From cppreference.com
| 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) |
template< /*execution-policy*/ Ep,
std::random_access_iterator I1, std::sized_sentinel_for<I1> S1,
std::random_access_iterator I2, std::sized_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>
I1 find_first_of( Ep&& policy, I1 first1, S1 last1, I2 first2, S2 last2,
Pred pred = {}, Proj1 proj1 = {}, Proj2 proj2 = {} );
|
(3) | (since C++26) |
template< /*execution-policy*/ Ep,
/*sized-random-access-range*/ R1,
/*sized-random-access-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>
ranges::borrowed_iterator_t<R1>
find_first_of( Ep&& policy, R1&& r1, R2&& r2, Pred pred = {},
Proj1 proj1 = {}, Proj2 proj2 = {} );
|
(4) | (since C++26) |
For the definition of /*execution-policy*/, see this page; for the definition of /*sized-random-access-range*/, see this page.
Searches the source range for any of the elements in the target range. The elements (projected by proj1 and proj2 respectively) are compared using the binary predicate pred.
1) The source range is
[first1, last1), and the target range is [first2, last2).2) The source range is
r1, and the target range is r2.3,4) Same as (1,2), but executed according to
policy.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.
Parameters
| first1, last1 | - | the iterator-sentinel pair defining the source range |
| first2, last2 | - | the iterator-sentinel pair defining the target range |
| r1 | - | the source range |
| r2 | - | the target range |
| pred | - | the predicate to be applied to the (projected) elements |
| proj1 | - | the projection to be applied to the elements in the source range |
| proj2 | - | the projection to be applied to the elements in the target range |
| policy | - | the execution policy to use |
Return value
Iterator to the first element in the source range that matches an element from the target range.
If the target range is empty or if no such element is found, returns:
1,3)
last12,4)
ranges::next(ranges::begin(r1), ranges::end(r1))Complexity
Given \(\scriptsize N_1\)N1 as ranges::distance(first1, last1) or ranges::distance(r1), and \(\scriptsize N_2\)N2 as ranges::distance(first2, last2) or ranges::distance(r2):
1,2) At most \(\scriptsize N_1 \cdot N_2\)N1⋅N2 applications of
pred and proj.3,4) \(\scriptsize \mathcal{O}(N_1 \cdot N_2)\)𝓞(N1⋅N2) applications of
pred and proj.Exceptions
3,4) During the execution process:
- If the temporary memory resources required for parallelization are not available, std::bad_alloc is thrown.
- If an uncaught exception is thrown while accessing objects via an algorithm argument, the behavior is determined by the execution policy (for standard policies, std::terminate is invoked).
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::next(ranges::begin(r2), ranges::end(r2)),
std::move(pred), std::move(proj1), std::move(proj2));
}
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 ranges::borrowed_iterator_t<R1>
operator()(R1&& r1, R2&& r2, Pred pred = {},
Proj1 proj1 = {}, Proj2 proj2 = {}) const
{
return (*this)(ranges::begin(r1),
ranges::next(ranges::begin(r1), ranges::end(r1)),
ranges::begin(r2),
ranges::next(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
Run this code
#include <algorithm>
#include <iostream>
#include <iterator>
int main()
{
using std::ranges::find_first_of;
constexpr static auto haystack = {1, 2, 3, 4};
constexpr static auto needles = {0, 3, 4, 3};
constexpr auto found1 = find_first_of(haystack.begin(), haystack.end(),
needles.begin(), needles.end());
static_assert(std::distance(haystack.begin(), found1) == 2);
constexpr auto found2 = 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 = find_first_of(haystack, negatives);
static_assert(not_found == haystack.end());
constexpr auto found3 = 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 = 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 equivalent 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) (algorithm function object) |
(C++20)(C++20)(C++20) |
finds the first element satisfying specific criteria (algorithm function object) |
(C++20) |
finds the last sequence of elements in a certain range (algorithm function object) |
(C++20) |
searches for the first occurrence of a range of elements (algorithm function object) |
(C++20) |
searches for the first occurrence of a number consecutive copies of an element in a range (algorithm function object) |