Get the indices of an array after sorting

Question

Your challenge today is to write a program or function which takes a list l and gives the positions in l at which each successive element of l sorted appears.

In other words, output the index of the smallest value, followed by the index of the second smallest value, etc.

You can assume that the input array will contain only positive integers, and will contain at least one element.

Test cases:

Input | Output (1-indexed)
[7, 4, 5] | [2, 3, 1]
[1, 2, 3] | [1, 2, 3]
[2, 6, 1, 9, 1, 2, 3] | [3, 5, 1, 6, 7, 2, 4]
[4] | [1]

When two or more elements with the same value appear, their indices should appear next to each other from smallest to largest.

This is code-golf, fewest bytes wins!

• Cody Gray: -1 for a trivial challenge that can be solved with built-ins in common golfing languages, and for accepting an answer in less than 24 hours. This was neither a fair challenge, nor an interesting one.
• Adalynn: Well, I get why he accepted an answer within 24 hours, it's impossible to beat.
• Dada: @CodyGray I thought downvoting when I saw the 1-2 bytes answer, but actually, I don't think it's a bad challenge for more standard programming languages. Of course, it's not a hard challenge, but still, there is definitely some golfing possibilities. Of course, it's unpleasant to see 1-byte built-ins, but I don't think that it's fair to blame the challenge for that.
• JAD: Using a 1 character builtin is hardly practice. Easy doesn't necessarily mean solvable using only builtins.
• Mr. Xcoder: The best solution in such cases is to forget about te accept feature, which isn't really relevant anyway here.

Answer 1

Haskell, (削除) 43 (削除ここまで) 42 bytes

1-indexed:

import Data.List
map snd.sort.(`zip`[1..])

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-1 byte thanks to @ØrjanJohansen!

• Ørjan Johansen: Pointfree version saves one byte: map snd.sort.(`zip`[1..]).

Answer 2

Dyalog APL, 1 byte

⍋

Dyalog APL has a built in (削除) operator (削除ここまで) function (thank you Zacharý for clearing this up) to do this.

Example

⍋11 2 4 15
 2 3 1 4 
{⍵[⍋⍵]}11 4 2 15
 2 4 11 15

Here I'm indexing into the list by the sorted indices to return the list in ascending order.

• Adalynn: Oh, just to alert you to some confusing terminology, in APL, builtins like ⍋ are considered functions, while things like ¨⍨⍣.∘/\⌿⍀⌸⍤ are operators.
• Razetime: polyglot with BQN.

Answer 3

Jelly, 1 byte

Ụ

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• Erik the Outgolfer: Heh that was too obvious...
• Adalynn: APL deserved this one, +1 though for your speed.
• Adám: @Zacharý I'm sure Jelly picked this one up from J, which in turn inherited it from APL.

Answer 4

Python 2, 56 bytes

This solution is 0-indexed. This abuses the fact that sorted() creates a copy of the original list.

l=input()
for k in sorted(l):a=l.index(k);print a;l[a]=0

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• Erik the Outgolfer: Why did you ungolf this?
• Mr. Xcoder: @EriktheOutgolfer Fixed, Rollback.

Answer 5

Javascript (ES6), 39 bytes

-2 bytes thanks to @powelles

This only works in browsers where Array.prototype.sort is stable.

a=>[...a.keys()].sort((b,c)=>a[b]-a[c])

1-indexed version (47 bytes):

a=>a.map((_,i)=>i+1).sort((b,c)=>a[b-1]-a[c-1])

Example code snippet:

f=
a=>[...a.keys()].sort((b,c)=>a[b]-a[c])
console.log("7,4,5 => "+f([7,4,5]))
console.log("1,2,3 => "+f([1,2,3]))
console.log("2,6,1,9,1,2,3 => "+f([2,6,1,9,1,2,3]))
console.log("4 -> "+f([4]))

• powelles: [...a.keys()] instead of a.map((_,i)=>i) will save you a couple of bytes.

Answer 6

Python 2, 48 bytes

lambda x:sorted(range(len(x)),key=x.__getitem__)

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• Mr. Xcoder: Nice, I got outgolfed >_<. I switched my answer to Python 3 such that I don't feel that bad
• Neil: @Mr.Xcoder Well, that's his job...
• Erik the Outgolfer: @Mr.Xcoder Come on, you shouldn't feel bad for that! You made a full program, I made a function, and my approach is a bit different.
• Mr. Xcoder: I don't feel bad, I knew this will appear (I personally hate the __<blahblah>__ syntax). I will do some Jelly, I don't want to lose my training :)
• Erik the Outgolfer: @Mr.Xcoder Codegolf doesn't mean pretty syntax and formatting. ;)

Answer 7

Perl 6, (削除) 27 (削除ここまで) 21 bytes

*.pairs.sort(*.value)».key

Test it

->\x{sort {x[$_]},^x}

Test it

Inspired by a Python answer

Expanded:

-> # pointy block lambda
 \x # declare sigilless parameter
{
 sort
 { x[$_] }, # sort by the value in 「x」 at the given position
 ^x # Range up-to the number of elements in 「x」
}

Answer 8

Bash + coreutils, 20

nl|sort -nk2|cut -f1

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Answer 9

Swift 4, 82 bytes

func f(l:[Int]){var l=l;for k in l.sorted(){let a=l.index(of:k)!;print(a);l[a]=0}}

Test Suite.

Explanation

In Swift, l.sorted() creates a sorted copy of the original Array. We loop through the sorted elements in the list and after printing each item's index in the original Array with let a=l.index(of:k)!;print(a), and then, in order to keep the correct indexes in the Array, we assign l[a] to 0, because it does not affect our normal output.

---

Take note that this is 0-indexed, since it is a port of my Python solution. If you want it to be 1-indexed, replace print(a) with print(a+1) or Try it online!.

Answer 10

J, 2 bytes

/:

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Zero-based indexing.

• Cyoce: Was going to post this...

Answer 11

R, 5 bytes

There is a builtin function for this.

order

• JAD: Standard rules is to provide a program of function. order is already a function, so you don't have to handle input using scan(). This would be 5 bytes.
• gstats: rank() would save a byte
• djhurio: I am sure there was a rank answer by @JarkoDubbeldam, but I do not see it anymore.
• JAD: Correct, it does not follow the spec so I deleted it.

Answer 12

Ruby, 40 bytes

->a{a.zip([*1..a.size]).sort.map &:last}

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Answer 13

Octave, 17 bytes

@(i)[~,y]=sort(i)

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Octave is like MATLAB but with inline assignment, making things possible that gives the folks at Mathworks HQ headaches. It doesn't matter what you call y, but you can't do without that dummy variable, as far as I know.

Answer 14

Julia, 8 bytes

sortperm

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Answer 15

MATL, 2 bytes

&S

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Input and output are implicit.

Answer 16

MY, 3 bytes

MY also has a builtin for this!

⎕⍋↵

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How?

Evaluated input, grade up, then output with a newline.

Indexed however you set the index, with ⌶/0x48. (Can even be some weird integer like -1 or 2, the default is 1).

Answer 17

Java 8, 128 + 19 = 147 bytes

Based on Mr. Xcoder's solution. 0-based. Lambda takes input as an Integer[] and returns Integer[]. Byte count includes lambda expression and required import.

import java.util.*;

l->{Integer o[]=l.clone(),s[]=l.clone(),i=0;for(Arrays.sort(s);i<l.length;)l[o[i]=Arrays.asList(l).indexOf(s[i++])]=0;return o;}

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Ungolfed lambda

l -> {
 Integer
 o[] = l.clone(),
 s[] = l.clone(),
 i = 0
 ;
 for (Arrays.sort(s); i < l.length; )
 l[o[i] = Arrays.asList(l).indexOf(s[i++])] = 0;
 return o;
}

Notes

I use Integer[] instead of int[] to allow use of Arrays.asList, which has no primitive versions. Integer is preferred to Long because values are used as array indices and would require casting.

This ended up being shorter than my best procedural-style List solution because of the cost of class and method names.

This also beat a solution I tried that streamed the inputs, mapped to (value, index) pairs, sorted on values, and mapped to indices, mostly because of the baggage needed to collect the stream.

Acknowledgments

• -5 bytes thanks to Nevay

• Nevay: You don't need j: l->{Integer o[]=l.clone(),s[]=l.clone(),i=0;for(Arrays.sort(s);i<l.length;l[o[i]=Arrays.asList(l).indexOf(s[i++])]=0);return o;} (19+128 bytes).

Answer 18

Factor, 8 bytes

arg-sort

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0-indexed

Answer 19

Common Lisp, 82 bytes

(lambda(l)(loop as i in(sort(copy-seq l)'<)do(setf(elt l(print(position i l)))0)))

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Answer 20

Clojure, 39 bytes

#(map key(sort-by val(zipmap(range)%)))

• Brad Gilbert b2gills: Translated to Perl 6 {map *.key,(sort *.value,(0..* Z=> @_))}

Answer 21

CJam, 12 bytes

{ee{1=}0ドルf=}

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Answer 22

MATLAB / Octave, 29 bytes

[~,y]=sort(input(''));disp(y)

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• Sanchises: While your answer is perfect MATLAB, you can actually do inline assignment in anonymous functions in Octave.
• Luis Mendo: Good one! I knew about in-line assignment, but I didn't know you could output directly like that
• Sanchises: To be honest, me neither. I started with something like @(X)([~,y]=sort(X)), and while I was looking of a way to get y from this, I realized y was actually the return value from the assignment, and closer inspection revealed that brackets weren't even needed. MATLAB likes everything explicit; Octave is happy when it's unambiguous.

Answer 23

JavaScript (ES6), 69 bytes

0-indexed. Works for lists containing up to 65,536 elements.

a=>[...a=a.map((n,i)=>n<<16|i)].sort((a,b)=>a-b).map(n=>a.indexOf(n))

Test cases

let f =
a=>[...a=a.map((n,i)=>n<<16|i)].sort((a,b)=>a-b).map(n=>a.indexOf(n))
console.log(JSON.stringify(f([7, 4, 5])))
console.log(JSON.stringify(f([1, 2, 3])))
console.log(JSON.stringify(f([2, 6, 1, 9, 1, 2, 3])))
console.log(JSON.stringify(f([4])))

• Adalynn: Can you change n=>a.indexOf(n) to just a.indexOf?
• Arnauld: @Zacharý Unfortunately not. A method of an instanced object cannot be used as a callback.
• kamoroso94: @Zacharý Even worse is that Array#map passes 3 arguments to the callback function, and Array#indexOf expects 2, so it will give undesirable results.

Answer 24

Python 3, 52 bytes

0-indexed. Based on Bruce Forte's Haskell answer here and G B's Ruby answer here.

lambda l:list(zip(*sorted(zip(l,range(len(l))))))[1]

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Answer 25

Husk, (削除) 10 (削除ここまで) 7 bytes

This is a direct port of my Haskell answer, also 1-indexed:

m→O`z,N

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Ungolfed/Explained

Code Description Example
 -- implicit input [2,6,1]
 N -- natural numbers [1,2,3,..]
 `z, -- zip, but keep input left [(2,1),(6,2),(1,3)]
 O -- sort [(1,3),(2,1),(6,2)]
m→ -- keep only indices [3,1,2]

Answer 26

Java (OpenJDK 8), 72 bytes

l->l.stream().sorted().map(i->{int j=l.indexOf(i);l.set(j,0);return j;})

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Takes a List<Integer>, returns a Stream<Integer> containing the results.

We get a Stream based off the initial list, sort it, then map each number to it's index in the list. In order to accommodate duplicate elements, we set the original element in the list to 0.

Answer 27

SmileBASIC, 67 bytes

DEF I(A)DIM B[0]FOR I=1TO LEN(A)PUSH B,I
NEXT
SORT A,B
RETURN B
END

Very simple, all it does is generate a list of numbers from 1 to (length of array) and sort this by the same order as the input.

Answer 28

Python 3 with Numpy, (削除) 38 (削除ここまで) 26 bytes

12 bytes saved thanks to Jo King (no need to give the function a name)

import numpy
numpy.argsort

Output is 0-based.

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• Jo King: The function could just be numpy.argsort without the lambda part
• Luis Mendo: @JoKing Thanks for the suggestion. I wrote it that way because with just numpy.argsort;import numpy I get an error (numpy has not been imported yet), and with import numpy;numpy.argsort I need to move f= to the code part. Do you know that the standard procedure is in these cases? Move the f= and not count it?
• Jo King: Yeah, I guess. Maybe just redefine f=numpy.argsort in the footer
• Luis Mendo: @JoKing Good idea. Done. Thanks!

Answer 29

Vyxal, 1 byte

⇧

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0-indexed

Answer 30

Thunno 2, 1 byte

ƙ

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Same as the other golflangs - built-in for "grade up".