Apache C++ Standard Library Reference Guide

## set_intersection()

Library: Algorithms

    Function

- Local Index
- Summary
- Synopsis
- Description
- Complexity
- Example
- See Also
- Standards Conformance

### Local Index

No Entries

### Summary

A basic set operation (algorithm) for constructing a sorted intersection

### Synopsis

    #include <algorithm>

    namespace std {
      template <class InputIterator1, class InputIterator2,
                class OutputIterator>
      OutputIterator
      set_intersection(InputIterator1 start1,
                       InputIterator1 finish1,
                       InputIterator2 start2,
                       InputIterator2 finish2,
                       OutputIterator result);
      template <class InputIterator1, class InputIterator2,
                class OutputIterator, class Compare>
      OutputIterator
      set_intersection(InputIterator1 start1,
                       InputIterator1 finish1,
                       InputIterator2 start2,
                       InputIterator2 finish2,
                       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 `operator<()`, unless an alternative comparison function object (`comp`) is provided.

### Complexity

At most `((finish1 - start1) + (finish2 - start2)) * 2 -1` comparisons are performed.

### Example

    //
    //  set_intr.cpp
    //

    #include <algorithm>
    #include <set>
    #include <iostream>

    int main ()
    {
    #ifndef _RWSTD_NO_NAMESPACE
      using namespace std;
    #endif

      // Initialize some sets.

      int a1[10] = {1,3,5,7,9,11};
      int a3[4]  = {3,5,7,8};
      set<int, less<int>,allocator<int>  > odd(a1+0, a1+6),
                                           result,
                                           smalll(a3+0, a3+4);

      // Create an insert_iterator for result.

      insert_iterator<set<int, less<int>,allocator<int> > >
                      res_ins(result, result.begin());

      // Demonstrate set_intersection.

      cout << "The result of:" << endl << "{";
      copy(smalll.begin(),smalll.end(),
           ostream_iterator<int,char,char_traits<char> >
                           (cout," "));
      cout << "} intersection {";
      copy(odd.begin(),odd.end(),
           ostream_iterator<int,char,char_traits<char> >
                           (cout," "));
      cout << "} =" << endl << "{";

      set_intersection(smalll.begin(),smalll.end(),
                       odd.begin(),odd.end(),res_ins);
      copy(result.begin(),result.end(),
           ostream_iterator<int,char,char_traits<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 }

### See Also

`includes()`, Iterators, `set_union()`, `set_difference()`, `set_symmetric_difference()`

### Standards Conformance

ISO/IEC 14882:1998 -- International Standard for Information Systems -- Programming Language C++, 25.3.5.3