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[alg.random.sample]
# 26 Algorithms library [[algorithms]](./#algorithms)
## 26.7 Mutating sequence operations [[alg.modifying.operations]](alg.modifying.operations#alg.random.sample)
### 26.7.12 Sample [alg.random.sample]
[🔗](#lib:sample)
`template<class PopulationIterator, class SampleIterator,
class Distance, class UniformRandomBitGenerator>
SampleIterator sample(PopulationIterator first, PopulationIterator last,
SampleIterator out, Distance n,
UniformRandomBitGenerator&& g);
template<[input_iterator](iterator.concept.input#concept:input_iterator "24.3.4.9Concept input_­iterator[iterator.concept.input]") I, [sentinel_for](iterator.concept.sentinel#concept:sentinel_for "24.3.4.7Concept sentinel_­for[iterator.concept.sentinel]")<I> S, [weakly_incrementable](iterator.concept.winc#concept:weakly_incrementable "24.3.4.4Concept weakly_­incrementable[iterator.concept.winc]") O, class Gen>
requires ([forward_iterator](iterator.concept.forward#concept:forward_iterator "24.3.4.11Concept forward_­iterator[iterator.concept.forward]")<I> || [random_access_iterator](iterator.concept.random.access#concept:random_access_iterator "24.3.4.13Concept random_­access_­iterator[iterator.concept.random.access]")<O>) &&
[indirectly_copyable](alg.req.ind.copy#concept:indirectly_copyable "24.3.7.3Concept indirectly_­copyable[alg.req.ind.copy]")<I, O> &&
[uniform_random_bit_generator](rand.req.urng#concept:uniform_random_bit_generator "29.5.3.3Uniform random bit generator requirements[rand.req.urng]")<remove_reference_t<Gen>>
O ranges::sample(I first, S last, O out, iter_difference_t<I> n, Gen&& g);
template<[input_range](range.refinements#concept:input_range "25.4.6Other range refinements[range.refinements]") R, [weakly_incrementable](iterator.concept.winc#concept:weakly_incrementable "24.3.4.4Concept weakly_­incrementable[iterator.concept.winc]") O, class Gen>
requires ([forward_range](range.refinements#concept:forward_range "25.4.6Other range refinements[range.refinements]")<R> || [random_access_iterator](iterator.concept.random.access#concept:random_access_iterator "24.3.4.13Concept random_­access_­iterator[iterator.concept.random.access]")<O>) &&
[indirectly_copyable](alg.req.ind.copy#concept:indirectly_copyable "24.3.7.3Concept indirectly_­copyable[alg.req.ind.copy]")<iterator_t<R>, O> &&
[uniform_random_bit_generator](rand.req.urng#concept:uniform_random_bit_generator "29.5.3.3Uniform random bit generator requirements[rand.req.urng]")<remove_reference_t<Gen>>
O ranges::sample(R&& r, O out, range_difference_t<R> n, Gen&& g);
`
[1](#1)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/algorithms.tex#L8410)
*Mandates*: For the overload in namespace std,Distance is an integer type and*first is writable ([[iterator.requirements.general]](iterator.requirements.general "24.3.1General")) to out[.](#1.sentence-1)
[2](#2)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/algorithms.tex#L8416)
*Preconditions*: out is not in the range [first, last)[.](#2.sentence-1)
For the overload in namespace std:
- [(2.1)](#2.1)
PopulationIterator meets
the [*Cpp17InputIterator*](input.iterators#:Cpp17InputIterator "24.3.5.3Input iterators[input.iterators]") requirements ([[input.iterators]](input.iterators "24.3.5.3Input iterators"))[.](#2.1.sentence-1)
- [(2.2)](#2.2)
SampleIterator meets
the [*Cpp17OutputIterator*](output.iterators#:Cpp17OutputIterator "24.3.5.4Output iterators[output.iterators]") requirements ([[output.iterators]](output.iterators "24.3.5.4Output iterators"))[.](#2.2.sentence-1)
- [(2.3)](#2.3)
SampleIterator meets
the [*Cpp17RandomAccessIterator*](random.access.iterators#:Cpp17RandomAccessIterator "24.3.5.7Random access iterators[random.access.iterators]") requirements ([[random.access.iterators]](random.access.iterators "24.3.5.7Random access iterators"))
unless PopulationIterator models [forward_iterator](iterator.concept.forward#concept:forward_iterator "24.3.4.11Concept forward_­iterator[iterator.concept.forward]") ([[iterator.concept.forward]](iterator.concept.forward "24.3.4.11Concept forward_­iterator"))[.](#2.3.sentence-1)
- [(2.4)](#2.4)
remove_reference_t<UniformRandomBitGenerator> meets
the requirements of a uniform random bit generator type ([[rand.req.urng]](rand.req.urng "29.5.3.3Uniform random bit generator requirements"))[.](#2.4.sentence-1)
[3](#3)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/algorithms.tex#L8437)
*Effects*: Copies min(last - first, n) elements (the [*sample*](#def:sample "26.7.12Sample[alg.random.sample]"))
from [first, last) (the [*population*](#def:population "26.7.12Sample[alg.random.sample]")) to out such that each possible sample has equal probability of appearance[.](#3.sentence-1)
[*Note [1](#note-1)*:
Algorithms that obtain such effects include [*selection sampling*](#def:selection_sampling) and [*reservoir sampling*](#def:reservoir_sampling)[.](#3.sentence-2)
— *end note*]
[4](#4)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/algorithms.tex#L8447)
*Returns*: The end of the resulting sample range[.](#4.sentence-1)
[5](#5)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/algorithms.tex#L8451)
*Complexity*: O(last - first)[.](#5.sentence-1)
[6](#6)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/algorithms.tex#L8455)
*Remarks*:
- [(6.1)](#6.1)
For the overload in namespace std,
stable if and only if PopulationIterator models [forward_iterator](iterator.concept.forward#concept:forward_iterator "24.3.4.11Concept forward_­iterator[iterator.concept.forward]")[.](#6.1.sentence-1)
For the first overload in namespace ranges,
stable if and only if I models [forward_iterator](iterator.concept.forward#concept:forward_iterator "24.3.4.11Concept forward_­iterator[iterator.concept.forward]")[.](#6.1.sentence-2)
- [(6.2)](#6.2)
To the extent that the implementation of this function makes use
of random numbers, the object g serves as
the implementation's source of randomness[.](#6.2.sentence-1)

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[alg.random.shuffle]
# 26 Algorithms library [[algorithms]](./#algorithms)
## 26.7 Mutating sequence operations [[alg.modifying.operations]](alg.modifying.operations#alg.random.shuffle)
### 26.7.13 Shuffle [alg.random.shuffle]
[🔗](#lib:shuffle)
`template<class RandomAccessIterator, class UniformRandomBitGenerator>
void shuffle(RandomAccessIterator first,
RandomAccessIterator last,
UniformRandomBitGenerator&& g);
template<[random_access_iterator](iterator.concept.random.access#concept:random_access_iterator "24.3.4.13Concept random_­access_­iterator[iterator.concept.random.access]") I, [sentinel_for](iterator.concept.sentinel#concept:sentinel_for "24.3.4.7Concept sentinel_­for[iterator.concept.sentinel]")<I> S, class Gen>
requires [permutable](alg.req.permutable#concept:permutable "24.3.7.6Concept permutable[alg.req.permutable]")<I> &&
[uniform_random_bit_generator](rand.req.urng#concept:uniform_random_bit_generator "29.5.3.3Uniform random bit generator requirements[rand.req.urng]")<remove_reference_t<Gen>>
I ranges::shuffle(I first, S last, Gen&& g);
template<[random_access_range](range.refinements#concept:random_access_range "25.4.6Other range refinements[range.refinements]") R, class Gen>
requires [permutable](alg.req.permutable#concept:permutable "24.3.7.6Concept permutable[alg.req.permutable]")<iterator_t<R>> &&
[uniform_random_bit_generator](rand.req.urng#concept:uniform_random_bit_generator "29.5.3.3Uniform random bit generator requirements[rand.req.urng]")<remove_reference_t<Gen>>
borrowed_iterator_t<R> ranges::shuffle(R&& r, Gen&& g);
`
[1](#1)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/algorithms.tex#L8491)
*Preconditions*: For the overload in namespace std:
- [(1.1)](#1.1)
RandomAccessIterator meets
the [*Cpp17ValueSwappable*](swappable.requirements#:Cpp17ValueSwappable "16.4.4.3Swappable requirements[swappable.requirements]") requirements ([[swappable.requirements]](swappable.requirements "16.4.4.3Swappable requirements"))[.](#1.1.sentence-1)
- [(1.2)](#1.2)
The type remove_reference_t<UniformRandomBitGenerator> meets
the uniform random bit generator ([[rand.req.urng]](rand.req.urng "29.5.3.3Uniform random bit generator requirements")) requirements[.](#1.2.sentence-1)
[2](#2)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/algorithms.tex#L8503)
*Effects*: Permutes the elements in the range [first, last)
such that each possible permutation of those elements
has equal probability of appearance[.](#2.sentence-1)
[3](#3)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/algorithms.tex#L8509)
*Returns*: last for the overloads in namespace ranges[.](#3.sentence-1)
[4](#4)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/algorithms.tex#L8513)
*Complexity*: Exactly (last - first) - 1 swaps[.](#4.sentence-1)
[5](#5)
[#](http://github.com/Eelis/draft/tree/9adde4bc1c62ec234483e63ea3b70a59724c745a/source/algorithms.tex#L8517)
*Remarks*: To the extent that the implementation of this function makes use
of random numbers, the object referenced by g shall serve as
the implementation's source of randomness[.](#5.sentence-1)