std::ranges::find, std::ranges::find_if, std::ranges::find_if_not
From cppreference.com
| Defined in header <algorithm>
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| Call signature |
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| (1) | ||
template< std::input_iterator I, std::sentinel_for<I> S, class T, class Proj = std::identity > |
(since C++20) (until C++26) |
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| template< std::input_iterator I, std::sentinel_for<I> S, class Proj = std::identity, |
(since C++26) | |
| (2) | ||
| template< ranges::input_range R, class T, class Proj = std::identity > requires std::indirect_binary_predicate |
(since C++20) (until C++26) |
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| template< ranges::input_range R, class Proj = std::identity, class T = std::projected_value_t<ranges::iterator_t<R>, Proj> > |
(since C++26) | |
template< std::input_iterator I, std::sentinel_for<I> S, class Proj = std::identity, |
(3) | (since C++20) |
| template< ranges::input_range R, class Proj = std::identity, std::indirect_unary_predicate |
(4) | (since C++20) |
template< std::input_iterator I, std::sentinel_for<I> S, class Proj = std::identity, |
(5) | (since C++20) |
| template< ranges::input_range R, class Proj = std::identity, std::indirect_unary_predicate |
(6) | (since C++20) |
Returns the first element in the range [first, last) that satisfies specific criteria:
1)
find searches for an element equal to value.3)
find_if searches for an element for which predicate pred returns true.5)
find_if_not searches for an element for which predicate pred returns false.2,4,6) Same as (1,3,5), 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 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.
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
Iterator to the first element satisfying the condition or iterator equal to last if no such element is found.
Complexity
At most last - first applications of the predicate and projection.
Possible implementation
| find |
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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)); } }; inline constexpr find_fn find; |
| 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)); } }; 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)); } }; inline constexpr find_if_not_fn find_if_not; |
Notes
| Feature-test macro | Value | Std | Feature |
|---|---|---|---|
__cpp_lib_algorithm_default_value_type |
202403 | (C++26) | List-initialization for algorithms (1,2) |
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: 4See also
| (C++20) |
finds the first two adjacent items that are equal (or satisfy a given predicate) (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) |
finds the first position where two ranges differ (niebloid) |
| (C++20) |
searches for the first occurrence of a range of elements (niebloid) |
| (C++11) |
finds the first element satisfying specific criteria (function template) |