Program to calculate the Sum of Digits in C

Program Description:

Write a program to calculate the Sum of Digits in C language. The program should accept a number from the user and calculate the sum of all the individual digits. We are going to use the C Loops ( while, for, etc) along with the division and modulus operators to calculate the sum of digits.

The program should show display the error message if the user enters an Invalid input ( i.e Negative numbers, etc). Here is an example expected inputs and outputs of the program.

Expected Input and Output:

Example 1:

Input:

Enter a Positive Number: 1234

Output:

Sum of Digits of 1234 is: 10

Example 2:

Input:

Enter a Positive Number: -9

Output:

ERROR: Please enter valid number

Program Pre-Requisites:

We are going to use the while, for loops, modulus, and division operators to calculate the sum of digits. Please go through the following articles to learn more

• While Loop in C Language
• For Loop in C Language with Example Programs
• Division Operator
• Modulus Operator

Sum of digits in C Program Algorithm:

• The idea is to get the individual digits of the given number ( num) and Add them to get the sum of the digits.
• We will maintain one variable sum to hold the sum of the digits. This sum variable will be initialized to zero. We need a way to go through all digits one by one and add them to “sum” variable.
• Then, We will use the modulus operator and division operators to get each digit.
  • The modulus operator is useful to get the last digit of any number. If we do the modulus of any number with 10, We will get the last digit.
    • For example 123 % 10 = 3
  • Similarly, We can remove the last digit from the number by dividing the given number by 10.
    • For example:  123 / 10  = 12
  • So we are going to use the above two operators to get the last digit and remove the last digit.
• Our loop will start from the last digit of the number and continue till we reach the first digit.
• At each iteration,
  • Get the last digit using the modulus operator and then Add the last digit to the sum variable.
  • Remove the last digit from the number ( num or temp) by using the division operator.
  • Repeat the above steps until our number( temp) becomes less than 0. Which is the stopping condition for our loop. i.e temp > 0
• Once all iterations are complete, The sum variable contains the sum of all digits in a given number.

Sum of Digits in C Program using while loop:

Program Output:

Example Walk-Through – Calculate Sum of digits in C Program:

Let’s take an example number, num = 1234. and here is the step-by-step walkthrough of the program.

At the first iteration:

• Initially the  sum variable will be zero and we used temp variable, which holds the value of num.
• Get the last digit using the modulus operator, Present temp value is 1234
  • temp % 10  which is 1234 % 10 = 4
• Add the last digit to  sum variable. The present value of sum = 0
  • sum = 0 + 4 the sum becomes 4 (sum = 4)
• Remove the last digit from the number using the division operator.
  • temp = temp / 10  which is equal to temp = 1234 / 10 and the temp becomes 123 ( i.e temp = 123)
• After the first iteration, The sum is 4, and the temp is 123.

Second Iteration:

• Get the last digit using the modulus operator, Present temp value is 123
  • temp % 10  which is 123 % 10 = 3
• Add the last digit to  sum variable. The present value of sum = 4
  • sum = 4 + 3 the sum becomes 7 (sum = 7)
• Remove the last digit from the number using the division operator.
  • temp = temp / 10  which is equal to temp = 123 / 10 and the temp becomes 12( i.e temp = 12)
• After the second iteration, The sum is 7, and the temp is 12.

Third Iteration:

• Get the last digit using the modulus operator, Present temp value is 12
  • temp % 10  which is 12 % 10 = 2
• Add the last digit to  sum variable. The present value of sum = 7
  • sum = 7 + 2the sum becomes 9 (sum = 9)
• Remove the last digit from the number using the division operator.
  • temp = temp / 10  which is equal to temp = 12 / 10 and the temp becomes 1 ( i.e temp = 1)
• After the Third iteration, The sum is 9, and the temp is 1.

Fourth Iteration:

• Get the last digit using the modulus operator, Present temp value is 1
  • temp % 10  which is 1 % 10 = 1
• Add the last digit to  sum variable. The present value of sum = 9
  • sum = 9 + 1the sum becomes 10 (i.e sum = 10)
• Remove the last digit from the number using the division operator.
  • temp = temp / 10  which is equal to temp = 1 / 10 and the temp becomes ( i.e temp = 0)
• After the fourth iteration, The sum is 10, and the temp is .

Fifth Iteration:

• The loop condition num > 0 will become False and The loop will terminate.
• The final value of the Sum of all digits is sum = 10.

Program to calculate Sum of digits in C using for loop:

Let’s rewrite the program using the for loop. The program logic will remain the same but this will differ in syntax.

Program Output:

If the user enters a Negative number.

Exercise Programs:

• Rewrite the above sum of digits program using the do while loop.

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• Armstrong Number Program in C Language – SillyCodes
• C program to print all ASCII characters and ASCII values/numbers – SillyCodes