NAME¶

std::ranges::search_n - std::ranges::search_n

Synopsis¶

Defined in header <algorithm>
Call signature
template< std::forward_iterator I, std::sentinel_for<I> S,
class T,

class Pred = ranges::equal_to, class Proj =
std::identity > (since
requires std::indirectly_comparable<I, const T*, Pred, Proj> C++20)
constexpr ranges::subrange<I> (until
search_n( I first, S last, std::iter_difference_t<I> C++26)
count,

const T& value, Pred pred = {}, Proj proj = {}
);
template< std::forward_iterator I, std::sentinel_for<I> S,

class Pred = ranges::equal_to, class Proj =
std::identity,
class T = std::projected_value_t<I, Proj> >
requires std::indirectly_comparable<I, const T*, Pred, Proj> (since
constexpr ranges::subrange<I> C++26)
search_n( I first, S last, std::iter_difference_t<I>
count,

const T& value, Pred pred = {}, Proj proj = {}
); (1)
template< ranges::forward_range R, class T,

class Pred = ranges::equal_to, class Proj =
std::identity > (since
requires std::indirectly_comparable C++20)
<ranges::iterator_t<R>, const T*, Pred, Proj> (until
constexpr ranges::borrowed_subrange_t<R> C++26)
search_n( R&& r, ranges::range_difference_t<R> count,

const T& value, Pred pred = {}, Proj proj = {}
);
template< ranges::forward_range R, (2)

class Pred = ranges::equal_to, class Proj =
std::identity,
class T =
std::projected_value_t<ranges::iterator_t<R>, Proj> > (since
requires std::indirectly_comparable C++26)
<ranges::iterator_t<R>, const T*, Pred, Proj>
constexpr ranges::borrowed_subrange_t<R>
search_n( R&& r, ranges::range_difference_t<R> count,

const T& value, Pred pred = {}, Proj proj = {}
);

1) Searches the range [first, last) for the first sequence of count elements whose
projected values are each equal to the given value according to the binary predicate
pred.
2) Same as (1), but uses r as the source range, as if using ranges::begin(r) as
first and ranges::end(r) as last.

The function-like entities described on this page are niebloids, that is:

* Explicit template argument lists cannot be specified when calling any of them.
* None of them are visible to argument-dependent lookup.
* When any of them are found by normal unqualified lookup as the name to the left
of the function-call operator, argument-dependent lookup is inhibited.

In practice, they may be implemented as function objects, or with special compiler
extensions.

Parameters¶

first, last - the range of elements to examine (aka haystack)
r - the range of elements to examine (aka haystack)
count - the length of the sequence to search for
value - the value to search for (aka needle)
pred - the binary predicate that compares the projected elements with value
proj - the projection to apply to the elements of the range to examine

Return value¶

1) Returns std::ranges::subrange object that contains a pair of iterators in the
range [first, last) that designate the found subsequence.

If no such subsequence is found, returns std::ranges::subrange{last, last}.

If count <= 0, returns std::ranges::subrange{first, first}.
2) Same as (1) but the return type is ranges::borrowed_subrange_t<R>.

Complexity¶

Linear: at most ranges::distance(first, last) applications of the predicate and the
projection.

Notes¶

An implementation can improve efficiency of the search in average if the iterators
model std::random_access_iterator.

Feature-test macro Value Std Feature
__cpp_lib_algorithm_default_value_type 202403 (C++26) List-initialization for
algorithms

Possible implementation¶

struct search_n_fn
{
template<std::forward_iterator I, std::sentinel_for<I> S,
class Pred = ranges::equal_to, class Proj = std::identity,
class T = std::projected_value_t<I, Proj>>
requires std::indirectly_comparable<I, const T*, Pred, Proj>
constexpr ranges::subrange<I>
operator()(I first, S last, std::iter_difference_t<I> count,
const T& value, Pred pred = {}, Proj proj = {}) const
{
if (count <= 0)
return {first, first};
for (; first != last; ++first)
if (std::invoke(pred, std::invoke(proj, *first), value))
{
I start = first;
std::iter_difference_t<I> n{1};
for (;;)
{
if (n++ == count)
return {start, std::next(first)}; // found
if (++first == last)
return {first, first}; // not found
if (!std::invoke(pred, std::invoke(proj, *first), value))
break; // not equ to value
}
}
return {first, first};
}

template<ranges::forward_range R,
class Pred = ranges::equal_to, class Proj = std::identity,
class T = std::projected_value_t<ranges::iterator_t<R>, Proj>>
requires std::indirectly_comparable<ranges::iterator_t<R>, const T*, Pred, Proj>
constexpr ranges::borrowed_subrange_t<R>
operator()(R&& r, ranges::range_difference_t<R> count,
const T& value, Pred pred = {}, Proj proj = {}) const
{
return (*this)(ranges::begin(r), ranges::end(r),
std::move(count), value,
std::move(pred), std::move(proj));
}
};

inline constexpr search_n_fn search_n {};

Example¶

// Run this code

#include <algorithm>
#include <cassert>
#include <complex>
#include <iomanip>
#include <iostream>
#include <iterator>
#include <string>
#include <vector>

int main()
{
namespace ranges = std::ranges;

static constexpr auto nums = {1, 2, 2, 3, 4, 1, 2, 2, 2, 1};
constexpr int count{3};
constexpr int value{2};
typedef int count_t, value_t;

constexpr auto result1 = ranges::search_n
(
nums.begin(), nums.end(), count, value
);
static_assert // found
(
result1.size() == count &&
std::distance(nums.begin(), result1.begin()) == 6 &&
std::distance(nums.begin(), result1.end()) == 9
);

constexpr auto result2 = ranges::search_n(nums, count, value);
static_assert // found
(
result2.size() == count &&
std::distance(nums.begin(), result2.begin()) == 6 &&
std::distance(nums.begin(), result2.end()) == 9
);

constexpr auto result3 = ranges::search_n(nums, count, value_t{5});
static_assert // not found
(
result3.size() == 0 &&
result3.begin() == result3.end() &&
result3.end() == nums.end()
);

constexpr auto result4 = ranges::search_n(nums, count_t{0}, value_t{1});
static_assert // not found
(
result4.size() == 0 &&
result4.begin() == result4.end() &&
result4.end() == nums.begin()
);

constexpr char symbol{'B'};
auto to_ascii = [](const int z) -> char { return 'A' + z - 1; };
auto is_equ = [](const char x, const char y) { return x == y; };

std::cout << "Find a sub-sequence " << std::string(count, symbol) << " in the ";
std::ranges::transform(nums, std::ostream_iterator<char>(std::cout, ""), to_ascii);
std::cout << '\n';

auto result5 = ranges::search_n(nums, count, symbol, is_equ, to_ascii);
if (not result5.empty())
std::cout << "Found at position "
<< ranges::distance(nums.begin(), result5.begin()) << '\n';

std::vector<std::complex<double>> nums2{{4, 2}, {4, 2}, {1, 3}};
#ifdef __cpp_lib_algorithm_default_value_type
auto it = ranges::search_n(nums2, 2, {4, 2});
#else
auto it = ranges::search_n(nums2, 2, std::complex<double>{4, 2});
#endif
assert(it.size() == 2);
}

Output:¶

Find a sub-sequence BBB in the ABBCDABBBA
Found at position 6

See also¶

ranges::adjacent_find finds the first two adjacent items that are equal (or satisfy
(C++20) a given predicate)
(niebloid)
ranges::find
ranges::find_if
ranges::find_if_not finds the first element satisfying specific criteria
(C++20) (niebloid)
(C++20)
(C++20)
ranges::find_end finds the last sequence of elements in a certain range
(C++20) (niebloid)
ranges::find_first_of searches for any one of a set of elements
(C++20) (niebloid)
ranges::includes returns true if one sequence is a subsequence of another
(C++20) (niebloid)
ranges::mismatch finds the first position where two ranges differ
(C++20) (niebloid)
ranges::search searches for a range of elements
(C++20) (niebloid)
searches a range for a number of consecutive copies of an
search_n element
(function template)