std::find_first_of

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

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