Question

I’m learning C and I am having a hard time understanding loops and the usage of modulo. I know Loops are used to shorten up the program and Modulo’s are used to get out the remainder. My assignment was to “Write a C program to find the sum of an individual positive integer”.

I just spent few hours trying to understand this problem. I experimented too.

    int  n,d=0,s=0;
    printf("\nEnter a number\n\n");
    scanf("%d",&n);


while(n>0)
     { 
           d = n%10;
       s = s+d;
       n = n/10;
     }

printf("\n sum of the individual digits = %d",s);

My questions are:

Can anyone help me understand the flow of this program? Why is Modulo being used? and Why is there a n = n/10

Experiements I’ve done:

When I removed d = n%10; line the output prints out the digits seperatley. thus it’s not calculating.

i.e 123 = 6 –> It’s giving me 136

When I removed the line n = n/10 It’s not showing me an output. The printf statement has a parameter ‘s’

Thanks in Advance!

Answer 1

Taking the modulo in d = n % 10 makes d equal to the last digit of n in base 10. n = n / 10 removes the last digit from n.

Modulo is essentially taking the remainder, so let’s say n = 123. Then n / 10 is 12 and n % 10 is 3.

Removing the n = n / 10 means that n doesn’t change between each run of the loop, so the loop condition n > 0 is always true and therefore the loop keeps going until you manually kill the program.

Here is a trace of the program with n = 123. Initially d and s are both zero.

 while (n > 0) {  // n is 123, which is greater than zero, so we enter the loop
     d = n % 10;  // 123 % 10 is 3, so d is now 3
     s = s + d;   // 0 + 3 is 3, so s is now 3
     n = n / 10;  // 123 / 10 is 12, so n is now 12.
 }                // go back to the top of the loop
 while (n > 0) {  // n is 12, which is still greater than zero
     d = n % 10;  // 12 % 10 is 2, so d is now 2
     s = s + d    // 3 + 2 is 5, so s is now 5
     n = n / 10;  // 12 / 10 is 1, so n is now 1
 }                // go back to the top again
 while (n > 0) {  // n is 1, which is still greater than zero
     d = n % 10;  // 1 % 10 is 1, so d is now 1
     s = s + d;   // 5 + 1 is 6, so s is now 6
     n = n / 10;  // 1 / 10 is 0, so n is now 0
 }                // go back to the top
 while (n > 0) {  // n is 0, which is not greater than zero, so we skip
                  // to after the loop body
 printf("\n sum of the individual digits = %d",s);