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C Loop Programs

This C tutorial helps you understand Happy Numbers and guides you in writing a C program to verify them. Here, you will learn how to determine whether the number entered by the user is a Happy Number.

What is a Happy Number?

A happy number is defined in mathematics as a process that replaces a number with the sum of the squares of its digits. This process is repeated until the number either equals 1 (indicating a "happy" ending) or loops indefinitely in a cycle that does not contain 1 (indicating the number is not "happy").

For example, consider the number 23:

  • First, we find the sum of the squares of its digits: (2^2) + (3^2) = 4 + 9 = 13
  • For the resulting 13, we repeat the process: (1^2) + (3^2) = 1 + 9 = 10
  • We do the same for 10: (1^2) + (0^2) = 1

In the above example, we arrive at the number 1 after three processes, indicating that 23 is a happy number. The number is not happy if the process enters a loop that does not result in a 1.

C Program to Check if a Number Is Happy

Below is the complete C program to check whether a number is a Happy Number or not:


int main() {
    int num, digit, sum = 0;
    // Prompt user for input
    printf("Enter any number: ");
    scanf("%d", &num);
    // While the number has more than one digit, calculate the sum of squares
    while(num > 0 || sum > 9) {
        if(num == 0){
            num = sum;
            sum = 0;
        // Get the last digit of the number
        digit = num % 10;
        // Add the square of the digit to sum
        sum += digit * digit;
        // Remove the last digit from the number
        num /= 10;
    // If the sum is 1, it's a happy number. Otherwise, it's not.
    (sum == 1)? printf("It is a happy number.\n"): printf("It is not a happy number.\n");

    return 0;

Program Output:

Here are some examples of running the program and the corresponding output:

Enter any number: 23
It is a happy number.
Enter any number: 12
It is not a happy number.

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