How to Replace a Specific Pair in a Multimap in C++?

in C++, multimap is similar to a map that stores the data in the key-value format where duplicate keys are allowed. In this article, we will learn how to replace a specific pair in a multimap in C++.

Example

Input: 
myMultimap = {{1, “one”}, {2, “two”}, {2, “two”}, {3, “three”}};
Key-Value Pair to Replace = {2, “two”};
To be Replaced with = {5, "five"}

Output:
myMultimap = {{1, “one”}, {3, “three”}, {5, "five"}, {5, "five"}};

Replace a Specific Pair in a Multimap in C++

To replace a specific pair in a std::multimap, we can use the std::multimap::equal_range function to get a range of iterators representing all occurrences of the key, find the required pair, delete it, and then insert a new one.

Approach

• Get the range of the old key using the equal_range() function.
• Iterate over the range of the old key.
• In each iteration, check if the value of the current pair matches the value you want to replace.
• If it matches, insert a new pair into the multimap with the new key and new value.
• Remove all occurrences of the old key-value pair from the multimap using erase() function.

C++ Program to Replace a Specific Pair in a Multimap

// C++ Program to illustrate how to replace a specific
// key-value pair
#include <iostream>
#include <map>
using namespace std;

int main()
{
    // Creating a multimap
    multimap<int, string> myMultimap = { { 1, "one" },
                                         { 2, "two" },
                                         { 2, "two" },
                                         { 3, "three" } };

    // Key-value pair to replace
    int oldKey = 2;
    string value = "two";

    // New key
    int newKey = 4;
    string newValue = "four";

    // Getting the range of the old key
    auto range = myMultimap.equal_range(oldKey);

    // Inserting new pairs with the new key and the same
    // value
    for (auto it = range.first; it != range.second; ++it) {
        if (it->second == value) {
            myMultimap.insert({ newKey, newValue });
        }
    }

    // Removing all occurrences of the old key-value pair
    myMultimap.erase(oldKey);

    // Printing the multimap after replacing the key
    cout << "Multimap after replacing the key:" << endl;
    for (const auto& pair : myMultimap) {
        cout << pair.first << " => " << pair.second << endl;
    }

    return 0;
}

Multimap after replacing the key:
1 => one
3 => three
4 => four
4 => four

Time Complexity: O(K Log N), where K is the number of matching elements, and N is the size of the multimap.
Space Complexity: O(K)