Efficient Counting with HashMaps in C++

In this lesson, we will explore the concept and practical application of HashMaps in C++. HashMaps, known as unordered_map in C++, are a powerful and efficient data structure used for storing key-value pairs. You will learn how to utilize unordered_map to count the frequency of elements in a collection, understand the underlying mechanics, and analyze the time and space efficiency of this approach. This lesson includes a step-by-step demonstration with detailed code examples and a discussion on the practical applications of using HashMaps for counting occurrences in various contexts.

We begin in a library, where we want to count book copies. With a small collection, we might be able to tally each one manually. However, as the collection grows, this approach becomes cumbersome and inefficient. A more efficient method uses a HashMap, known as unordered_map in C++.

For a quick illustration, consider this list of colors:

#include <vector>
#include <string>

std::vector<std::string> colors = {"red", "blue", "red", "green", "blue", "blue"};

If we count manually, red appears twice, blue appears thrice, and green appears once. We can employ HashMaps for a more efficient counting process.

Simple yet powerful, HashMaps allow us to store and retrieve data using keys. The unique colors in our list act as keys, and the count of each color becomes its corresponding value. Let's demonstrate how we can count elements in our colors list using a C++ unordered_map:

#include <iostream>
#include <unordered_map>
#include <vector>
#include <string>

int main() {
    std::vector<std::string> colors = {"red", "blue", "red", "green", "blue", "blue"};
    std::unordered_map<std::string, int> color_map;

    // Start the loop to iterate over each color
    for (const auto& color : colors) {

        // If the color is present in our unordered_map, increment its value by 1
        if (color_map.find(color) != color_map.end()) {
            color_map[color] += 1;
        } else {
            // If the color isn't present, it means we're encountering this color in our list for the first time.
            // In this case, we add it to our unordered_map and set its value to 1
            color_map[color] = 1;
        }
    }

    // Print our unordered_map with counts
    for (const auto& pair : color_map) {
        std::cout << pair.first << ": " << pair.second << std::endl;
    }

    return 0;
}

When the above code executes, it displays the counts for each color:

red: 2
blue: 3
green: 1