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

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< cpp‎ | algorithm

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 (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 do not participate in overload resolution unless

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)

### 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

The first iterator it in the range `[`first`,` last`)` 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

### 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

### 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

find
    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;
    }
find_if
    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;
    }
find_if_not
    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 std::vector.

Run this code

    #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";
    }

Output:

    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.

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

### See also

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

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