std::sample

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

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

Sorting and related operations
Partitioning operations
(哋它亢++11)
(哋它亢++11)

Sorting operations
(哋它亢++11)
(哋它亢++11)

Binary search operations
(on partitioned ranges)
Set operations (on sorted ranges)
Merge operations (on sorted ranges)
Heap operations
(哋它亢++11)
(哋它亢++11)
Minimum/maximum operations
(哋它亢++11)
(哋它亢++17)
Lexicographical comparison operations
(哋它亢++20)
Permutation operations
(哋它亢++11)


C library
Numeric operations
(哋它亢++17)
(哋它亢++17)
(哋它亢++17)   
(哋它亢++17)
(哋它亢++17)
(哋它亢++17)

Operations on uninitialized memory
(哋它亢++17)
(哋它亢++17)
(哋它亢++17)
(哋它亢++20)      
 
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 哋它亢++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 哋它亢++20)*first(since 哋它亢++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 哋它亢++23)
 (since 哋它亢++23)
  • The return type of T is not convertible to Distance.
(until 哋它亢++20)

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.

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).

Possible implementation

See the implementations in libstd哋它亢++, lib哋它亢++ and MSVC STL.

Notes

This function may implement selection sampling or reservoir sampling.

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

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

(until 哋它亢++17)(哋它亢++11)
 randomly re-orders elements in a range
(function template)
(哋它亢++20)
 selects N random elements from a sequence
(niebloid)
Retrieved from "https://en.cppreference.com/mwiki/index.php?title=cpp/algorithm/sample&oldid=170521"