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Difference between revisions of "cpp/algorithm/find first of"

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Revision as of 02:35, 28 July 2015

 
C++
 
Algorithm library
Constrained algorithms and algorithms on ranges (C++20)
Constrained algorithms, e.g. ranges::copy, ranges::sort, ...
Execution policies (C++17)
Non-modifying sequence operations
Batch operations
(C++17)
Search operations
(C++11)                (C++11)(C++11)

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
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
C library
Numeric operations
Operations on uninitialized memory
 
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)
(2)
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)

Searches the range [first, last) for any of the elements in the range [s_first, s_last). The first version uses operator== to compare the elements, the second version uses the given binary predicate p.

Contents

Parameters

first, last - the range of elements to examine
s_first, s_last - the range of elements to search for
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

Template:par req concept Template:par req concept Template:par req concept

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).

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

(C++11)
 finds the first element satisfying specific criteria
(function template) [edit]
(parallelism TS)
 parallelized version of std::find_first_of
(function template) [edit]
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