Apache C++ Standard Library Reference Guide

## set_difference()

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 difference

### Synopsis

    #include <algorithm>

    namespace std {
      template <class InputIterator1, class InputIterator2,
                class OutputIterator>
      OutputIterator
      set_difference(InputIterator1 start1,
                     InputIterator1 finish1,
                     InputIterator2 start2,
                     InputIterator2 finish2,
                     OutputIterator result);
      template <class InputIterator1, class InputIterator2,
                class OutputIterator, class Compare>
      OutputIterator
      set_difference(InputIterator1 start1,
                     InputIterator1 finish1,
                     InputIterator2 start2,
                     InputIterator2 finish2,
                     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 `[start1, finish1)` but are not present in the range `[start2, finish2)`. 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 `operator<()`, unless an alternative comparison function object (`comp`) is provided.

Use the `set_symetric_difference()` algorithm to obtain a result that contains all elements that are not in common between the two sets.

### Complexity

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

### Example

    //
    //  set_diff.cpp
    //

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

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

      // 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>,allocator<int> > all(a1+0, a1+10),
                                          even(a2+0, a2+6), odd;

      // Create an insert_iterator for odd.

      insert_iterator<set <int, less<int>,allocator<int> > >
                     odd_ins(odd, odd.begin());

      // Demonstrate set_difference.

      cout << "The result of:" << endl << "{";
      copy(all.begin(),all.end(),
           ostream_iterator<int,char,char_traits<char> >
                           (cout," "));
      cout << "} - {";
      copy(even.begin(),even.end(),
           ostream_iterator<int,char,char_traits<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,char_traits<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 }

### See Also

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

### Standards Conformance

ISO/IEC 14882:1998 -- International Standard for Information Systems -- Programming Language C++, Section 25.3.5.4