How to Check if a Key Exists in a Map in C++ - Step by Step Examples

How to Check if a Key Exists in a Map in C++ ?

Answer

To check if a key exists in a map in C++, you can use the find method or the count method. These methods allow you to determine if a specific key is present in the map.

✐ Examples

1 Checking if a Key Exists Using find Method

We can check if a key exists in a map in C++ using the find method, which returns an iterator to the element if it is found, or the map's end iterator if it is not.

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 the find method to search for a specific key in the map. The find method returns an iterator to the element if it is found, or the map's end iterator if it is not.
We check if the iterator is equal to the map's end iterator. If it is not, the key exists in the map. Otherwise, the key does not exist.
We print the result to the console using std::cout.

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"}
    };

    // Check if a key exists using find method
    auto it = myMap.find(2);
    if (it != myMap.end()) {
        std::cout << "Key 2 exists with value: " << it->second << std::endl;
    } else {
        std::cout << "Key 2 does not exist." << std::endl;
    }

    return 0;
}

Output

Key 2 exists with value: two

2 Checking if a Key Exists Using count Method

We can check if a key exists in a map in C++ using the count method, which returns the number of elements with a specific key. Since maps do not allow duplicate keys, the result will be either 0 (key does not exist) or 1 (key exists).

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 the count method to check for a specific key in the map. The count method returns the number of elements with the specified key.
We check if the result is greater than 0. If it is, the key exists in the map. Otherwise, the key does not exist.
We print the result to the console using std::cout.

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"}
    };

    // Check if a key exists using count method
    if (myMap.count(2) > 0) {
        std::cout << "Key 2 exists with value: " << myMap[2] << std::endl;
    } else {
        std::cout << "Key 2 does not exist." << std::endl;
    }

    return 0;
}

Output

Key 2 exists with value: two

3 Checking if a Key Does Not Exist Using find Method

We can check if a key does not exist in a map in C++ using the find method, which returns an iterator to the element if it is found, or the map's end iterator if it is not.

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 the find method to search for a specific key in the map. The find method returns an iterator to the element if it is found, or the map's end iterator if it is not.
We check if the iterator is equal to the map's end iterator. If it is, the key does not exist. Otherwise, the key exists in the map.
We print the result to the console using std::cout.

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"}
    };

    // Check if a key does not exist using find method
    auto it = myMap.find(4);
    if (it == myMap.end()) {
        std::cout << "Key 4 does not exist." << std::endl;
    } else {
        std::cout << "Key 4 exists with value: " << it->second << std::endl;
    }

    return 0;
}

Output

Key 4 does not exist.

Summary

In this tutorial, we learned How to Check if a Key Exists in a Map in C++ language with well detailed examples.

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How to Check if a Key Exists in a Map in C++ - Step by Step Examples