Multimap Find and Replace Operator in C++: Step-by-Step Guide

Article by: Manish Methani

Last Updated: September 8, 2021 at 2:04pm IST
6 min 5 sec

A multimap in C++ allows you to store key-value pairs where multiple values can be associated with the same key. Finding and replacing values associated with a specific key is a common operation when working with multimap. In this step-by-step guide, we'll explore how to use the find and replace operator to manipulate multimap entries in C++. We'll also provide a FAQ section to address common questions related to this topic.

Step 1: Include Necessary Headers

To begin, you need to include the required C++ Standard Library headers to work with multimap. Ensure you have #include <iostream> and #include <map> included in your code for input/output operations and multimap functionality.

#include <iostream>
#include <map>

Step 2: Create a Multimap

Next, create a multimap that will store your key-value pairs. You can define the key and the associated values' data types in your multimap declaration.

std::multimap<std::string, int> myMultimap;

In this example, we've created a multimap where the key is a string, and the associated values are integers.

Step 3: Insert Key-Value Pairs

Insert key-value pairs into the multimap using the insert method. You can add as many entries as needed.

myMultimap.insert(std::make_pair("apple", 5));
myMultimap.insert(std::make_pair("banana", 3));
myMultimap.insert(std::make_pair("apple", 8));
myMultimap.insert(std::make_pair("cherry", 10));
myMultimap.insert(std::make_pair("banana", 2));

Step 4: Find a Key in the Multimap

Use the find() method to locate a specific key in the multimap. The find() method returns an iterator to the found key.

std::string keyToFind = "apple";
auto it = myMultimap.find(keyToFind);

if (it != myMultimap.end()) {
    // Key found
    std::cout << "Key '" << keyToFind << "' found." << std::endl;
} else {
    // Key not found
    std::cout << "Key '" << keyToFind << "' not found." << std::endl;
}

Step 5: Replace Values for a Key

To replace values associated with a key, you can use the iterator obtained from the find() method. In this example, we replace values for the key "apple."

if (it != myMultimap.end()) {
    it->second = 15;
    std::cout << "Values for key '" << keyToFind << "' replaced." << std::endl;
} else {
    std::cout << "Key '" << keyToFind << "' not found." << std::endl;
}

Step 6: Full Example Code

Here's the full example code to find and replace values for a key in a multimap in C++:

#include <iostream>
#include <map>

int main() {
    std::multimap myMultimap;

    myMultimap.insert(std::make_pair("apple", 5));
    myMultimap.insert(std::make_pair("banana", 3));
    myMultimap.insert(std::make_pair("apple", 8));
    myMultimap.insert(std::make_pair("cherry", 10));
    myMultimap.insert(std::make_pair("banana", 2));

    std::string keyToFind = "apple";
    auto it = myMultimap.find(keyToFind);

    if (it != myMultimap.end()) {
        it->second = 15;
        std::cout << "Values for key '" << keyToFind << "' replaced." << std::endl;
    } else {
        std::cout << "Key '" << keyToFind << "' not found." << std::endl;
    }

    return 0;
}

Output:

Values for key "apple" replaced.

This code demonstrates how to find and replace values associated with a key in a multimap in C++.

Remember that multimap allows duplicate keys, and you can use this approach to handle multiple entries for the same key efficiently.