std::ranges::find, std::ranges::find_if, std::ranges::find_if_not - cppreference.com (original) (raw)

Returns the first element in the range [first, last) that satisfies specific criteria:

1. find searches for an element equal to value.

2. find_if searches for an element for which predicate pred returns true.

3. 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 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.

Contents

• 1 Parameters
• 2 Return value
• 3 Complexity
• 4 Possible implementation
• 5 Notes
• 6 Example
• 7 See also

[edit] Parameters

[edit] Return value

Iterator to the first element satisfying the condition or iterator equal to last if no such element is found.

[edit] Complexity

At most last - first applications of the predicate and projection.

[edit] Possible implementation

find (1)
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 (3)
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 (5)
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 ((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;

[edit] Notes

[edit] Example

#include #include #include #include #include #include #include #include   void projector_example() { struct folk_info { unsigned uid; std::string name, position; };   std::vector 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> 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{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

[edit] See also