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

Defined in header <algorithm>
(1) 
template< class InputIt, class T >
InputIt find( InputIt first, InputIt last, const T& value );
 (until C++20) 
template< class InputIt, class T >
constexpr InputIt find( InputIt first, InputIt last, const T& value );
 (since C++20) 
template< class ExecutionPolicy, class ForwardIt, class T >
ForwardIt find( ExecutionPolicy&& policy, ForwardIt first, ForwardIt last,
                const T& value );
 (2) (since C++17)
(3) 
template< class InputIt, class UnaryPredicate >
InputIt find_if( InputIt first, InputIt last, 
                 UnaryPredicate p );
 (until C++20) 
template< class InputIt, class UnaryPredicate >
constexpr InputIt find_if( InputIt first, InputIt last, 
                           UnaryPredicate p );
 (since C++20) 
template< class ExecutionPolicy, class ForwardIt, class UnaryPredicate >
ForwardIt find_if( ExecutionPolicy&& policy, ForwardIt first, ForwardIt last, 
                   UnaryPredicate p );
 (4) (since C++17)
(5) 
template< class InputIt, class UnaryPredicate >
InputIt find_if_not( InputIt first, InputIt last, 
                     UnaryPredicate q );
 (since C++11)
(until C++20) 
template< class InputIt, class UnaryPredicate >
constexpr InputIt find_if_not( InputIt first, InputIt last, 
                               UnaryPredicate q );
 (since C++20) 
template< class ExecutionPolicy, class ForwardIt, class UnaryPredicate >
ForwardIt find_if_not( ExecutionPolicy&& policy, ForwardIt first, ForwardIt last, 
                       UnaryPredicate q );
 (6) (since C++17)

Returns an iterator to 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 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 do not participate in overload resolution unless std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> (until C++20) std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>> (since C++20) is true.

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.

Return value

Iterator to the first element satisfying the condition or last if no such element is found.

Complexity

At most N applications of the predicate, where N = std::distance(first, last).

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.

Possible implementation

First version 
template<class InputIt, class T>
constexpr InputIt find(InputIt first, InputIt last, const T& value)
{
    for (; first != last; ++first) {
        if (*first == value) {
            return first;
        }
    }
    return last;
}
Second version 
template<class InputIt, class UnaryPredicate>
constexpr InputIt find_if(InputIt first, InputIt last, UnaryPredicate p)
{
    for (; first != last; ++first) {
        if (p(*first)) {
            return first;
        }
    }
    return last;
}
Third version 
template<class InputIt, class UnaryPredicate>
constexpr InputIt find_if_not(InputIt first, InputIt last, UnaryPredicate q)
{
    for (; first != last; ++first) {
        if (!q(*first)) {
            return first;
        }
    }
    return last;
}

Notes

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

template<class InputIt, class UnaryPredicate>
InputIt find_if_not(InputIt first, InputIt last, UnaryPredicate q)
{
    return std::find_if(first, last, std::not1(q));
}

Example

The following example finds integers in given vector.

#include <iostream>
#include <algorithm>
#include <vector>
#include <iterator>
 
int main()
{
    std::vector<int> v{1, 2, 3, 4};
    int n1 = 3;
    int n2 = 5;
    auto is_even = [](int i){ return i%2 == 0; };
 
    auto result1 = std::find(begin(v), end(v), n1);
    auto result2 = std::find(begin(v), end(v), n2);
    auto result3 = std::find_if(begin(v), end(v), is_even);
 
    (result1 != std::end(v))
        ? std::cout << "v contains " << n1 << '\n'
        : std::cout << "v does not contain " << n1 << '\n';
 
    (result2 != std::end(v))
        ? std::cout << "v contains " << n2 << '\n'
        : std::cout << "v does not contain " << n2 << '\n';
 
    (result3 != std::end(v))
        ? std::cout << "v contains an even number: " << *result3 << '\n'
        : std::cout << "v does not contain even numbers\n";
}

Output:

v contains 3
v does not contain 5
v contains an even number: 2

See also

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

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Last Updated 2022-06-02T22:02:33Z