std::ranges::search_n
From cppreference.com
| Defined in header <algorithm>
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| Call signature |
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| template< std::forward_iterator I, std::sentinel_for<I> S, class T, class Pred = ranges::equal_to, class Proj = std::identity > |
(1) | (since C++20) |
| template< ranges::forward_range R, class T, class Pred = ranges::equal_to, class Proj = std::identity > |
(2) | (since C++20) |
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 may not be specified when calling any of them.
- None of them is visible to argument-dependent lookup.
- When one of them is found by normal unqualified lookup for the name to the left of the function-call operator, it inhibits argument-dependent lookup.
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
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| 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}.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.
Possible implementation
struct search_n_fn { 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> 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 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> 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 <iomanip> #include <iostream> #include <string> int main() { static constexpr auto nums = {1, 2, 2, 3, 4, 1, 2, 2, 2, 1}; constexpr int count{ 3 }; constexpr int value{ 2 }; constexpr auto result1 = std::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 = std::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 = std::ranges::search_n(nums, count, /* value */ 5); static_assert( // not found result3.size() == 0 && result3.begin() == result3.end() && result3.end() == nums.end() ); constexpr auto result4 = std::ranges::search_n(nums, /* count */ 0, /* value */ 1); static_assert( // not found result4.size() == 0 && result4.begin() == result4.end() && result4.end() == nums.begin() ); constexpr char symbol{'B'}; std::cout << std::boolalpha << "Find a sub-sequence: " << std::quoted(std::string(count, symbol)) << '\n'; auto result5 = std::ranges::search_n(nums, count, symbol, [](const char x, const char y) { // binary predicate const bool o{ x == y }; std::cout << "bin_op(" << x << ", " << y << ") == " << o << "\n"; return o; }, [](const int z) { return 'A' + z - 1; } // projects nums -> ASCII ); std::cout << "Found: " << !result5.empty() << '\n'; }
Output:
Find a sub-sequence: "BBB" bin_op(A, B) == false bin_op(B, B) == true bin_op(B, B) == true bin_op(C, B) == false bin_op(D, B) == false bin_op(A, B) == false bin_op(B, B) == true bin_op(B, B) == true bin_op(B, B) == true Found: true
See also
| (C++20) |
finds the first two adjacent items that are equal (or satisfy a given predicate) (niebloid) |
| (C++20)(C++20)(C++20) |
finds the first element satisfying specific criteria (niebloid) |
| (C++20) |
finds the last sequence of elements in a certain range (niebloid) |
| (C++20) |
searches for any one of a set of elements (niebloid) |
| (C++20) |
returns true if one sequence is a subsequence of another (niebloid) |
| (C++20) |
finds the first position where two ranges differ (niebloid) |
| (C++20) |
searches for a range of elements (niebloid) |
| searches a range for a number of consecutive copies of an element (function template) |