set_difference()

Library:  Algorithms

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

#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);
}

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.

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

//
//  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 }

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

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