std::sample

template< class PopulationIterator, class SampleIterator,
          class Distance, class URBG >
SampleIterator sample( PopulationIterator first, PopulationIterator last,
                       SampleIterator out, Distance n,
                       URBG&& g );

Selects n elements from the sequence [first, last) (without replacement) such that each possible sample has equal probability of appearance, and writes those selected elements into the output iterator out. Random numbers are generated using the random number generator g.

If n is greater than the number of elements in the sequence, selects last - first elements.

The algorithm is stable (preserves the relative order of the selected elements) only if PopulationIterator meets the requirements of LegacyForwardIterator.

The behavior is undefined if out is in [first, last).

Parameters

Return value

Returns a copy of out after the last sample that was output, that is, end of the sample range.

Complexity

Linear in std::distance(first, last).

Notes

This function may implement selection sampling or reservoir sampling.

Possible implementation

See the implementations in libstdc++, libc++ and MSVC STL.

Example

#include <algorithm>
#include <iostream>
#include <iterator>
#include <random>
#include <string>
 
int main()
{
    std::string in {"ABCDEFGHIJK"}, out;
    std::sample(in.begin(), in.end(), std::back_inserter(out), 4,
                std::mt19937 {std::random_device{}()});
    std::cout << "Four random letters out of " << in << " : " << out << '\n';
}

Four random letters out of ABCDEFGHIJK: EFGK

See also