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Challenges

# Shuffle a subset of a list

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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.

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# Jelly, 7 bytes

œPżịẊ¥F


Try it online!

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
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# 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]])


Try it online!

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

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# Japt, 17 bytes

ô@VøYÃíUgV öx)c f


Try it

# Japt, 17 bytes

ô@VøYÃíUgV öx)c f


Try it

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
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# APL(Dyalog Unicode), 13 bytes SBCS

{⍵[?⍨≢⍵]}@⎕⊢⎕


Try it on APLgolf!

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

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