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

From cppreference.com
< cpp‎ | algorithm‎ | ranges
 
 
Algorithm library
Constrained algorithms and algorithms on ranges (C++20)
Constrained algorithms, e.g. ranges::copy, ranges::sort, ...
Execution policies (C++17)
Non-modifying sequence operations
Batch operations
(C++17)
Search operations
(C++11)                (C++11)(C++11)

Modifying sequence operations
Copy operations
(C++11)
(C++11)
Swap operations
Transformation operations
Generation operations
Removing operations
Order-changing operations
(until C++17)(C++11)
(C++20)(C++20)
Sampling operations
(C++17)

Sorting and related operations
Partitioning operations
Sorting operations
Binary search operations
(on partitioned ranges)
Set operations (on sorted ranges)
Merge operations (on sorted ranges)
Heap operations
Minimum/maximum operations
(C++11)
(C++17)
Lexicographical comparison operations
Permutation operations
C library
Numeric operations
Operations on uninitialized memory
 
Constrained algorithms
All names in this menu belong to namespace std::ranges
Non-modifying sequence operations
Modifying sequence operations
Partitioning operations
Sorting operations
Binary search operations (on sorted ranges)
       
       
Set operations (on sorted ranges)
Heap operations
Minimum/maximum operations
       
       
Permutation operations
Fold operations
Numeric operations
(C++23)            
Operations on uninitialized storage
Return types
 
Defined in header <algorithm>
Call signature
(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 = {} );
(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 = {} );
(since C++26)
(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 = {} );
(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 = {} );
(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 = {} );
(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 = {} );
(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 = {} );
(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 = {} );
(6) (since C++20)

Returns the first element in the range [firstlast) 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:

In practice, they may be implemented as function objects, or with special compiler extensions.

Contents

[edit] 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

[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
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;

[edit] Notes

Feature-test macro Value Std Feature
__cpp_lib_algorithm_default_value_type 202403 (C++26) List-initialization for algorithms (1,2)

[edit] Example

#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

[edit] See also

finds the first two adjacent items that are equal (or satisfy a given predicate)
(niebloid)[edit]
finds the last sequence of elements in a certain range
(niebloid)[edit]
searches for any one of a set of elements
(niebloid)[edit]
finds the first position where two ranges differ
(niebloid)[edit]
searches for a range of elements
(niebloid)[edit]
finds the first element satisfying specific criteria
(function template) [edit]