}, the pattern prints every line it matches. This pattern in particular is NR>1: that is, print whenever the line number is greater than 1, which happens to be line 2. This runs after the swapping of values has taken place, thus completing the task.

Answer 30

Japt, 15 bytes

£gY¶V?W:Y¶W?V:Y

Test it

---

Explanation

       :Implicit input of array U and integers V & W
£      :Map over the array (with Y being the index of the current element), replacing each element with...
g      :  The element in the array at index...
Y¶V?W  :    W, if Y equals V
:Y¶W?V :    or V, if Y equals W
:Y     :    or just Y otherwise
       :Implicit output of modified array

Answer 31

q/kdb+, 17 bytes

Solution:

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

Example:

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

Explanation:

A q version of the k answer by Simon. Apply the assign : function to x at indices reverse-y with value of x indexed at y. Broken down you can see more clearly:

q)x:1 2 3 4
q)y:0 1
q)x y
1 2
q)(|)y
1 0
q)x(|)y
2 1
q)@[x;(|)y;:;x y]
2 1 3 4

Answer 32

Powershell, 64 49 bytes

$f=$args;$t=$f[0];($t[$f[1]],$t[$f[2]]=$t[$f[2]],$t[$f[1]])>$;$t

param($a,$b,$c)($a[$b],$a[$c]=$a[$c],$a[$b])>$;$a

At least somewhat better solution.

Also I have no idea why redirecting to $ even works ^^

Answer 33

V, 10 bytes

ÀGYÀGVpVp

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This takes input with each number to a line, and indices as program arguments. 1-indexed.

Answer 34

Julia 0.5, 20 bytes

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

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Answer 35

C, 42 bytes (one extra argument on function call)

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

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C, 44 bytes

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