std::sample

 

C++

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Technical Specifications

 

Algorithm library

Constrained algorithms and algorithms on ranges(C++20)
Constrained algorithms: std::ranges::copy, std::ranges::sort, ...
Execution policies (C++17)
is_execution_policy (C++17)	execution::seqexecution::parexecution::par_unseqexecution::unseq (C++17)(C++17)(C++17)(C++20)	execution::sequenced_policyexecution::parallel_policyexecution::parallel_unsequenced_policyexecution::parallel_unsequenced (C++17)(C++17)(C++17)(C++20)
Non-modifying sequence operations
all_ofany_ofnone_of (C++11)(C++11)(C++11) for_each for_each_n (C++17)	countcount_if mismatch equal adjacent_find	findfind_iffind_if_not (C++11) find_end find_first_of	search search_n lexicographical_compare lexicographical_compare_three_way (C++20)
Modifying sequence operations
copycopy_if (C++11) copy_n (C++11) copy_backward move (C++11) move_backward (C++11) shift_leftshift_right (C++20)(C++20) transform	fill fill_n generate generate_n swap iter_swap swap_ranges sample (C++17)	removeremove_if replacereplace_if reverse rotate unique random_shuffle (until C++17)	remove_copyremove_copy_if replace_copyreplace_copy_if reverse_copy rotate_copy unique_copy shuffle (C++11)
Operations on uninitialized storage
uninitialized_copy uninitialized_move (C++17) uninitialized_fill destroy (C++17)	uninitialized_copy_n (C++11) uninitialized_move_n (C++17) uninitialized_fill_n destroy_n (C++17)	uninitialized_default_construct (C++17) uninitialized_value_construct (C++17) destroy_at (C++17)	uninitialized_default_construct_n (C++17) uninitialized_value_construct_n (C++17) construct_at (C++20)
Partitioning operations
is_partitioned (C++11) partition_point (C++11)	partition partition_copy (C++11)	stable_partition
Sorting operations
is_sorted (C++11) is_sorted_until (C++11)	sort stable_sort	partial_sort partial_sort_copy	nth_element
Binary search operations
lower_bound	upper_bound	binary_search	equal_range
Set operations (on sorted ranges)
merge inplace_merge	set_difference set_intersection	set_symmetric_difference set_union	includes
Heap operations
is_heap (C++11) is_heap_until (C++11)	make_heap sort_heap	push_heap pop_heap
Minimum/maximum operations
max max_element	min min_element	minmax (C++11) minmax_element (C++11)	clamp (C++17)
Permutations
is_permutation (C++11)	next_permutation	prev_permutation
Numeric operations
iota (C++11) inner_product adjacent_difference	accumulate reduce (C++17) transform_reduce (C++17)	partial_sum inclusive_scan (C++17) exclusive_scan (C++17)	transform_inclusive_scan (C++17) transform_exclusive_scan (C++17)
C library
qsort	bsearch

 

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

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.

Possible implementation

See the implementations in libstdc++, libc++ and msvc.

Example

Run this code

#include <iostream>#include <random>#include <string>#include <iterator>#include <algorithm>   int main(){std::string in ="hgfedcba", 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 hgfedcba: gfcba

See also

random_shuffleshuffle (until C++17)(C++11)

randomly re-orders elements in a range (function template)