# 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 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`. This overload only participates in overload resolution if std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> 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 `last` - `first` applications of the predicate

### 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 an integer in a vector of integers.

Run this code

    #include <iostream>
    #include <algorithm>
    #include <vector>
    #include <iterator>

    int main()
    {
        int n1 = 3;
        int n2 = 5;

        std::vector<int> v{0, 1, 2, 3, 4};

        auto result1 = std::find(std::begin(v), std::end(v), n1);
        auto result2 = std::find(std::begin(v), std::end(v), n2);

        if (result1 != std::end(v)) {
            std::cout << "v contains: " << n1 << '\n';
        } else {
            std::cout << "v does not contain: " << n1 << '\n';
        }

        if (result2 != std::end(v)) {
            std::cout << "v contains: " << n2 << '\n';
        } else {
            std::cout << "v does not contain: " << n2 << '\n';
        }
    }

Output:

    v contains: 3
    v does not contain: 5

### 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)

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