# 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`)` (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` does not meet LegacyForwardIterator
- `PopulationIterator`'s value type must be writable 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 | Value | Std | Comment
--- | --- | --- | ---
`__cpp_lib_sample` | 201603L | (C++17) | `std::sample`

### Possible implementation

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

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

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