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std::sample

From cppreference.com
< 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
(C++17)
Search operations
(C++11)                (C++11)(C++11)

Modifying sequence operations Copy operations
(C++11)
(C++11)
Swap operations Transformation operations Generation operations Removing operations Order-changing operations
(until C++17)(C++11)
(C++20)(C++20)
Sampling operations
sample
(C++17)

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
(C++11)
Minimum/maximum operations
(C++11)
(C++17)
Lexicographical comparison operations Permutation operations


C library
Numeric operations
 
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 [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:
(until C++23)
(since C++23)
  • The return type of T is not convertible to Distance.
(until C++20)

Contents

[edit] Parameters

first, last - the pair of iterators defining the range of elements 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

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

[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
(algorithm function object)[edit]

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