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

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< cpp‎ | algorithm‎ | ranges
 
C++
 
Algorithm library
Constrained algorithms and algorithms on ranges (C++20)
Constrained algorithms, e.g. ranges::copy, ranges::sort, ...
Execution policies (C++17)
(C++17)  
Non-modifying sequence operations
(C++11)(C++11)(C++11)
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Modifying sequence operations
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(C++11)
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(C++11)
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(C++17)
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(C++17)
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C library
 
Constrained algorithms
Non-modifying sequence operations
ranges::findranges::find_ifranges::find_if_not
(C++23)(C++23)(C++23)

Modifying sequence operations
Partitioning operations
Sorting operations
Binary search operations
Set operations (on sorted ranges)
Heap operations
Minimum/maximum operations
Permutations
Constrained numeric operations
(C++23)
Fold operations
Operations on uninitialized storage
Return types
 
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.

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

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;

Example

Run this code
#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";
    }
}

Output: 

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

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