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

template< class InputIt, class T >
InputIt find( InputIt first, InputIt last, const T& value );

template< class InputIt, class T >
constexpr InputIt find( InputIt first, InputIt last, const T& value );

template< class ExecutionPolicy, class ForwardIt, class T >
ForwardIt find( ExecutionPolicy&& policy,
                ForwardIt first, ForwardIt last, const T& value );

template< class InputIt, class UnaryPredicate >
InputIt find_if( InputIt first, InputIt last, UnaryPredicate p );

template< class InputIt, class UnaryPredicate >
constexpr InputIt find_if( InputIt first, InputIt last, UnaryPredicate p );

template< class ExecutionPolicy, class ForwardIt, class UnaryPredicate >
ForwardIt find_if( ExecutionPolicy&& policy,
                   ForwardIt first, ForwardIt last, UnaryPredicate p );

template< class InputIt, class UnaryPredicate >
InputIt find_if_not( InputIt first, InputIt last, UnaryPredicate q );

template< class InputIt, class UnaryPredicate >
constexpr InputIt find_if_not( InputIt first, InputIt last, UnaryPredicate q );

template< class ExecutionPolicy, class ForwardIt, class UnaryPredicate >
ForwardIt find_if_not( ExecutionPolicy&& policy,
                       ForwardIt first, ForwardIt last, UnaryPredicate q );

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

Parameters

Return value

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

Complexity

Given N as 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

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;
}

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;
}

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

#include <algorithm>
#include <array>
#include <iostream>
 
int main()
{
    const auto v = {1, 2, 3, 4};
 
    for (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 (auto const& 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";
}

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

Defect reports

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

See also