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).

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` |

Returns a copy of `out`

after the last sample that was output, that is, end of the sample range.

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

.

This function may implement selection sampling or reservoir sampling.

Feature-test macro |
---|

`__cpp_lib_sample` |

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

#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

(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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