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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