## set_intersection

    Algorithm

- Summary
- Data Type and Member Function Indexes
- Synopsis
- Description
- Complexity
- Example
- Warnings
- See Also

### Summary

A basic set operation for constructing a sorted intersection.

### Data Type and Member Function Indexes
(exclusive of constructors and destructors)

None

### Synopsis

    #include <algorithm>
    template <class InputIterator1, class InputIterator2,
              class OutputIterator>
      OutputIterator
        set_intersection (InputIterator1 first1,
                          InputIterator1 last1,
                          InputIterator2 first2,
                          InputIterator last2,
                          OutputIterator result);
    template <class InputIterator1, class InputIterator2,
              class OutputIterator, class Compare>
      OutputIterator
        set_intersection (InputIterator1 first1,
                          InputIterator1 last1,
                          InputIterator2 first2,
                          InputIterator2 last2,
                          OutputIterator result, Compare comp);

### Description

The set_intersection algorithm constructs a sorted intersection of elements from the two ranges. It returns the end of the constructed range. When it finds an element present in both ranges, set_intersection always copies the element from the first range into `result`. This means that the result of set_intersection is guaranteed to be stable. The result of set_intersection is undefined if the result range overlaps with either of the original ranges.

set_intersection assumes that the ranges are sorted using the default comparison operator less than (`<`), unless an alternative comparison operator (`comp`) is provided.

### Complexity

At most `((last1 - first1) + (last2 - first2)) * 2 -1` comparisons are performed.

### Example

    //
    // set_intr.cpp
    //
    #include <algorithm>
    #include <set>
    #include <iostream>
    using namespace std;
    int main()
     {

    //Initialize some sets
    int a1[10] = {1,3,5,7,9,11};
    int a3[4]  = {3,5,7,8};
    set<int, less<int> > odd(a1+0, a1+6),
        result, small(a3+0,a3+4);
    //Create an insert_iterator for result
    insert_iterator<set<int, less<int> > >
        res_ins(result, result.begin());
    //Demonstrate set_intersection
    cout << "The result of:" << endl << "{";
    copy(small.begin(),small.end(),
         ostream_iterator<int,char>(cout," "));
    cout << "} intersection {";
    copy(odd.begin(),odd.end(),
         ostream_iterator<int,char>(cout," "));
    cout << "} =" << endl << "{";
    set_intersection(small.begin(), small.end(),
                     odd.begin(), odd.end(), res_ins);
    copy(result.begin(),result.end(),
         ostream_iterator<int,char>(cout," "));
    cout << "}" << endl << endl;
    return 0;
     }

Program Output

    The result of:
    {3 5 7 8 } intersection {1 3 5 7 9 11 } =
    {3 5 7 }

### Warnings

If your compiler does not support default template parameters, then you always need to supply the `Compare` template argument and the `Allocator` template argument. For instance, you need to write:

`set<int, less<int> allocator<int> >`

instead of:

`set<int>`

If your compiler does not support namespaces, then you do not need the using declaration for `std`.

### See Also

includes, set, set_union, set_difference, set_symmetric_difference

Copyright (c) 1998, Rogue Wave Software, Inc.
Send mail to report errors or comment on the documentation.
OEM Release, June 1998