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

From cppreference.com
< cpp‎ | algorithm

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

### [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 signature of the predicate function should be equivalent to the following: bool pred(const Type &a); The signature does not need to have const &, but the function must not modify the objects passed to it. The type Type must be such that an object of type InputIt can be dereferenced and then implicitly converted to Type. ​
q | - | unary predicate which returns ​false for the required element. The signature of the predicate function should be equivalent to the following: bool pred(const Type &a); The signature does not need to have const &, but the function must not modify the objects passed to it. The type Type must be such that an object of type InputIt can be dereferenced and then implicitly converted to Type. ​
Type requirements
- `InputIt` must meet the requirements of `InputIterator`.
- `ForwardIt` must meet the requirements of `ForwardIterator`.
- `UnaryPredicate` must meet the requirements of `Predicate`.

### [edit] Return value

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

### [edit] Complexity

At most `last` - `first` applications of the predicate

### [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 three 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

First version
    template<class InputIt, class T>
    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>
    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>
    InputIt find_if_not(InputIt first, InputIt last, UnaryPredicate q)
    {
        for (; first != last; ++first) {
            if (!q(*first)) {
                return first;
            }
        }
        return last;
    }

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));
    }

### [edit] 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

### [edit] See also

adjacent_find | finds the first two adjacent items that are equal (or satisfy a given predicate) (function template) [edit]
find_end | finds the last sequence of elements in a certain range (function template) [edit]
find_first_of | searches for any one of a set of elements (function template) [edit]
mismatch | finds the first position where two ranges differ (function template) [edit]
search | searches for a range of elements (function template) [edit]

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