Example: Count Frequency with Direct Mapping in C

// Count Frequency with Direct Mapping
#include <stdio.h>

// Assuming max element value is less than 100 for this example.
// For larger ranges, dynamically allocate or adjust MAX_VALUE.
#define MAX_VALUE 100 

int main() {
    // Step 1: Initialize the array and its size
    int arr[] = {10, 20, 20, 10, 10, 30, 40, 30, 5, 20, 5};
    int n = sizeof(arr) / sizeof(arr[0]);

    // Step 2: Declare and initialize the frequency array
    // Its size should be MAX_VALUE + 1 to accommodate elements up to MAX_VALUE
    int freq[MAX_VALUE] = {0}; // All elements initialized to 0

    // Step 3: Iterate through the input array and increment counts
    for (int i = 0; i < n; i++) {
        // Check if element is within the valid range
        if (arr[i] >= 0 && arr[i] < MAX_VALUE) {
            freq[arr[i]]++; // Increment count for the element's value
        } else {
            printf("Warning: Element %d out of defined range [0, %d).\n", arr[i], MAX_VALUE);
        }
    }

    // Step 4: Print the frequencies of elements that appeared
    printf("Element | Frequency\n");
    printf("--------|-----------\n");
    for (int i = 0; i < MAX_VALUE; i++) {
        if (freq[i] > 0) { // Only print if the element appeared at least once
            printf("%7d | %9d\n", i, freq[i]);
        }
    }

    return 0;
}