< cpp‎ | algorithm
Algorithm library
Constrained algorithms and algorithms on ranges (C++20)
Constrained algorithms, e.g. ranges::copy, ranges::sort, ...
Execution policies (C++17)
Non-modifying sequence operations
Batch operations
Search operations
(C++11)                (C++11)(C++11)

Modifying sequence operations
Copy operations
Swap operations
Transformation operations
Generation operations
Removing operations
Order-changing operations
(until C++17)(C++11)
Sampling operations

Sorting and related operations
Partitioning operations
Sorting operations
Binary search operations
(on partitioned ranges)
Set operations (on sorted ranges)
Merge operations (on sorted ranges)
Heap operations
Minimum/maximum operations
Lexicographical comparison operations
Permutation operations
C library
Numeric operations
Operations on uninitialized memory
Defined in header <algorithm>
template< class PopulationIt, class SampleIt, class Distance, class URBG >

SampleIterator sample( PopulationIt first, PopulationIt last,

                       SampleIt out, Distance n, URBG&& g );
(since C++17)

Selects n elements from the sequence [firstlast) (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 [firstlast).
  • PopulationIt does not meet the requirements of LegacyInputIterator.
  • SampleIt does not meet the requirements of LegacyOutputIterator.
  • All following conditions are satisfied:
(until C++23)
(since C++23)
  • The return type of T is not convertible to Distance.
(until C++20)


[edit] 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.

[edit] Return value

Returns a copy of out after the last sample that was output, that is, end of the sample range.

[edit] Complexity

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

[edit] Possible implementation

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

[edit] Notes

This function may implement selection sampling or reservoir sampling.

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

[edit] Example

#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

[edit] See also

(until C++17)(C++11)
randomly re-orders elements in a range
(function template) [edit]
selects N random elements from a sequence