# std::sample

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< cpp‎ | algorithm

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) 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

### 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. Must not be in the [first;last) range
n | - | number of samples to make
g | - | the random number generator used as the source of randomness
- `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.

### Example

Run this code

    #include <iostream>
    #include <random>
    #include <string>
    #include <iterator>
    #include <algorithm>

    int main()
    {
        std::string in = "abcdefgh", 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 abcdefgh : adfgh

### See also

random_shuffle shuffle (until C++17)(C++11) | randomly re-orders elements in a range (function template)

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