NAME¶

std::sample - std::sample

Synopsis¶

Defined in header <algorithm>
template< class PopulationIt, class SampleIt, class Distance, class
URBG >
(since C++17)
SampleIterator sample( PopulationIt first, PopulationIt last,

SampleIt 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 all elements in
the sequence.

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

If
the value type of first
(until C++20)
*first
(since C++20) is not writable to out, the program is ill-formed.

If any of the following conditions is satisfied, the behavior is undefined:

* out is in [first, last).
* PopulationIt does not meet the requirements of LegacyInputIterator.
* SampleIt does not meet the requirements of LegacyOutputIterator.
* All following conditions are satisfied:

* PopulationIt does not meet the requirements of (until C++23)
LegacyForwardIterator.
* PopulationIt does not model forward_iterator. (since C++23)

* SampleIt does not meet the requirements of LegacyRandomAccessIterator.
* Given the type T as std::remove_reference_t<URBG>, any of the following
conditions is satisfied:

* T does not meet the requirements of UniformRandomBitGenerator.

* The return type of T is not convertible to Distance. (until C++20)

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¶

-
Distance must be an integer type.

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).

Possible implementation¶

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

Notes¶

This function may implement selection sampling or reservoir sampling.

Feature-test macro Value Std Feature
__cpp_lib_sample 201603L (C++17) std::sample

Example¶

// Run this code

#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';
}

Possible output:¶

Four random letters out of ABCDEFGHIJK: EFGK

See also¶

random_shuffle
shuffle randomly re-orders elements in a range
(until C++17) (function template)
(C++11)
ranges::sample selects N random elements from a sequence
(C++20) (niebloid)