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# std::find_first_of

From cppreference.com
< 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)
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)
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

### [edit] 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); The signature does not need to have const &, but the function must not modify the objects passed to it. 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 `InputIterator`.
- `ForwardIt1` must meet the requirements of `ForwardIterator`.
- `ForwardIt2` must meet the requirements of `ForwardIterator`.

### [edit] 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.

### [edit] Complexity

Does at most `(S*N)` comparisons where S = distance(s_first, s_last) and N = distance(first, last).

### [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 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;
    }

### [edit] 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

### [edit] See also

find find_if find_if_not (C++11) | finds the first element satisfying specific criteria (function template) [edit]

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