◐ Shell
reader mode source ↗
From cppreference.com
 
C++
 
Algorithm library
Constrained algorithms and algorithms on ranges (C++20)
Constrained algorithms, e.g. ranges::copy, ranges::sort, ...
Non-modifying sequence operations    
Batch operations
(C++17)
Search operations
Modifying sequence operations
Copy operations
(C++11)
(C++11)
(C++11)
Swap operations
Transformation operations
Generation operations
Removing operations
Order-changing operations
(until C++17)(C++11)
(C++20)(C++20)
Sampling operations
(C++17)

Sorting and related operations
Partitioning operations
(C++11)    
(C++11)

Sorting operations
Binary search operations
(on partitioned ranges)
Set operations (on sorted ranges)
Merge operations (on sorted ranges)
Heap operations
Minimum/maximum operations
(C++11)
(C++17)
Lexicographical comparison operations
(C++20)
Permutation operations


 
Defined in header <algorithm> 
template< class InputIt, class ForwardIt >
InputIt find_first_of( InputIt first1, InputIt last1,
                       ForwardIt first2, ForwardIt last2 );
(1) (constexpr since C++20) 
template< class InputIt, class ForwardIt, class BinaryPred >
InputIt find_first_of( InputIt first1, InputIt last1,
                       ForwardIt first2, ForwardIt last2,
                       BinaryPred p );
(2) (constexpr since C++20) 
template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2 >
ForwardIt1 find_first_of( ExecutionPolicy&& policy,
                          ForwardIt1 first1, ForwardIt1 last1,
                          ForwardIt2 first2, ForwardIt2 last2 );
(3) (since C++17) 
template< class ExecutionPolicy,
          class ForwardIt1, class ForwardIt2, class BinaryPred >
ForwardIt1 find_first_of( ExecutionPolicy&& policy,
                          ForwardIt1 first1, ForwardIt last1,
                          ForwardIt2 first2, ForwardIt2 last2,
                          BinaryPred p );
(4) (since C++17)

Searches the source range [first1, last1) for any of the elements in the target range [first2, last2).

1) Elements are compared using operator==.
2) Elements are compared using the given binary predicate p.
3,4) Same as (1,2), but executed according to policy.
These overloads participate in overload resolution only if the value of the following expression is true:

std::is_execution_policy_v<std::decay_t<ExecutionPolicy>>

(until C++20)

std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>>

(since C++20)

Parameters

first1, last1 - the pair of iterators defining the source range
first2, last2 - the pair of iterators defining the target range
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. ​

policy - the execution policy to use
Type requirements
-
InputIt must meet the requirements of LegacyInputIterator.
-
ForwardIt must meet the requirements of LegacyForwardIterator.
-
ForwardIt1 must meet the requirements of LegacyForwardIterator.
-
ForwardIt2 must meet the requirements of LegacyForwardIterator.
-
BinaryPred must meet the requirements of BinaryPredicate.

Return value

Iterator to the first element in the source range that matches an element from the target range.

If the target range is empty or if no such element is found, last1 is returned.

Complexity

Given \(\scriptsize N_1\)N1 as std::distance(first1, last1) and \(\scriptsize N_2\)N2 as std::distance(first2, last2):

1) At most \(\scriptsize N_1 \cdot N_2\)N1⋅N2 comparisons using operator==.
2) At most \(\scriptsize N_1 \cdot N_2\)N1⋅N2 applications of p.
3) \(\scriptsize \mathcal{O}(N_1 \cdot N_2)\)𝓞(N1⋅N2) comparisons using operator==.
4) \(\scriptsize \mathcal{O}(N_1 \cdot N_2)\)𝓞(N1⋅N2) applications of p.

Exceptions

3,4) During the execution process:
  • If the temporary memory resources required for parallelization are not available, std::bad_alloc is thrown.
  • If an uncaught exception is thrown while accessing objects via an algorithm argument, the behavior is determined by the execution policy (for standard policies, std::terminate is invoked).

Possible implementation

find_first_of (1) 
template<class InputIt, class ForwardIt>
InputIt find_first_of(InputIt first1, InputIt last1,
                      ForwardIt first2, ForwardIt last2)
{
    for (; first1 != last1; ++first1)
        for (ForwardIt it = first2; it != last2; ++it)
            if (*first1 == *it)
                return first1;
    return last1;
}
find_first_of (2) 
template<class InputIt, class ForwardIt, class BinaryPred>
InputIt find_first_of(InputIt first1, InputIt last1,
                      ForwardIt first2, ForwardIt last2,
                      BinaryPred p)
{
    for (; first1 != last1; ++first1)
        for (ForwardIt it = first2; it != last2; ++it)
            if (p(*first1, *it))
                return first1;
    return last1;
}

Example

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

Run this code
#include <algorithm>
#include <iostream>
#include <vector>

auto print_sequence = [](const auto id, const auto& seq, int pos = -1)
{
    std::cout << id << "{ ";
    for (int i{}; const auto& e : seq)
    {
        const bool mark{i == pos};
        std::cout << (i++ ? ", " : "");
        std::cout << (mark ? "[ " : "") << e << (mark ? " ]" : "");
    }
    std::cout << " }\n";
};

int main()
{
    const std::vector<int> v{0, 2, 3, 25, 5};
    const auto t1 = {19, 10, 3, 4};
    const auto t2 = {1, 6, 7, 9};
    
    auto find_any_of = [](const auto& v, const auto& t)
    {
        const auto result = std::find_first_of(v.begin(), v.end(),
                                               t.begin(), t.end());
        if (result == v.end())
        {
            std::cout << "No elements of v are equal to any element of ";
            print_sequence("t = ", t);
            print_sequence("v = ", v);
        }
        else
        {
            const auto pos = std::distance(v.begin(), result);
            std::cout << "Found a match (" << *result << ") at position " << pos;
            print_sequence(", where t = ", t);
            print_sequence("v = ", v, pos);
        }
    };
    
    find_any_of(v, t1);
    find_any_of(v, t2);
}

Output: 

Found a match (3) at position 2, where t = { 19, 10, 3, 4 }
v = { 0, 2, [ 3 ], 25, 5 }
No elements of v are equal to any element of t = { 1, 6, 7, 9 }
v = { 0, 2, 3, 25, 5 }

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR Applied to Behavior as published Correct behavior
LWG 576 C++98 first1 and last1 needed to be LegacyForwardIterators they only need to be LegacyInputIterators
LWG 1205 C++98 the return value was unclear if the target range is empty returns last1 in this case

See also

(C++20)
 searches for any one of a set of elements
(algorithm function object)[edit]
(C++11)
 finds the first element satisfying specific criteria
(function template & algorithm function object)[edit]
(C++20)(C++20)(C++20)