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

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< 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)
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(C++17)

Sorting and related operations
Partitioning operations
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(on partitioned ranges) Set operations (on sorted ranges) Merge operations (on sorted ranges) Heap operations
(C++11)
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(C++11)
(C++17)
Lexicographical comparison operations Permutation operations


C library
Numeric operations
 
Constrained algorithms
All names in this menu belong to namespace std::ranges
Non-modifying sequence operations
findfind_iffind_if_not
(C++23)(C++23)(C++23)  

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
(C++23)
(C++23)  
(C++23)
(C++23)  
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 [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 algorithm function objects (informally known as niebloids), that is:

Contents

[edit] Parameters

first, last - the iterator-sentinel pair defining 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 (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 (!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

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

[edit] See also

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

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