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

How to Sort a Map by Keys in C++ ?

Answer

To sort a map by keys in C++, you can use the default behavior of the `std::map` container, which stores key-value pairs in sorted order based on the keys. This method ensures that the map is always sorted by keys.

✐ Examples

1 Sorting a Map by Keys Using std::map

We can sort a map by keys in C++ by using the `std::map` container, which automatically sorts the keys.

For example,

We start by including the <map> header file, which provides the necessary functions and data structures for working with maps.
We also include the <iostream> header for 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.
Since `std::map` automatically sorts the keys, we do not need to perform any additional sorting operations.
We print the sorted map to the console using a range-based for loop and the std::cout function to verify the sorting.

C++ Program

#include <map>
#include <iostream>

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

    // Print the sorted map
    std::cout << "Sorted Map by Keys: \n";
    for (const auto& pair : myMap) {
        std::cout << pair.first << ": " << pair.second << std::endl;
    }

    return 0;
}

Output

Sorted Map by Keys:
1: one
2: two
3: three

2 Sorting a Map by Keys in Descending Order

We can sort a map by keys in descending order in C++ by using `std::map` with a custom comparator.

For example,

We start by including the <map> and <functional> header files, which provide the necessary functions and data structures for working with maps and custom comparators.
We also include the <iostream> header for input-output operations.
We declare a map named myMap with a custom comparator that sorts the keys in descending order.
We initialize the map with some key-value pairs. In this example, the map has integer keys and string values.
We print the sorted map to the console using a range-based for loop and the std::cout function to verify the sorting.

C++ Program

#include <map>
#include <functional>
#include <iostream>

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

    // Print the sorted map
    std::cout << "Sorted Map by Keys (Descending): \n";
    for (const auto& pair : myMap) {
        std::cout << pair.first << ": " << pair.second << std::endl;
    }

    return 0;
}

Output

Sorted Map by Keys (Descending):
3: three
2: two
1: one

3 Sorting a Map by Keys Using std::map and Custom Comparator Function

We can sort a map by keys in C++ using a custom comparator function with `std::map` to specify a specific sorting order.

For example,

We start by including the <map> and <functional> header files, which provide the necessary functions and data structures for working with maps and custom comparators.
We also include the <iostream> header for input-output operations.
We declare a custom comparator function named customComparator that sorts the keys in a specific order.
We declare a map named myMap with the custom comparator function.
We initialize the map with some key-value pairs. In this example, the map has integer keys and string values.
We print the sorted map to the console using a range-based for loop and the std::cout function to verify the sorting.

C++ Program

#include <map>
#include <functional>
#include <iostream>

// Custom comparator function
struct customComparator {
    bool operator()(const int& lhs, const int& rhs) const {
        return lhs > rhs; // Example: descending order
    }
};

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

    // Print the sorted map
    std::cout << "Sorted Map by Keys (Custom Order): \n";
    for (const auto& pair : myMap) {
        std::cout << pair.first << ": " << pair.second << std::endl;
    }

    return 0;
}

Output

Sorted Map by Keys (Custom Order):
3: three
2: two
1: one

Summary

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

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