## set_difference

    Algorithm

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

### Summary

A basic set operation for constructing a sorted difference.

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

None

### Synopsis

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

### Description

The set_difference algorithm constructs a sorted difference that includes copies of the elements that are present in the range `[first1, last1)` but are not present in the range `[first2, last2)`. It returns the end of the constructed range.

As an example, assume we have the following two sets:

`1 2 3 4 5`

and

`3 4 5 6 7`

The result of applying set_difference is the set:

`1 2`

The result of set_difference is undefined if the result range overlaps with either of the original ranges.

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

Use the set_symetric_difference algorithm to return a result that contains all elements that are not in common between the two sets.

### Complexity

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

### Example

    //
    // set_diff.cpp
    //
    #include <algorithm>
    #include <set>
    #include <iostream>
    using namespace std;
    int main()
     {
    //Initialize some sets
    int a1[10] = {1,2,3,4,5,6,7,8,9,10};
    int a2[6]  = {2,4,6,8,10,12};
    set<int, less<int> > all(a1+0, a1+10), even(a2+0, a2+6),
                         odd;

    //Create an insert_iterator for odd
    insert_iterator<set<int, less<int> > >
                    odd_ins(odd, odd.begin());

    //Demonstrate set_difference
    cout << "The result of:" << endl << "{";
    copy(all.begin(),all.end(),
         ostream_iterator<int,char>(cout," "));
    cout << "} - {";
    copy(even.begin(),even.end(),
         ostream_iterator<int,char>(cout," "));
    cout << "} =" << endl << "{";
    set_difference(all.begin(), all.end(),
                   even.begin(), even.end(), odd_ins);
    copy(odd.begin(),odd.end(),
         ostream_iterator<int,char>(cout," "));
    cout << "}" << endl << endl;
    return 0;
     }

Program Output

    The result of:
    {1 2 3 4 5 6 7 8 9 10 } - {2 4 6 8 10 12 } =
    {1 3 5 7 9 }

### 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_intersection, set_symmetric_difference

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