std::ranges::search_n - cppreference.com (original) (raw)

| Defined in header | | | | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ | | --------------------------- | | Call signature | | | | (1) | | | | template< std::forward_iterator I, std::sentinel_for<I> S, class T, class Pred = ranges::equal_to, class Proj = std::identity >requires std::indirectly_comparable<I, const T*, Pred, Proj> constexpr ranges::subrange<I> search_n( I first, S last, std::iter_difference_t<I> count, const T& value, Pred pred = {}, Proj proj = {} ); | | (since C++20) (until C++26) | | 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> search_n( I first, S last, std::iter_difference_t<I> count, const T& value, Pred pred = {}, Proj proj = {} ); | | (since C++26) | | (2) | | | | template< ranges::forward_range R, class T, class Pred = ranges::equal_to, class Proj = std::identity >requires std::indirectly_comparable <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 = {} ); | | (since C++20) (until C++26) | | 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> search_n( R&& r, ranges::range_difference_t<R> count, const T& value, Pred pred = {}, Proj proj = {} ); | | (since C++26) |

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 algorithm function objects (informally known as 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.

Contents

• 1 Parameters
• 2 Return value
• 3 Complexity
• 4 Notes
• 5 Possible implementation
• 6 Example
• 7 See also

[edit] Parameters

[edit] 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}.

[edit] Complexity

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

[edit] Notes

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

[edit] Possible implementation

struct search_n_fn { template<std::forward_iterator I, std::sentinel_for 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 operator()(I first, S last, std::iter_difference_t 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 n{1}; for (;;) { if (n++ == count) return {start, std::next(first)}; // found if (++first == last) return {first, first}; // not found if ((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, Proj>> requires std::indirectly_comparable<ranges::iterator_t, const T, Pred, Proj> constexpr ranges::borrowed_subrange_t operator()(R&& r, ranges::range_difference_t 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 {};

[edit] Example

#include #include #include #include #include #include #include #include   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(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> 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{4, 2}); #endif assert(it.size() == 2); }

Output:

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

[edit] See also