From cppreference.com
| 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 >
requires std::indirect_binary_predicate
<ranges::equal_to, std::projected<I, Proj>, const T*>
constexpr I find( I first, S last, const T& value, Proj proj = {} );
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(1) | (since C++20) (until C++26) |
template< std::input_iterator I, std::sentinel_for<I> S,
class Proj = std::identity,
class T = std::projected_value_t<I, Proj> >
requires std::indirect_binary_predicate
<ranges::equal_to, std::projected<I, Proj>, const T*>
constexpr I find( I first, S last, const T& value, Proj proj = {} );
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(since C++26) | |
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 ranges::borrowed_iterator_t<R>
find( R&& r, const T& value, Proj proj = {} );
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(2) | (since C++20) (until C++26) |
template< ranges::input_range R, class Proj = std::identity,
class T = std::projected_value_t<ranges::iterator_t<R>, Proj> >
requires std::indirect_binary_predicate
<ranges::equal_to,
std::projected<ranges::iterator_t<R>, Proj>, const T*>
constexpr ranges::borrowed_iterator_t<R>
find( R&& r, const T& value, Proj proj = {} );
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(since C++26) | |
template< std::input_iterator I, std::sentinel_for<I> S,
class Proj = std::identity,
std::indirect_unary_predicate<std::projected<I, Proj>> Pred >
constexpr I find_if( I first, S last, Pred pred, Proj proj = {} );
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(3) | (since C++20) |
template< ranges::input_range R, class Proj = std::identity,
std::indirect_unary_predicate
<std::projected<ranges::iterator_t<R>, Proj>> Pred >
constexpr ranges::borrowed_iterator_t<R>
find_if( R&& r, Pred pred, Proj proj = {} );
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(4) | (since C++20) |
template< std::input_iterator I, std::sentinel_for<I> S,
class Proj = std::identity,
std::indirect_unary_predicate<std::projected<I, Proj>> Pred >
constexpr I find_if_not( I first, S last, Pred pred, Proj proj = {} );
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(5) | (since C++20) |
template< ranges::input_range R, class Proj = std::identity,
std::indirect_unary_predicate
<std::projected<ranges::iterator_t<R>, Proj>> Pred >
constexpr ranges::borrowed_iterator_t<R>
find_if_not( R&& r, Pred pred, Proj proj = {} );
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(6) | (since C++20) |
template< /*execution-policy*/ Ep,
std::random_access_iterator I, std::sized_sentinel_for<I> S,
class Proj = std::identity,
class T = std::projected_value_t<I, Proj> >
requires std::indirect_binary_predicate
<ranges::equal_to, std::projected<I, Proj>, const T*>
I find( Ep&& policy, I first, S last, const T& value, Proj proj = {} );
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(7) | (since C++26) |
template< /*execution-policy*/ Ep, /*sized-random-access-range*/ R,
class Proj = std::identity,
class T = std::projected_value_t<ranges::iterator_t<R>, Proj> >
requires std::indirect_binary_predicate
<ranges::equal_to,
std::projected<ranges::iterator_t<R>, Proj>, const T*>
ranges::borrowed_iterator_t<R>
find( Ep&& policy, R&& r, const T& value, Proj proj = {} );
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(8) | (since C++26) |
template< /*execution-policy*/ Ep,
std::random_access_iterator I, std::sized_sentinel_for<I> S,
class Proj = std::identity,
std::indirect_unary_predicate<std::projected<I, Proj>> Pred >
I find_if( Ep&& policy, I first, S last, Pred pred, Proj proj = {} );
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(9) | (since C++26) |
template< /*execution-policy*/ Ep, /*sized-random-access-range*/ R,
class Proj = std::identity,
std::indirect_unary_predicate
<std::projected<ranges::iterator_t<R>, Proj>> Pred >
ranges::borrowed_iterator_t<R>
find_if( Ep&& policy, R&& r, Pred pred, Proj proj = {} );
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(10) | (since C++26) |
template< /*execution-policy*/ Ep,
std::random_access_iterator I, std::sized_sentinel_for<I> S,
class Proj = std::identity,
std::indirect_unary_predicate<std::projected<I, Proj>> Pred >
I find_if_not( Ep&& policy, I first, S last, Pred pred, Proj proj = {} );
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(11) | (since C++26) |
template< /*execution-policy*/ Ep, /*sized-random-access-range*/ R,
class Proj = std::identity,
std::indirect_unary_predicate
<std::projected<ranges::iterator_t<R>, Proj>> Pred >
ranges::borrowed_iterator_t<R>
find_if_not( Ep&& policy, R&& r, Pred pred, Proj proj = {} );
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(12) | (since C++26) |
For the definition of /*execution-policy*/, see this page; for the definition of /*sized-random-access-range*/, see this page.
Search for the first element (projected by proj) in the source range [first, last) or r that satisfies specific criteria:
1,2)
find searches for the first element equal to the target value value.3,4)
find_if searches for the first element for which predicate pred returns true.5,6)
find_if_not searches for the first element for which predicate pred returns false.7-12) Same as (1-6), 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
| first, last | - | the iterator-sentinel pair defining the source range |
| r | - | the source range |
| value | - | the target value |
| pred | - | the predicate to be applied to the (projected) elements |
| proj | - | the projection to be applied to the elements |
| policy | - | the execution policy to use |
Return value
Iterator to the first element satisfying the condition, or last if no such element is found.
Complexity
Given \(\scriptsize N\)N as ranges::distance(first, last) or ranges::distance(r):
1,2) At most \(\scriptsize N\)N comparisons and applications of
proj.3-6) At most \(\scriptsize N\)N applications of
pred and proj.7,8) \(\scriptsize \mathcal{O}(N)\)𝓞(N) comparisons and applications of
proj.9-12) \(\scriptsize \mathcal{O}(N)\)𝓞(N) applications of
pred and proj.Exceptions
7-12) 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).
Notes
| Feature-test macro | Value | Std | Feature |
|---|---|---|---|
__cpp_lib_algorithm_default_value_type |
202403 |
(C++26) | List-initialization for algorithms (1,2) |
Possible implementation
| find |
|---|
struct find_fn
{
template<std::input_iterator I, std::sentinel_for<I> S,
class Proj = std::identity,
class T = std::projected_value_t<I, Proj>>
requires std::indirect_binary_predicate
<ranges::equal_to, std::projected<I, Proj>, const T*>
constexpr I operator()(I first, S last, const T& value, Proj proj = {}) const
{
for (; first != last; ++first)
if (std::invoke(proj, *first) == value)
return first;
return first;
}
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 ranges::borrowed_iterator_t<R>
operator()(R&& r, const T& value, Proj proj = {}) const
{
return (*this)(ranges::begin(r), ranges::end(r), value, std::ref(proj));
}
template<ranges::forward_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 ranges::borrowed_iterator_t<R>
operator()(R&& r, const T& value, Proj proj = {}) const
{
return (*this)(ranges::begin(r),
ranges::next(ranges::begin(r), ranges::end(r)),
value, std::ref(proj));
}
};
inline constexpr find_fn find;
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| find_if |
struct find_if_fn
{
template<std::input_iterator I, std::sentinel_for<I> S, class Proj = std::identity,
std::indirect_unary_predicate<std::projected<I, Proj>> Pred>
constexpr I operator()(I first, S last, Pred pred, Proj proj = {}) const
{
for (; first != last; ++first)
if (std::invoke(pred, std::invoke(proj, *first)))
return first;
return first;
}
template<ranges::input_range R, class Proj = std::identity,
std::indirect_unary_predicate
<std::projected<ranges::iterator_t<R>, Proj>> Pred>
constexpr ranges::borrowed_iterator_t<R>
operator()(R&& r, Pred pred, Proj proj = {}) const
{
return (*this)(ranges::begin(r), ranges::end(r), std::ref(pred), std::ref(proj));
}
template<ranges::forward_range R, class Proj = std::identity,
std::indirect_unary_predicate
<std::projected<ranges::iterator_t<R>, Proj>> Pred>
constexpr ranges::borrowed_iterator_t<R>
operator()(R&& r, Pred pred, Proj proj = {}) const
{
return (*this)(ranges::begin(r),
ranges::next(ranges::begin(r), ranges::end(r)),
std::ref(pred), std::ref(proj));
}
};
inline constexpr find_if_fn find_if;
|
| find_if_not |
struct find_if_not_fn
{
template<std::input_iterator I, std::sentinel_for<I> S, class Proj = std::identity,
std::indirect_unary_predicate<std::projected<I, Proj>> Pred>
constexpr I operator()(I first, S last, Pred pred, Proj proj = {}) const
{
for (; first != last; ++first)
if (!std::invoke(pred, std::invoke(proj, *first)))
return first;
return first;
}
template<ranges::input_range R, class Proj = std::identity,
std::indirect_unary_predicate
<std::projected<ranges::iterator_t<R>, Proj>> Pred>
constexpr ranges::borrowed_iterator_t<R>
operator()(R&& r, Pred pred, Proj proj = {}) const
{
return (*this)(ranges::begin(r), ranges::end(r), std::ref(pred), std::ref(proj));
}
template<ranges::forward_range R, class Proj = std::identity,
std::indirect_unary_predicate
<std::projected<ranges::iterator_t<R>, Proj>> Pred>
constexpr ranges::borrowed_iterator_t<R>
operator()(R&& r, Pred pred, Proj proj = {}) const
{
return (*this)(ranges::begin(r),
ranges::next(ranges::begin(r), ranges::end(r)),
std::ref(pred), std::ref(proj));
}
};
inline constexpr find_if_not_fn find_if_not;
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Example
Run this code
#include <algorithm>
#include <cassert>
#include <complex>
#include <format>
#include <iostream>
#include <iterator>
#include <string>
#include <vector>
void projector_example()
{
struct folk_info
{
unsigned uid;
std::string name, position;
};
std::vector<folk_info> folks
{
{0, "Ana", "dev"},
{1, "Bob", "devops"},
{2, "Eve", "ops"}
};
const auto who{"Eve"};
if (auto it = std::ranges::find(folks, who, &folk_info::name); it != folks.end())
std::cout << std::format("Profile:\n"
" UID: {}\n"
" Name: {}\n"
" Position: {}\n\n",
it->uid, it->name, it->position);
}
int main()
{
namespace ranges = std::ranges;
projector_example();
const int n1 = 3;
const int n2 = 5;
const auto v = {4, 1, 3, 2};
if (ranges::find(v, n1) != v.end())
std::cout << "v contains: " << n1 << '\n';
else
std::cout << "v does not contain: " << n1 << '\n';
if (ranges::find(v.begin(), v.end(), n2) != v.end())
std::cout << "v contains: " << n2 << '\n';
else
std::cout << "v does not contain: " << n2 << '\n';
auto is_even = [](int x) { return x % 2 == 0; };
if (auto result = ranges::find_if(v.begin(), v.end(), is_even); result != v.end())
std::cout << "First even element in v: " << *result << '\n';
else
std::cout << "No even elements in v\n";
if (auto result = ranges::find_if_not(v, is_even); result != v.end())
std::cout << "First odd element in v: " << *result << '\n';
else
std::cout << "No odd elements in v\n";
auto divides_13 = [](int x) { return x % 13 == 0; };
if (auto result = ranges::find_if(v, divides_13); result != v.end())
std::cout << "First element divisible by 13 in v: " << *result << '\n';
else
std::cout << "No elements in v are divisible by 13\n";
if (auto result = ranges::find_if_not(v.begin(), v.end(), divides_13);
result != v.end())
std::cout << "First element indivisible by 13 in v: " << *result << '\n';
else
std::cout << "All elements in v are divisible by 13\n";
std::vector<std::complex<double>> nums{{4, 2}};
#ifdef __cpp_lib_algorithm_default_value_type
// T gets deduced in (2) making list-initialization possible
const auto it = ranges::find(nums, {4, 2});
#else
const auto it = ranges::find(nums, std::complex<double>{4, 2});
#endif
assert(it == nums.begin());
}
Output:
Profile:
UID: 2
Name: Eve
Position: ops
v contains: 3
v does not contain: 5
First even element in v: 4
First odd element in v: 1
No elements in v are divisible by 13
First element indivisible by 13 in v: 4
See also
(C++11) |
finds the first element satisfying specific criteria (function template) |
(C++20) |
finds the first two adjacent items that are equal (or satisfy a given predicate) (algorithm function object) |
(C++20) |
finds the last sequence of elements in a certain range (algorithm function object) |
(C++20) |
searches for any one of a set of elements (algorithm function object) |
(C++20) |
finds the first position where two ranges differ (algorithm function object) |
(C++20) |
searches for the first occurrence of a range of elements (algorithm function object) |