How to Filter a Map Based on Keys in C++ - Step by Step Examples

How to Filter a Map Based on Keys in C++ ?

Answer

To filter a map based on keys in C++, you can iterate over the map and insert the key-value pairs that meet the filter criteria into a new map. This method allows you to create a subset of the original map based on specific key conditions.

✐ Examples

1 Filtering a Map Based on Key Range

We can filter a map based on a range of keys in C++ by iterating over the map and inserting key-value pairs that fall within the specified range into a new map.

For example,

We start by including the <map> and <iostream> headers, 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 empty map named filteredMap to store the filtered key-value pairs.
We iterate over the original map using a range-based for loop and check if each key falls within the specified range.
We insert the key-value pairs that meet the condition into the filteredMap.
We print the filtered map to the console using a range-based for loop and the std::cout function to verify the filtering.

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"},
        {4, "four"},
        {5, "five"}
    };

    // Declare an empty map to store the filtered key-value pairs
    std::map<int, std::string> filteredMap;

    // Define the key range
    int lowerBound = 2;
    int upperBound = 4;

    // Iterate over the original map and filter based on the key range
    for (const auto& pair : myMap) {
        if (pair.first >= lowerBound && pair.first <= upperBound) {
            filteredMap[pair.first] = pair.second;
        }
    }

    // Print the filtered map
    std::cout << "Filtered Map by Key Range:\n";
    for (const auto& pair : filteredMap) {
        std::cout << pair.first << ": " << pair.second << std::endl;
    }

    return 0;
}

Output

Filtered Map by Key Range:
2: two
3: three
4: four

2 Filtering a Map Based on Specific Keys

We can filter a map based on specific keys in C++ by iterating over the map and inserting key-value pairs that match the specified keys into a new map.

For example,

We start by including the <map> and <iostream> headers, 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 empty map named filteredMap to store the filtered key-value pairs.
We define a set of specific keys that we want to filter the map by.
We iterate over the original map using a range-based for loop and check if each key matches one of the specified keys.
We insert the key-value pairs that meet the condition into the filteredMap.
We print the filtered map to the console using a range-based for loop and the std::cout function to verify the filtering.

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"},
        {4, "four"},
        {5, "five"}
    };

    // Declare an empty map to store the filtered key-value pairs
    std::map<int, std::string> filteredMap;

    // Define the specific keys to filter by
    std::set<int> keysToFilter = {1, 3, 5};

    // Iterate over the original map and filter based on the specific keys
    for (const auto& pair : myMap) {
        if (keysToFilter.find(pair.first) != keysToFilter.end()) {
            filteredMap[pair.first] = pair.second;
        }
    }

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

    return 0;
}

Output

Filtered Map by Specific Keys:
1: one
3: three
5: five

3 Filtering a Map with String Keys Based on Prefix

We can filter a map with string keys based on a specific prefix in C++ by iterating over the map and inserting key-value pairs that have keys starting with the given prefix into a new map.

For example,

We start by including the <map> and <iostream> headers, 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 string keys and string values.
We declare an empty map named filteredMap to store the filtered key-value pairs.
We define a prefix string that we want to filter the map by.
We iterate over the original map using a range-based for loop and check if each key starts with the specified prefix.
We insert the key-value pairs that meet the condition into the filteredMap.
We print the filtered map to the console using a range-based for loop and the std::cout function to verify the filtering.

C++ Program

#include <map>
#include <iostream>

int main() {
    // Declare and initialize a map
    std::map<std::string, std::string> myMap = {
        {"apple", "fruit"},
        {"banana", "fruit"},
        {"carrot", "vegetable"},
        {"apricot", "fruit"},
        {"blueberry", "fruit"}
    };

    // Declare an empty map to store the filtered key-value pairs
    std::map<std::string, std::string> filteredMap;

    // Define the prefix to filter by
    std::string prefix = "ap";

    // Iterate over the original map and filter based on the prefix
    for (const auto& pair : myMap) {
        if (pair.first.find(prefix) == 0) {
            filteredMap[pair.first] = pair.second;
        }
    }

    // Print the filtered map
    std::cout << "Filtered Map by Prefix:\n";
    for (const auto& pair : filteredMap) {
        std::cout << pair.first << ": " << pair.second << std::endl;
    }

    return 0;
}

Output

Filtered Map by Prefix:
apple: fruit
apricot: fruit

Summary

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

◀ Previous Next ▶

C++ Tutorials

C++ How-Tos

How to Filter a Map Based on Keys in C++ - Step by Step Examples