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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.

import "math/rand"
y := make([]T, k)
perm := rand.Perm(len(x))
for i, v := range perm[:k] {
	y[i] = x[v]
}
Types, Math
function RandArr(Max: Integer): TIntegerDynArray;
var
  i, j, temp: Integer;
begin
  SetLength(Result, Max+1);
  for i := Low(Result) to High(Result) do Result[i] := i;
  i := Length(Result);
  while i > 0 do
  begin
    Dec(i);
    j := RandomRange(0,i);
    temp := Result[i];
    Result[i] := Result[j];
    Result[j] := temp;
  end;
end;
  
var
  Idx: TIntegerDynArray;
begin
  Idx := RandArr(High(X));
  SetLength(Y, k);
  for i := 0 to k-1 do Y[i] := X[Idx];
end.
y = x.sample(k)

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