# std::find_first_of

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< cpp‎ | algorithm

Defined in header `<algorithm>` |  |
| (1) |
template< class ForwardIt1, class ForwardIt2 > ForwardIt1 find_first_of( ForwardIt1 first, ForwardIt1 last, ForwardIt2 s_first, ForwardIt2 s_last ); |  | (until C++11)
template< class InputIt, class ForwardIt > InputIt find_first_of( InputIt first, InputIt last, ForwardIt s_first, ForwardIt s_last ); |  | (since C++11) (until C++20)
template< class InputIt, class ForwardIt > constexpr InputIt find_first_of( InputIt first, InputIt last, ForwardIt s_first, ForwardIt s_last ); |  | (since C++20)
template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2 > ForwardIt1 find_first_of( ExecutionPolicy&& policy, ForwardIt1 first, ForwardIt1 last, ForwardIt2 s_first, ForwardIt2 s_last ); | (2) | (since C++17)
| (3) |
template< class ForwardIt1, class ForwardIt2, class BinaryPredicate > ForwardIt1 find_first_of( ForwardIt1 first, ForwardIt1 last, ForwardIt2 s_first, ForwardIt2 s_last, BinaryPredicate p ); |  | (until C++11)
template< class InputIt, class ForwardIt, class BinaryPredicate > InputIt find_first_of( InputIt first, InputIt last, ForwardIt s_first, ForwardIt s_last, BinaryPredicate p ); |  | (since C++11) (until C++20)
template< class InputIt, class ForwardIt, class BinaryPredicate > constexpr InputIt find_first_of( InputIt first, InputIt last, ForwardIt s_first, ForwardIt s_last, BinaryPredicate p ); |  | (since C++20)
template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2, class BinaryPredicate > ForwardIt1 find_first_of( ExecutionPolicy&& policy, ForwardIt1 first, ForwardIt last, ForwardIt2 s_first, ForwardIt2 s_last, BinaryPredicate p ); | (4) | (since C++17)

Searches the range `[first, last)` for any of the elements in the range `[s_first, s_last)`.

1) Elements are compared using `operator==`.
3) Elements are compared using the given binary predicate `p`.
2,4) Same as (1,3), 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
s_first, s_last | - | the range of elements to search for
policy | - | the execution policy to use. See execution policy for details.

p

-

binary predicate which returns ​true if the elements should be treated as equal.

The signature of the predicate function should be equivalent to the following:

bool pred(const Type1 &a, const Type2 &b);

While the signature does not need to have const &, the function must not modify the objects passed to it and must be able to accept all values of type (possibly const) `Type1` and `Type2` regardless of value category (thus, Type1 & is not allowed, nor is Type1 unless for `Type1` a move is equivalent to a copy (since C++11)).
The types Type1 and Type2 must be such that objects of types ForwardIt1 and ForwardIt2 can be dereferenced and then implicitly converted to Type1 and Type2 respectively. ​

Type requirements
- `InputIt` must meet the requirements of LegacyInputIterator.
- `ForwardIt1` must meet the requirements of LegacyForwardIterator.
- `ForwardIt2` must meet the requirements of LegacyForwardIterator.

### Return value

Iterator to the first element in the range `[first, last)` that is equal to an element from the range `[s_first; s_last)`. If no such element is found, `last` is returned.

### Complexity

Does at most `(S*N)` comparisons where S = distance(s_first, s_last) and N = 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 ForwardIt>
    InputIt find_first_of(InputIt first, InputIt last,
                          ForwardIt s_first, ForwardIt s_last)
    {
        for (; first != last; ++first) {
            for (ForwardIt it = s_first; it != s_last; ++it) {
                if (*first == *it) {
                    return first;
                }
            }
        }
        return last;
    }

Second version

    template<class InputIt, class ForwardIt, class BinaryPredicate>
    InputIt find_first_of(InputIt first, InputIt last,
                          ForwardIt s_first, ForwardIt s_last,
                          BinaryPredicate p)
    {
        for (; first != last; ++first) {
            for (ForwardIt it = s_first; it != s_last; ++it) {
                if (p(*first, *it)) {
                    return first;
                }
            }
        }
        return last;
    }

### Example

The following code searches for any of specified integers in a vector of integers:

Run this code

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

    int main()
    {
        std::vector<int> v{0, 2, 3, 25, 5};
        std::vector<int> t{3, 19, 10, 2};

        auto result = std::find_first_of(v.begin(), v.end(), t.begin(), t.end());

        if (result == v.end()) {
            std::cout << "no elements of v were equal to 3, 19, 10 or 2\n";
        } else {
            std::cout << "found a match at "
                      << std::distance(v.begin(), result) << "\n";
        }
     }

Output:

    found a match at 1

### See also

findfind_iffind_if_not (C++11) | finds the first element satisfying specific criteria (function template)

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