std::ranges::find_first_of
Defined in header <algorithm> |
||
---|---|---|
Call signature | ||
template< std::input_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 I1 find_first_of( I1 first1, S1 last1, I2 first2, S2 last2, Pred pred = {}, Proj1 proj1 = {}, Proj2 proj2 = {} ); |
(1) | (since C++20) |
template< ranges::input_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 ranges::borrowed_iterator_t<R1> find_first_of( R1&& r1, R2&& r2, Pred pred = {}, Proj1 proj1 = {}, Proj2 proj2 = {} ); |
(2) | (since C++20) |
[first1, last1)
for
any of the elements in the range
[first2, last2)
, after projecting the ranges with
proj1
and
proj2
respectively. The projected elements are compared using the binary predicate
pred
.
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 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
first1, last1 | - | the range of elements to examine (aka haystack) |
first2, last2 | - | the range of elements to search for (aka needles) |
r1 | - | the range of elements to examine (aka haystack) |
r2 | - | the range of elements to search for (aka needles) |
pred | - | binary predicate to compare the elements |
proj1 | - | projection to apply to the elements in the first range |
proj2 | - | projection to apply to the elements in the second range |
Return value
Iterator to the first element in the range [first1, last1)
that is equal to an element from the range [first2, last2)
after projection. If no such element is found, an iterator comparing equal to last1
is returned.
Complexity
At most (S*N)
applications of the predicate and each projection, where
(1) S = ranges::distance(first2, last2)
and N = ranges::distance(first1, last1)
;
(2) S = ranges::distance(r2)
and N = ranges::distance(r1)
.
Possible implementation
struct find_first_of_fn { template<std::input_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 I1 operator()(I1 first1, S1 last1, I2 first2, S2 last2, Pred pred = {}, Proj1 proj1 = {}, Proj2 proj2 = {}) const { for (; first1 != last1; ++first1) for (auto i = first2; i != last2; ++i) if (std::invoke(pred, std::invoke(proj1, *first1), std::invoke(proj2, *i))) return first1; return first1; } template<ranges::input_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 ranges::borrowed_iterator_t<R1> 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 find_first_of_fn find_first_of{}; |
Example
#include <algorithm> #include <iostream> #include <iterator> int main() { namespace rng = std::ranges; constexpr static auto haystack = {1, 2, 3, 4}; constexpr static auto needles = {0, 3, 4, 3}; constexpr auto found1 = rng::find_first_of(haystack.begin(), haystack.end(), needles.begin(), needles.end()); static_assert(std::distance(haystack.begin(), found1) == 2); constexpr auto found2 = rng::find_first_of(haystack, needles); static_assert(std::distance(haystack.begin(), found2) == 2); constexpr static auto negatives = {-6, -3, -4, -3}; constexpr auto not_found = rng::find_first_of(haystack, negatives); static_assert(not_found == haystack.end()); constexpr auto found3 = rng::find_first_of(haystack, negatives, [](int x, int y) { return x == -y; }); // uses a binary comparator static_assert(std::distance(haystack.begin(), found3) == 2); struct P { int x, y; }; constexpr static auto p1 = { P{1, -1}, P{2, -2}, P{3, -3}, P{4, -4} }; constexpr static auto p2 = { P{5, -5}, P{6, -3}, P{7, -5}, P{8, -3} }; // Compare only P::y data members by projecting them: const auto found4 = rng::find_first_of(p1, p2, {}, &P::y, &P::y); std::cout << "First equivalent element {" << found4->x << ", " << found4->y << "} was found at position " << std::distance(p1.begin(), found4) << ".\n"; }
Output:
First equavalent element {3, -3} was found at position 2.
See also
searches for any one of a set of elements (function template) |
|
(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 a range of elements (niebloid) |
(C++20)
|
searches for a number consecutive copies of an element in a range (niebloid) |
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