How to Set a Default Value for a Key in a Map in C++ - Step by Step Examples

How to Set a Default Value for a Key in a Map in C++ ?

Answer

To set a default value for a key in a map in C++, you can use the subscript operator, the insert method, or the emplace method.

✐ Examples

1 Setting a Default Value Using Subscript Operator

We can set a default value for a key in a map in C++ using the subscript operator, which allows us to insert a key-value pair if the key does not already exist 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 the subscript operator to set a default value for a key. If the key does not exist in the map, the subscript operator inserts the key with the specified value.
We print the updated map to the console using a range-based for loop and 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"}
    };

    // Set a default value using subscript operator
    myMap[4] = myMap[4].empty() ? "default" : myMap[4];

    // Print the updated map
    for (const auto& pair : myMap) {
        std::cout << pair.first << ": " << pair.second << std::endl;
    }

    return 0;
}

Output

1: one
2: two
3: three
4: default

2 Setting a Default Value Using insert Method

We can set a default value for a key in a map in C++ using the insert method, which inserts the key-value pair if the key does not already exist 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 the insert method to set a default value for a key. The insert method takes a std::pair object, which contains the key and value to be inserted. If the key does not exist in the map, the insert method inserts the key-value pair.
We print the updated map to the console using a range-based for loop and 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"}
    };

    // Set a default value using insert method
    myMap.insert(std::pair<int, std::string>(4, "default"));

    // Print the updated map
    for (const auto& pair : myMap) {
        std::cout << pair.first << ": " << pair.second << std::endl;
    }

    return 0;
}

Output

1: one
2: two
3: three
4: default

3 Setting a Default Value Using emplace Method

We can set a default value for a key in a map in C++ using the emplace method, which constructs an element in place within the map if the key does not already exist.

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 emplace method to set a default value for a key. The emplace method takes the key and value as arguments and constructs the element in place if the key does not exist in the map.
We print the updated map to the console using a range-based for loop and 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"}
    };

    // Set a default value using emplace method
    myMap.emplace(4, "default");

    // Print the updated map
    for (const auto& pair : myMap) {
        std::cout << pair.first << ": " << pair.second << std::endl;
    }

    return 0;
}

Output

1: one
2: two
3: three
4: default

Summary

In this tutorial, we learned How to Set a Default Value for a Key in a Map in C++ language with well detailed examples.

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How to Set a Default Value for a Key in a Map in C++ - Step by Step Examples