std::ranges::find() algorithm
- od C++20
- Simplified
- Detailed
// (1)
constexpr I
find( I first, S last, const T& value, Proj proj = {} );
// (2)
constexpr ranges::borrowed_iterator_t<R>
find( R&& r, const T& value, Proj proj = {} );
The type of arguments are generic and have the following constraints:
I
-std::input_iterator
S
-std::sentinel_for<I>
T
- (none)Proj
- (none)- (2) -
R
-std::ranges::input_range
The Proj
template argument has a default type of std::identity
for all overloads.
Additionally, each overload has the following constraints:
- (1) -
indirect_binary_predicate<ranges::equal_to, projected<I, Proj>, const T*>>
- (2) -
indirect_binary_predicate<ranges::equal_to, projected<ranges::iterator_t<R>, Proj>, const T*>
(The std::
namespace was ommitted here for readability)
// (1)
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 = {} );
// (2)
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 = {} );
Returns an iterator to the first element in the range satisfiying specific criteria (or last
iterator if there is no such iterator):
-
(1) Searches for an element equal to
value
(usingoperator==
) -
(2) Same as (1), but uses
r
as the source range, as if usingranges::begin(r)
asfirst
andranges::end(r)
aslast
.
The function-like entities described on this page are niebloids.
Parameters
first last | The range of elements to apply the function to. |
r | The range of elements to apply the function to. |
value | Value to compare the elements to. |
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
Exactly last - first
applications of the predicate and projection.
Exceptions
(none)
Possible implementation
find(1)
struct find_fn
{
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 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;
Examples
#include <algorithm>
#include <iostream>
#include <iterator>
int main()
{
namespace ranges = std::ranges;
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";
}
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
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