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std::set_intersection

From cppreference.com
< cpp‎ | algorithm
 
 
Algorithm library
Execution policies (C++17)
Non-modifying sequence operations
(C++11)(C++11)(C++11)
(C++17)
Modifying sequence operations
Operations on uninitialized storage
Partitioning operations
Sorting operations
(C++11)
Binary search operations
Set operations (on sorted ranges)
set_intersection
Heap operations
(C++11)
Minimum/maximum operations
(C++11)
(C++17)

Permutations
Numeric operations
C library
 
Defined in header <algorithm>
template< class InputIt1, class InputIt2, class OutputIt >

OutputIt set_intersection( InputIt1 first1, InputIt1 last1,
                           InputIt2 first2, InputIt2 last2,

                           OutputIt d_first );
(1)
template< class ExecutionPolicy, class InputIt1, class InputIt2, class OutputIt >

OutputIt set_intersection( ExecutionPolicy&& policy, InputIt1 first1, InputIt1 last1,
                           InputIt2 first2, InputIt2 last2,

                           OutputIt d_first );
(2) (since C++17)
template< class InputIt1, class InputIt2,

          class OutputIt, class Compare >
OutputIt set_intersection( InputIt1 first1, InputIt1 last1,
                           InputIt2 first2, InputIt2 last2,

                           OutputIt d_first, Compare comp );
(3)
template< class ExecutionPolicy, class InputIt1, class InputIt2,

          class OutputIt, class Compare >
OutputIt set_intersection( ExecutionPolicy&& policy, InputIt1 first1, InputIt1 last1,
                           InputIt2 first2, InputIt2 last2,

                           OutputIt d_first, Compare comp );
(4) (since C++17)

Constructs a sorted range beginning at d_first consisting of elements that are found in both sorted ranges [first1, last1) and [first2, last2). If some element is found m times in [first1, last1) and n times in [first2, last2), the first std::min(m, n) elements will be copied from the first range to the destination range. The order of equivalent elements is preserved. The resulting range cannot overlap with either of the input ranges.

1) Elements are compared using operator< and the ranges must be sorted with respect to the same.
3) Elements are compared using the given binary comparison function comp and the ranges must be sorted with respect to the same.
2,4) Same as (1,3), but executed according to policy. These overloads do not participate in overload resolution unless std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is true

Contents

[edit] Parameters

first1, last1 - the first range of elements to examine
first2, last2 - the second range of elements to examine
policy - the execution policy to use. See execution policy for details.
comp - comparison function object (i.e. an object that satisfies the requirements of Compare) which returns ​true if the first argument is less than (i.e. is ordered before) the second.

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

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

The signature does not need to have const &, but the function object must not modify the objects passed to it.
The types Type1 and Type2 must be such that objects of types InputIt1 and InputIt2 can be dereferenced and then implicitly converted to both Type1 and Type2. ​

Type requirements
-
InputIt1 must meet the requirements of InputIterator.
-
InputIt2 must meet the requirements of InputIterator.
-
OutputIt must meet the requirements of OutputIterator.

[edit] Return value

Iterator past the end of the constructed range.

[edit] Complexity

At most 2·(N1+N2-1) comparisons, where N1 = std::distance(first1, last1) and N2 = std::distance(first2, last2).

[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, std::terminate is called.
  • If the algorithm fails to allocate memory, std::bad_alloc is thrown.

[edit] Possible implementation

First version
template<class InputIt1, class InputIt2, class OutputIt>
OutputIt set_intersection(InputIt1 first1, InputIt1 last1,
                          InputIt2 first2, InputIt2 last2,
                          OutputIt d_first)
{
    while (first1 != last1 && first2 != last2) {
        if (*first1 < *first2) {
            ++first1;
        } else  {
            if (!(*first2 < *first1)) {
                *d_first++ = *first1++;
            }
            ++first2;
        }
    }
    return d_first;
}
Second version
template<class InputIt1, class InputIt2,
         class OutputIt, class Compare>
OutputIt set_intersection(InputIt1 first1, InputIt1 last1,
                          InputIt2 first2, InputIt2 last2,
                          OutputIt d_first, Compare comp)
{
    while (first1 != last1 && first2 != last2) {
        if (comp(*first1, *first2)) {
            ++first1;
        } else {
            if (!comp(*first2, *first1)) {
                *d_first++ = *first1++;
            }
            ++first2;
        }
    }
    return d_first;
}


[edit] Example

#include <iostream>
#include <vector>
#include <algorithm>
#include <iterator>
int main()
{
    std::vector<int> v1{1,2,3,4,5,6,7,8};
    std::vector<int> v2{        5,  7,  9,10};
    std::sort(v1.begin(), v1.end());
    std::sort(v2.begin(), v2.end());
 
    std::vector<int> v_intersection;
 
    std::set_intersection(v1.begin(), v1.end(),
                          v2.begin(), v2.end(),
                          std::back_inserter(v_intersection));
    for(int n : v_intersection)
        std::cout << n << ' ';
}

Output:

5 7

[edit] See also

computes the union of two sets
(function template) [edit]
parallelized version of std::set_intersection
(function template) [edit]