Example: Count Frequency with Visited Marker in C

// Count Frequency with Visited Marker
#include <stdio.h>

int main() {
    // Step 1: Initialize the array and its size
    int arr[] = {1, 2, 8, 3, 2, 2, 2, 5, 1};
    int n = sizeof(arr) / sizeof(arr[0]);

    // Step 2: Declare a frequency array of the same size, initialized to 0
    int freq[n];
    for (int i = 0; i < n; i++) {
        freq[i] = 0;
    }

    // Step 3: Iterate through the array to count frequencies
    for (int i = 0; i < n; i++) {
        int count = 1; // Start count for the current element

        // If the element hasn't been counted yet
        if (arr[i] != -1) {
            // Compare it with subsequent elements
            for (int j = i + 1; j < n; j++) {
                if (arr[i] == arr[j]) {
                    count++;
                    freq[j] = -1; // Mark element as visited by setting its freq to -1
                }
            }
            freq[i] = count; // Store the frequency of the current element
        }
    }

    // Step 4: Print the frequencies of unique elements
    printf("Element | Frequency\n");
    printf("--------|-----------\n");
    for (int i = 0; i < n; i++) {
        if (freq[i] != -1) { // Only print if the element was not a duplicate
            printf("%7d | %9d\n", arr[i], freq[i]);
        }
    }

    return 0;
}