Difference between revisions of "cpp/algorithm/ranges/contains"
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Revision as of 04:52, 29 July 2022
| Defined in header <algorithm>
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| Call signature |
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| template< std::input_iterator I, std::sentinel_for<I> S, class T, class Proj = std::identity > |
(1) | (since C++23) |
| template< ranges::input_range R, class T, class Proj = std::identity > requires std::indirect_binary_predicate<ranges::equal_to, |
(2) | (since C++23) |
| template< std::forward_iterator I1, std::sentinel_for<I1> S1, std::forward_iterator I2, std::sentinel_for<I2> S2, |
(3) | (since C++23) |
| template< ranges::forward_range R1, ranges::forward_range R2, class Pred = ranges::equal_to, |
(4) | (since C++23) |
r as the source range, as if using ranges::begin(r) as first and ranges::end(r) as last.r1 as the first source range and r2 as the second source range, as if using ranges::begin(r1) as first1, ranges::end(r1) as last1, ranges::begin(r2) as first2, and ranges::end(r2) as last2.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.
Contents |
Parameters
| first, last | - | the range of elements to examine |
| r | - | the range of the elements to examine |
| value | - | value to compare the elements to |
| pred | - | predicate to apply to the projected elements |
| proj | - | projection to apply to the elements |
Return value
Complexity
At most last - first applications of the predicate and projection.
Notes
Up until C++20, we've had to write std::ranges::find(r, value) != std::ranges::end(r) to determine if a single value is inside a range. And to check if a range contains a subrange of interest, we use not std::ranges::search(haystack, needle).empty(). While this is accurate, it isn't necessarily convenient, and it hardly expresses intent (especially in the latter case). Being able to say std::ranges::contains(r, value) addresses both of these points. Further evidence for the existence of a contains algorithm is that the STL gave C++98 an obscurely-named contains algorithm called binary_search, and any_of is "contains with a predicate", showing that there's prior art for this contains to be an algorithm.
ranges::contains_subrange, same as ranges::search, but as opposed to std::search, provides no access to Searchers (such as Boyer-Moore).
| Feature-test macro | Value | Std | Feature |
|---|---|---|---|
__cpp_lib_ranges_contains |
Possible implementation
| First version |
|---|
struct __contains_fn { template< std::input_iterator I, std::sentinel_for<I> S, class T, class Proj = std::identity > requires std::indirect_binary_predicate<ranges::equal_to, std::projected<I, Proj>, const T*> constexpr bool operator()(I first, S last, const T& value, Proj proj = {}) const { return ranges::find(std::move(first), last, value, proj) != last; } template< ranges::input_range R, class T, class Proj = std::identity > requires std::indirect_binary_predicate<ranges::equal_to, std::projected<ranges::iterator_t<R>, Proj>, const T*> constexpr bool operator()(R&& r, const T& value, Proj proj = {}) const { return (*this)(ranges::begin(r), ranges::end(r), std::move(value), proj); } }; inline constexpr __contains_fn contains {}; |
| Second version |
struct __contains_subrange_fn { template< std::forward_iterator I1, std::sentinel_for<I1> S1, std::forward_iterator I2, std::sentinel_for<I2> S2, class Pred = ranges::equal_to, class Proj1 = std::identity, class Proj2 = std::identity > requires std::indirectly_comparable<I1, I2, Pred, Proj1, Proj2> constexpr bool operator()(I1 first1, S1 last1, I2 first2, S2 last2, Pred pred = {}, Proj1 proj1 = {}, Proj2 proj2 = {}) const { return (first2 == last2) || !ranges::search(first1, last1, first2, last2, pred, proj1, proj2).empty(); } template< ranges::forward_range R1, ranges::forward_range R2, class Pred = ranges::equal_to, class Proj1 = std::identity, class Proj2 = std::identity > requires std::indirectly_comparable<ranges::iterator_t<R1>, ranges::iterator_t<R2>, Pred, Proj1, Proj2> constexpr bool operator()(R1&& r1, R2&& r2, Pred pred = {}, Proj1 proj1 = {}, Proj2 proj2 = {}) const { return (*this)(ranges::begin(r1), ranges::end(r1), ranges::begin(r2), ranges::end(r2), std::move(pred), std::move(proj1), std::move(proj2)); } }; inline constexpr __contains_subrange_fn contains_subrange {}; |
Example
#include <algorithm> #include <array> int main() { constexpr auto haystack = std::array { 3, 1, 4, 1, 5 }; constexpr auto needle = std::array { 1, 4, 1 }; constexpr auto bodkin = std::array { 2, 5, 2 }; auto increment = [](int x) { return ++x; }; auto decrement = [](int x) { return --x; }; static_assert( std::ranges::contains(haystack, 4) and not std::ranges::contains(haystack, 6) and std::ranges::contains_subrange(haystack, needle) and not std::ranges::contains_subrange(haystack, bodkin) and std::ranges::contains(haystack, 6, increment) and not std::ranges::contains(haystack, 1, increment) and std::ranges::contains_subrange(haystack, bodkin, {}, increment) and not std::ranges::contains_subrange(haystack, bodkin, {}, decrement) and std::ranges::contains_subrange(haystack, bodkin, {}, {}, decrement) ); }
See also
| (C++20)(C++20)(C++20) |
finds the first element satisfying specific criteria (niebloid) |
| (C++23)(C++23)(C++23) |
finds the last element satisfying specific criteria (niebloid) |
| (C++20) |
searches for the first occurrence of a range of elements (niebloid) |
| (C++20) |
determines if an element exists in a partially-ordered range (niebloid) |
| (C++20)(C++20)(C++20) |
checks if a predicate is true for all, any or none of the elements in a range (niebloid) |
