C++ offers in its STL library a merge() which is quite useful to merge sort two containers into a single container. It is defined in header "algorithm". It is implemented in two ways. Syntax 1 : Using operator "<"
Template :
template
outiter merge (initer1 beg1, initer1 end1,
initer2 beg2, initer2 end2,
outiter res)
Parameters :
beg1 : Input iterator to initial position of first sequence.
end1 : Input iterator to final position of first sequence.
beg2 : Input iterator to initial position of second sequence.
end2 : Input iterator to final position of second sequence.
res : Output Iterator to initial position of resultant container.
Return value :
Iterator to last element of the resulting container.
CPP
// C++ code to demonstrate the working of
// merge() implementation 1
#include <bits/stdc++.h>
using namespace std;
int main()
{
// initializing 1st container
vector<int> arr1 = { 1, 4, 6, 3, 2 };
// initializing 2nd container
vector<int> arr2 = { 6, 2, 5, 7, 1 };
// declaring resultant container
vector<int> arr3(10);
// sorting initial containers
sort(arr1.begin(), arr1.end());
sort(arr2.begin(), arr2.end());
// using merge() to merge the initial containers
merge(arr1.begin(), arr1.end(), arr2.begin(), arr2.end(), arr3.begin());
// printing the resultant merged container
cout << "The container after merging initial containers is : ";
for (int i = 0; i < arr3.size(); i++)
cout << arr3[i] << " ";
return 0;
}
Output:
The container after merging initial containers is : 1 1 2 2 3 4 5 6 6 7
Syntax 2 : Using comparator function
Template :
template
outiter merge (initer1 beg1, initer1 end1,
initer2 beg2, initer2 end2,
outiter res, Compare comp)
Parameters :
beg1 : Input iterator to initial position of first sequence.
end1 : Input iterator to final position of first sequence.
beg2 : Input iterator to initial position of second sequence.
end2 : Input iterator to final position of second sequence.
res : Output Iterator to initial position of resultant container.
comp : The comparator function that returns a boolean
true/false of the each elements compared. This function
accepts two arguments. This can be function pointer or
function object and cannot change values.
Return value :
Iterator to last element of the resulting container.
CPP
// C++ code to demonstrate the working of
// merge() implementation 2
#include <bits/stdc++.h>
using namespace std;
// comparator function to reverse merge sort
struct greaters {
bool operator()(const long& a, const long& b) const
{
return a > b;
}
};
int main()
{
// initializing 1st container
vector<int> arr1 = { 1, 4, 6, 3, 2 };
// initializing 2nd container
vector<int> arr2 = { 6, 2, 5, 7, 1 };
// declaring resultant container
vector<int> arr3(10);
// sorting initial containers
// in descending order
sort(arr1.rbegin(), arr1.rend());
sort(arr2.rbegin(), arr2.rend());
// using merge() to merge the initial containers
// returns descended merged container
merge(arr1.begin(), arr1.end(), arr2.begin(), arr2.end(), arr3.begin(), greaters());
// printing the resultant merged container
cout << "The container after reverse merging initial containers is : ";
for (int i = 0; i < arr3.size(); i++)
cout << arr3[i] << " ";
return 0;
}
Output :
The container after reverse merging initial containers is : 7 6 6 5 4 3 2 2 1 1
Possible application : The merge function can be used to make a single stack of two stacks available in sorted order. These can be stack of books or notes. Let us discuss a simple example that merge orders two stack of notes in ascending order into one on basis of its value.
CPP
// C++ code to demonstrate the application of
// merge() stacking notes
#include <bits/stdc++.h>
using namespace std;
int main()
{
// initializing 1st container
// containing denominations
vector<int> stack1 = { 50, 20, 10, 100, 2000 };
// initializing 2nd container
// containing denominations
vector<int> stack2 = { 500, 2000, 10, 100, 50 };
// declaring resultant stack
vector<int> stack3(10);
cout << "The original 1st stack : ";
for (int i = 0; i < 5; i++)
cout << stack1[i] << " ";
cout << endl;
cout << "The original 2nd stack : ";
for (int i = 0; i < 5; i++)
cout << stack2[i] << " ";
cout << endl;
// sorting initial stacks of notes
// in descending order
sort(stack1.begin(), stack1.end());
sort(stack2.begin(), stack2.end());
// using merge() to merge the initial stacks
// of notes
merge(stack1.begin(), stack1.end(), stack2.begin(), stack2.end(), stack3.begin());
// printing the resultant stack
cout << "The resultant stack of notes is : ";
for (int i = 0; i < stack3.size(); i++)
cout << stack3[i] << " ";
return 0;
}
Output :
The original 1st stack : 50 20 10 100 2000
The original 2nd stack : 500 2000 10 100 50
The resultant stack of notes is : 10 10 20 50 50 100 100 500 2000 2000
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