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std::sample

Defined in header <algorithm>
template< class PopulationIterator, class SampleIterator,
          class Distance, class URBG >
SampleIterator sample( PopulationIterator first, PopulationIterator last,
                       SampleIterator out, Distance n, 
                       URBG&& g );
(since C++17)

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

first, last - pair of iterators forming the range from which to make the sampling (the population)
out - the output iterator where the samples are written
n - number of samples to make
g - the random number generator used as the source of randomness
Type requirements
-PopulationIterator must meet the requirements of LegacyInputIterator.
-SampleIterator must meet the requirements of LegacyOutputIterator.
-SampleIterator must also meet the requirements of LegacyRandomAccessIterator if PopulationIterator doesn't meet LegacyForwardIterator
-PopulationIterator's value type must be writeable to out
-Distance must be an integer type
-std::remove_reference_t<URBG> must meet the requirements of UniformRandomBitGenerator and its return type must be convertible to Distance

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.

Feature-test macro
__cpp_lib_sample

Possible implementation

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

Example

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

Possible output:

five random letters out of hgfedcba: gfcba

See also

(until C++17)(C++11)
randomly re-orders elements in a range
(function template)
(C++20)
selects n random elements from a sequence
(niebloid)

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