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

From cppreference.com
< cpp‎ | algorithm
 
 
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)

findfind_iffind_if_not
(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
 
Defined in header <algorithm>
(1)
template< class InputIt, class T >
InputIt find( InputIt first, InputIt last, const T& value );
(constexpr since C++20)
(until C++26)
template< class InputIt, class T = typename std::iterator_traits

                                       <InputIt>::value_type >

constexpr InputIt find( InputIt first, InputIt last, const T& value );
(since C++26)
(2)
template< class ExecutionPolicy, class ForwardIt, class T >

ForwardIt find( ExecutionPolicy&& policy,

                ForwardIt first, ForwardIt last, const T& value );
(since C++17)
(until C++26)
template< class ExecutionPolicy,

          class ForwardIt, class T = typename std::iterator_traits
                                         <ForwardIt>::value_type >
ForwardIt find( ExecutionPolicy&& policy,

                ForwardIt first, ForwardIt last, const T& value );
(since C++26)
template< class InputIt, class UnaryPred >
InputIt find_if( InputIt first, InputIt last, UnaryPred p );
(3) (constexpr since C++20)
template< class ExecutionPolicy, class ForwardIt, class UnaryPred >

ForwardIt find_if( ExecutionPolicy&& policy,

                   ForwardIt first, ForwardIt last, UnaryPred p );
(4) (since C++17)
template< class InputIt, class UnaryPred >
InputIt find_if_not( InputIt first, InputIt last, UnaryPred q );
(5) (since C++11)
(constexpr since C++20)
template< class ExecutionPolicy, class ForwardIt, class UnaryPred >

ForwardIt find_if_not( ExecutionPolicy&& policy,

                       ForwardIt first, ForwardIt last, UnaryPred q );
(6) (since C++17)

Returns an iterator to the first element in the range [firstlast) that satisfies specific criteria (or last if there is no such iterator).

1) find searches for an element equal to value (using operator==).
3) find_if searches for an element for which predicate p returns true.
5) find_if_not searches for an element for which predicate q returns false.
2,4,6) Same as (1,3,5), but executed according to policy.
These overloads participate in overload resolution only if

std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is true.

(until C++20)

std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>> is true.

(since C++20)

Contents

[edit] Parameters

first, last - the range of elements to examine
value - value to compare the elements to
policy - the execution policy to use. See execution policy for details.
p - unary predicate which returns ​true for the required element.

The expression p(v) must be convertible to bool for every argument v of type (possibly const) VT, where VT is the value type of InputIt, regardless of value category, and must not modify v. Thus, a parameter type of VT&is not allowed, nor is VT unless for VT a move is equivalent to a copy(since C++11). ​

q - unary predicate which returns ​false for the required element.

The expression q(v) must be convertible to bool for every argument v of type (possibly const) VT, where VT is the value type of InputIt, regardless of value category, and must not modify v. Thus, a parameter type of VT&is not allowed, nor is VT unless for VT a move is equivalent to a copy(since C++11). ​

Type requirements
-
InputIt must meet the requirements of LegacyInputIterator.
-
ForwardIt must meet the requirements of LegacyForwardIterator.
-
UnaryPredicate must meet the requirements of Predicate.

[edit] Return value

The first iterator it in the range [firstlast) satisfying the following condition or last if there is no such iterator:

1,2) *it == value is true.
3,4) p(*it) is true.
5,6) q(*it) is false.

[edit] Complexity

Given N as std::distance(first, last):

1,2) At most N comparisons with value using operator==.
3,4) At most N applications of the predicate p.
5,6) At most N applications of the predicate q.

[edit] Exceptions

The overloads with a template parameter named ExecutionPolicy report errors as follows:

  • If execution of a function invoked as part of the algorithm throws an exception and ExecutionPolicy is one of the standard policies, std::terminate is called. For any other ExecutionPolicy, the behavior is implementation-defined.
  • If the algorithm fails to allocate memory, std::bad_alloc is thrown.

[edit] Possible implementation

find
template<class InputIt, class T = typename std::iterator_traits<InputIt>::value_type>
constexpr InputIt find(InputIt first, InputIt last, const T& value)
{
    for (; first != last; ++first)
        if (*first == value)
            return first;
 
    return last;
}
find_if
template<class InputIt, class UnaryPred>
constexpr InputIt find_if(InputIt first, InputIt last, UnaryPred p)
{
    for (; first != last; ++first)
        if (p(*first))
            return first;
 
    return last;
}
find_if_not
template<class InputIt, class UnaryPred>
constexpr InputIt find_if_not(InputIt first, InputIt last, UnaryPred q)
{
    for (; first != last; ++first)
        if (!q(*first))
            return first;
 
    return last;
}

[edit] Notes

If C++11 is not available, an equivalent to std::find_if_not is to use std::find_if with the negated predicate.

template<class InputIt, class UnaryPred>
InputIt find_if_not(InputIt first, InputIt last, UnaryPred q)
{
    return std::find_if(first, last, std::not1(q));
}
Feature-test macro Value Std Feature
__cpp_lib_algorithm_default_value_type 202403 (C++26) List-initialization for algorithms (1,2)

[edit] Example

The following example finds numbers in given sequences.

#include <algorithm>
#include <array>
#include <cassert>
#include <complex>
#include <iostream>
#include <vector>
 
int main()
{
    const auto v = {1, 2, 3, 4};
 
    for (const int n : {3, 5})
        (std::find(v.begin(), v.end(), n) == std::end(v))
            ? std::cout << "v does not contain " << n << '\n'
            : std::cout << "v contains " << n << '\n';
 
    auto is_even = [](int i) { return i % 2 == 0; };
 
    for (const auto& w : {std::array{3, 1, 4}, {1, 3, 5}})
        if (auto it = std::find_if(begin(w), end(w), is_even); it != std::end(w))
            std::cout << "w contains an even number " << *it << '\n';
        else
            std::cout << "w does not contain even numbers\n";
 
    std::vector<std::complex<double>> nums{{4, 2}};
    #ifdef __cpp_lib_algorithm_default_value_type
        // T gets deduced making list-initialization possible
        const auto it = std::find(nums.begin(), nums.end(), {4, 2});
    #else
        const auto it = std::find(nums.begin(), nums.end(), std::complex<double>{4, 2});
    #endif
    assert(it == nums.begin());   
}

Output:

v contains 3
v does not contain 5
w contains an even number 4
w does not contain even numbers

[edit] Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR Applied to Behavior as published Correct behavior
LWG 283 C++98 T was required to be EqualityComparable, but
the value type of InputIt might not be T
removed the requirement

[edit] See also

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