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merge() in C++ STL

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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 



Last Updated : 14 Sep, 2022
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