Digit Frequency Counter (with Comments) in C

Summary: This C program counts the frequency of digits (0–9) in a given string and prints their occurrences. It includes inline comments for beginner-level clarity and understanding of logic behind each step.

#include <stdio.h>
#include <string.h>
#include <math.h>
#include <stdlib.h>

int main() {
    char input[1000];          // A place to store the text the user types
    int counter[10] = {0};     // This will count how many times each digit appears (0 to 9)

    scanf("%s", input);        // Read the text and store it in 'input'

    // Go through each character in the input
    for (int i = 0; input[i] != '\0'; i++) {
        // If the character is a digit (from '0' to '9')
        if (input[i] >= '0' && input[i] <= '9') {
            int digit = input[i] - '0';  // Change the character to the real number (e.g., '3' - '0' = 3)
            counter[digit]++;           // Add 1 to that digit's counter
        }
    }

    // Show how many times each digit appeared
    for (int i = 0; i < 10; i++) {
        printf("%d ", counter[i]);
    }

    return 0;
}

Explanation:

String Handling:
The input is stored in a character array. scanf("%s", input) reads input until a space is encountered.

Digit Detection:
The code checks if a character is a digit using input[i] >= '0' && input[i] <= '9'. This ensures that only numeric characters are processed.

Character to Integer Conversion:
input[i] - '0' converts a character digit (e.g., '3') into its integer value (3).

Frequency Tracking:
An integer array counter[10] is used to store the count of each digit from 0 to 9.

Output:
A simple loop prints the count of each digit in a single line, separated by spaces.