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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: std::ranges::copy, std::ranges::sort, ...
Execution policies (C++17)
Non-modifying sequence operations
(C++11)(C++11)(C++11)
(C++17)
Modifying sequence operations
Operations on uninitialized storage
Partitioning operations
Sorting operations
(C++11)
Binary search operations
Set operations (on sorted ranges)
Heap operations
(C++11)
Minimum/maximum operations
(C++11)
(C++17)

Permutations
Numeric operations
C library
 
Constrained algorithms
Non-modifying sequence operations
ranges::findranges::find_ifranges::find_if_not
Modifying sequence operations
Operations on uninitialized storage
Partitioning operations
Sorting operations
Binary search operations
Set operations (on sorted ranges)
Heap operations
Minimum/maximum operations
Permutations
 
Defined in header <algorithm>
Call signature
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 = {} );
(1) (since C++20)
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 = {} );
(2) (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( 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 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

First version
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;
Second version
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;
Third version
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] Example

The following example finds an integer in a vector of integers.

#include <iostream>
#include <algorithm>
#include <vector>
#include <iterator>
 
int main()
{
    int n1 = 3;
    int n2 = 5;
 
    std::vector<int> v{4, 1, 3, 2};
 
    namespace ranges = std::ranges;
 
    auto result1 = ranges::find(v, n1);
    auto result2 = ranges::find(v.begin(), v.end(), n2);
 
    if (result1 != v.end()) {
        std::cout << "v contains: " << n1 << '\n';
    } else {
        std::cout << "v does not contain: " << n1 << '\n';
    }
 
    if (result2 != 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; };
    auto divides_13 = [](int x) { return x % 13 == 0; };
 
    auto result3 = ranges::find_if(v.begin(), v.end(), is_even);
    auto result4 = ranges::find_if(v, divides_13);
    if (result3 != v.end()) {
      std::cout << "First even element in v: " << *result3 << '\n';
    } else {
      std::cout << "No even elements in v\n";
    }
 
    if (result4 != v.end()) {
      std::cout << "First element divisible by 13 in v: " << *result4 << '\n';
    } else {
      std::cout << "No elements in v are divisible by 13\n";
    }
 
    auto result5 = ranges::find_if_not(v.begin(), v.end(), divides_13);
    auto result6 = ranges::find_if_not(v, is_even);
    if (result5 != v.end()) {
      std::cout << "First element indivisible by 13 in v: " << *result5 << '\n';
    } else {
      std::cout << "No elements in v are divisible by 13\n";
    }
 
    if (result6 != v.end()) {
      std::cout << "First odd element in v: " << *result6 << '\n';
    } else {
      std::cout << "No even elements in v\n";
    }
}

Output:

v contains: 3
v does not contain: 5
First even element in v: 4
No elements in v are divisible by 13
First element indivisible by 13 in v: 4
First odd element in v: 1

[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]