NAME¶

std::ranges::find,std::ranges::find_if,std::ranges::find_if_not - std::ranges::find,std::ranges::find_if,std::ranges::find_if_not

Synopsis¶

Defined in header <algorithm>
Call signature
template< std::input_iterator I, std::sentinel_for<I> S,

class T, class Proj = std::identity > (since
requires std::indirect_binary_predicate C++20)
<ranges::equal_to, std::projected<I, Proj>, (until
const T*> C++26)

constexpr I find( I first, S last, const T& value, Proj proj
= {} );
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 (since
<ranges::equal_to, std::projected<I, Proj>, C++26)
const T*>

constexpr I find( I first, S last, const T& value, Proj proj
= {} );
template< ranges::input_range R, class T, class Proj =
std::identity >

requires std::indirect_binary_predicate (since
<ranges::equal_to, C++20)
std::projected<ranges::iterator_t<R>, Proj>, (until
const T*> C++26)
constexpr ranges::borrowed_iterator_t<R>

find( R&& r, const T& value, Proj proj = {} );
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 (since
<ranges::equal_to, C++26)
std::projected<ranges::iterator_t<R>, Proj>, (1)
const T*>
constexpr ranges::borrowed_iterator_t<R>

find( R&& r, const T& value, Proj proj = {} );
template< std::input_iterator I, std::sentinel_for<I> S,

class Proj = std::identity,
std::indirect_unary_predicate<std::projected<I, (3) (since
Proj>> Pred > C++20)
(2)
constexpr I find_if( I first, S last, Pred pred, Proj proj =
{} );
template< ranges::input_range R, class Proj = std::identity,

std::indirect_unary_predicate
<std::projected<ranges::iterator_t<R>, Proj>> (4) (since
Pred > C++20)
constexpr ranges::borrowed_iterator_t<R>

find_if( R&& r, Pred pred, Proj proj = {} );
template< std::input_iterator I, std::sentinel_for<I> S,

class Proj = std::identity,
std::indirect_unary_predicate<std::projected<I, (5) (since
Proj>> Pred > C++20)

constexpr I find_if_not( I first, S last, Pred pred, Proj
proj = {} );
template< ranges::input_range R, class Proj = std::identity,

std::indirect_unary_predicate
<std::projected<ranges::iterator_t<R>, Proj>> (6) (since
Pred > C++20)
constexpr ranges::borrowed_iterator_t<R>

find_if_not( R&& r, Pred pred, Proj proj = {} );

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
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: 4

See also¶

ranges::adjacent_find finds the first two adjacent items that are equal (or satisfy
(C++20) a given predicate)
(niebloid)
ranges::find_end finds the last sequence of elements in a certain range
(C++20) (niebloid)
ranges::find_first_of searches for any one of a set of elements
(C++20) (niebloid)
ranges::mismatch finds the first position where two ranges differ
(C++20) (niebloid)
ranges::search searches for a range of elements
(C++20) (niebloid)
find
find_if finds the first element satisfying specific criteria
find_if_not (function template)
(C++11)