merge

Algorithm

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• Data Type and Member Function Indexes
• Synopsis
• Description
• Complexity
• Example
• Warning
• See Also

Merge two sorted sequences into a third sequence.

None

#include <algorithm>

template <class InputIterator1, class InputIterator2,
          class OutputIterator>
  OutputIterator
  merge(InputIterator first1, InputIterator1 last1,
        InputIterator2 first2, InputIterator last2,
        OutputIterator result);

template <class InputIterator1, class InputIterator2,
          class OutputIterator, class Compare>
  OutputIterator
  merge(InputIterator1 first1, InputIterator1 last1,
        InputIterator2 first2, InputIterator last2,
        OutputIterator result, Compare comp);

The merge algorithm merges two sorted sequences, specified by [first1, last1) and [first2, last2), into the sequence specified by [result, result + (last1 - first1) + (last2 - first2)). The first version of the merge algorithm uses the less than operator (<) to compare elements in the two sequences. The second version uses the comparison function provided by the function call. If a comparison function is provided, merge assumes that both sequences were sorted using that comparison function.

The merge is stable. This means that if the two original sequences contain equivalent elements, the elements from the first sequence will always precede the matching elements from the second in the resulting sequence. The size of the result of a merge is equal to the sum of the sizes of the two argument sequences. merge returns an iterator that points to the end of the resulting sequence, i.e., result + (last1 - first1) + (last2 -first2). The result of merge is undefined if the resulting range overlaps with either of the original ranges.

merge assumes that there are at least (last1 - first1) + (last2 - first2) elements following result, unless result has been adapted by an insert iterator.

For merge at most (last - first1) + (last2 - first2) - 1 comparisons are performed.

//
// merge.cpp
//
 #include <algorithm>
 #include <vector>
 #include <iostream.h>

 int main()
 {
   int d1[4] = {1,2,3,4};
   int d2[8] = {11,13,15,17,12,14,16,18};

   // Set up two vectors
   vector<int> v1(d1,d1 + 4), v2(d1,d1 + 4);
   // Set up four destination vectors
   vector<int> v3(d2,d2 + 8),v4(d2,d2 + 8),
               v5(d2,d2 + 8),v6(d2,d2 + 8);
   // Set up one empty vector
   vector<int> v7;

   // Merge v1 with v2
   merge(v1.begin(),v1.end(),v2.begin(),v2.end(),v3.begin());
   // Now use comparator
   merge(v1.begin(),v1.end(),v2.begin(),v2.end(),v4.begin(),
         less<int>());

   // In place merge v5
   vector<int>::iterator mid = v5.begin();
   advance(mid,4);
   inplace_merge(v5.begin(),mid,v5.end());
   // Now use a comparator on v6
   mid = v6.begin();
   advance(mid,4);
   inplace_merge(v6.begin(),mid,v6.end(),less<int>()); 
   
   // Merge v1 and v2 to empty vector using insert iterator
   merge(v1.begin(),v1.end(),v2.begin(),v2.end(),
         back_inserter(v7));

   // Copy all cout
   ostream_iterator<int,char> out(cout," ");
   copy(v1.begin(),v1.end(),out);
   cout << endl;
   copy(v2.begin(),v2.end(),out);
   cout << endl;
   copy(v3.begin(),v3.end(),out);
   cout << endl;
   copy(v4.begin(),v4.end(),out);
   cout << endl;
   copy(v5.begin(),v5.end(),out);
   cout << endl;
   copy(v6.begin(),v6.end(),out);
   cout << endl;
   copy(v7.begin(),v7.end(),out);
   cout << endl;
    
   // Merge v1 and v2 to cout
   merge(v1.begin(),v1.end(),v2.begin(),v2.end(),
         ostream_iterator<int,char>(cout," "));
   cout << endl;

   return 0;
 }

Output :
1 2 3 4
1 2 3 4
1 1 2 2 3 3 4 4
1 1 2 2 3 3 4 4
11 12 13 14 15 16 17 18
11 12 13 14 15 16 17 18
1 1 2 2 3 3 4 4
1 1 2 2 3 3 4 4

If your compiler does not support default template parameters then you need to always supply the Allocator template argument. For instance you'll have to write:

vector<int,allocator<int> >

instead of:

vector<int>

Containers, inplace_merge

©Copyright 1996, Rogue Wave Software, Inc.