This language bar is your friend. Select your favorite languages!
Select your favorite languages :
  • Or search :

Idiom #158 Random sublist

Create a new list y from randomly picking exactly k elements from list x.

It is assumed that x has at least k elements.
Each element must have same probability to be picked.
Each element from x must be picked at most once.
Explain if the original ordering is preserved or not.

(def y (->> x shuffle (take k)))
using System.Collections.Generic;
Random rnd = new Random();
List<int> y = x.OrderBy(r => rnd.Next()).Take(k).ToList();
import std.random;
auto y = randomSample(x, k);
  allocate (sample(k))
  do i=1,k
     sample(i) = x(i)
  end do
  do i=k+1,n
    call random_number(a)
    j = 1 + int(i*a)
    if (j .le. k) sample(j) = x(i)
  end do
import "math/rand"
y := make([]T, k)
perm := rand.Perm(len(x))
for i, v := range perm[:k] {
	y[i] = x[v]
import System.Random (randomRIO)
randomSample :: Int -> [a] -> IO [a]
randomSample 0 x = pure []
randomSample k x = do
   i <- randomRIO (0, length x - 1)
   let (a, e:b) = splitAt i x
   l <- randomSample (k-1) (a ++ b)
   pure (e : l)
const idx = x.map((item, i) => i);
while (y.length < k) {
  const i = parseInt(Math.random() * idx.length, 10);
  idx.splice(i, 1);
$y = array_intersect_key($x, array_flip(array_rand($x, $k)));
uses Types, Math;
function RandArr(Max: Integer): TIntegerDynArray;
  i, j, temp: Integer;
  SetLength(Result, Max+1);
  for i := Low(Result) to High(Result) do Result[i] := i;
  i := Length(Result);
  while i > 0 do
    j := RandomRange(0,i);
    temp := Result[i];
    Result[i] := Result[j];
    Result[j] := temp;
  Idx: TIntegerDynArray;
  Idx := RandArr(High(X));
  SetLength(Y, k);
  for i := 0 to k-1 do Y[i] := X[Idx];
use List::Util qw(shuffle head);
my @y = head $k, shuffle @x;
import random
y = random.sample(x, k)
y = x.sample(k)
use rand::prelude::*;
let mut rng = &mut rand::thread_rng();
let y = x.choose_multiple(&mut rng, k).cloned().collect::<Vec<_>>();
y := x shuffled first: k.

New implementation...