How to Iterate Over Keys in a Map in C++ - Step by Step Examples

How to Iterate Over Keys in a Map in C++ ?

Answer

To iterate over keys in a map in C++, you can use a range-based for loop or an iterator. This allows you to access each key in the map.

✐ Examples

1 Iterating Over Keys Using Range-Based For Loop

We can iterate over keys in a map in C++ using a range-based for loop, which provides a convenient syntax for iterating over all keys in the map.

For example,

We start by including the <map> and <iostream> header files, which provide the necessary functions and data structures for working with maps and input-output operations.
We declare and initialize a map named myMap with some key-value pairs. In this example, the map has integer keys and string values.
We use a range-based for loop to iterate over the keys of the map. We access each key using the first member of the map's key-value pairs.
We print each key to the console using the std::cout function to verify that we are iterating over all keys.

C++ Program

#include <map>
#include <iostream>

int main() {
    // Declare and initialize a map
    std::map<int, std::string> myMap = {
        {1, "one"},
        {2, "two"},
        {3, "three"}
    };

    // Iterate over keys using range-based for loop
    for (const auto& pair : myMap) {
        std::cout << pair.first << std::endl;
    }

    return 0;
}

Output

1
2
3

2 Iterating Over Keys Using Iterator

We can iterate over keys in a map in C++ using an iterator, which provides a flexible way to access each key in the map.

For example,

We start by including the <map> and <iostream> header files, which provide the necessary functions and data structures for working with maps and input-output operations.
We declare and initialize a map named myMap with some key-value pairs. In this example, the map has integer keys and string values.
We declare an iterator for the map using the std::map<int, std::string>::iterator type.
We use a for loop to iterate over the keys of the map by incrementing the iterator. We access each key using the first member of the iterator's key-value pairs.
We print each key to the console using the std::cout function to verify that we are iterating over all keys.

C++ Program

#include <map>
#include <iostream>

int main() {
    // Declare and initialize a map
    std::map<int, std::string> myMap = {
        {1, "one"},
        {2, "two"},
        {3, "three"}
    };

    // Declare an iterator for the map
    std::map<int, std::string>::iterator it;

    // Iterate over keys using iterator
    for (it = myMap.begin(); it != myMap.end(); ++it) {
        std::cout << it->first << std::endl;
    }

    return 0;
}

Output

1
2
3

3 Iterating Over Keys Using Key Extraction

We can extract and iterate over keys in a map in C++ by first storing the keys in a separate container.

For example,

We start by including the <map>, <vector>, and <iostream> header files, which provide the necessary functions and data structures for working with maps, vectors, and input-output operations.
We declare and initialize a map named myMap with some key-value pairs. In this example, the map has integer keys and string values.
We declare a vector named keys to store the keys from the map.
We use a range-based for loop to iterate over the keys of the map, and we store each key in the keys vector.
We use another range-based for loop to iterate over the keys vector and print each key to the console using the std::cout function to verify that we are iterating over all keys.

C++ Program

#include <map>
#include <vector>
#include <iostream>

int main() {
    // Declare and initialize a map
    std::map<int, std::string> myMap = {
        {1, "one"},
        {2, "two"},
        {3, "three"}
    };

    // Declare a vector to store the keys
    std::vector<int> keys;

    // Extract keys from the map
    for (const auto& pair : myMap) {
        keys.push_back(pair.first);
    }

    // Iterate over the keys vector
    for (const auto& key : keys) {
        std::cout << key << std::endl;
    }

    return 0;
}

Output

1
2
3

Summary

In this tutorial, we learned How to Iterate Over Keys in a Map in C++ language with well detailed examples.

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