# Shuffle a subset of a list

^{Idea shamelessly stolen from caird and rak1507}

Shuffle a subset of a list of unique, positive integers with uniform randomness, given the indices of that subset. For example, given the list $[A, B, C, D, E, F, G, H]$ and the indices $[0, 3, 4, 5, 7]$ (0-indexed), you would extract the list $[A, D, E, F, H]$, shuffle it, and insert the shuffled elements back according to the list of indices. Some possible results of this are (elements that stayed in place are bolded) $$[H, \textbf{B}, \textbf{C}, D, A, E, \textbf{G}, F]$$ $$[A, \textbf{B}, \textbf{C}, D, E, F, \textbf{G}, H]$$ $$[F, \textbf{B}, \textbf{C}, E, A, H, \textbf{G}, D]$$.

## Rules

- Every possible rearrangement of the list should have a non-zero chance of being chosen.
- You may use zero- or one-indexing, but please specify which you use.
- The indices in the input are guaranteed to be unique and valid.
- This is code golf, so least number of bytes wins.

## Test cases

All of these examples use 0-indexing.

```
List
Indices
Possible output
[1,2,3,4,5,6,7]
[0,1,2,3,4,5,6]
[2,3,6,1,0,4,5], etc.
[1,2,3,4,5,6,7]
[0,2,4,6]
[3,2,7,4,5,6,1], etc.
[93,6,10,1,200,41,78,31,34,27]
[0,3,4,8,9]
[1,6,10,27,93,41,78,31,200,34], etc.
```

[JavaScript (Node.js)], 75 68 …

~1mo ago

[Jelly], 7 bytes œPżịẊ¥ …

~1mo ago

Japt, 17 bytes ô@VøYÃíU …

25d ago

APL(Dyalog Unicode), 13 byte …

~1mo ago

## 4 answers

#
JavaScript (Node.js), ~~75~~ 68 bytes

-7 bytes thanks to Hakerh400

```
(a,b)=>b.map(i=>[a[i],a[j]]=[a[j=b[Math.random()*b.length|0]],a[i]])
```

Basic random swap algorithm. Unfortunately, JavaScript doesn't have a built in shuffle function.

# Jelly, 7 bytes

```
œPżịẊ¥F
```

Takes the 1-indexed indices on the left and the list on the right

## How it works

```
œPżịẊ¥F - Main link. Takes I on the left and L on the right
œP - Partition L after the indices in I, removing the borders
¥ - Group the previous 2 links as a dyad f(I, L):
ị - Get the elements of L at the indices in I
Ẋ - Shuffle uniformly
ż - Zip
F - Flatten
```

#### 0 comment threads

# Japt, 17 bytes

```
ô@VøYÃíUgV öx)c f
```

# Japt, 17 bytes

```
ô@VøYÃíUgV öx)c f
```

```
ô@VøYÃíUgV öx)c f :Implicit input of arrays U=integers & V=indices
ô :Split U on elements ...
@ :That return true when their indices are passed through the following function as Y
VøY : Does V contain Y?
Ã :End split
í :Interleave with
UgV : Elements in U at indices in V
öx : Random permutation
) :End interleave
c :Flatten
f :Filter
```

#### 0 comment threads

#
APL(Dyalog Unicode), ^{}13 bytes ^{SBCS}

```
{⍵[?⍨≢⍵]}@⎕⊢⎕
```

A tradfn submission which takes both lists on STDIN, second input is 1-indexed.

## 0 comment threads