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 …
3y ago
[Jelly], 7 bytes œPżịẊ¥ …
3y ago
Japt, 17 bytes ô@VøYÃíU …
3y ago
APL(Dyalog Unicode), 13 byte …
3y ago
4 answers
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APL(Dyalog Unicode),
{⍵[?⍨≢⍵]}@⎕⊢⎕
A tradfn submission which takes both lists on STDIN, second input is 1-indexed.
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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]])
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
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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?
à :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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