Sum till a Single Digit

Q) Write a program that accepts integer as input and repeatedly sums it digits till it reaches single digit. It the outputs that single digit.

Code:

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#include<stdio.h>
int main()
{
  int num,r,sum=0;
  printf("Enter a no.");
  scanf("%d",&num);
    do{
        while(num!=0){
        r=num%10;
        num/=10;
        sum +=r;
            }
        if(sum >9){
            num=sum;
            sum=0;
            }
      }while(num!=0);
    printf("the sum of digits is %d",sum);
}

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I would like to know whether this is optimum or some better program can be written.(I haven't been taught functions yet).
Thanks for constructive remarks.

Mathematically speaking, it's pretty awesome. I would consider a recursive function only you wrote that you have not learned functions yet, so... :cool:

Quote:

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whether this is optimum or some better program can be written.

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perhaps...

Code:

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#include <stdio.h>

int main() {
    int num, r, sum=0;
    printf("Enter a no: ");
    scanf("%d",&num);
    while( num != 0 ) {
        r = num % 10;
        num /= 10;
        sum += r;
        if ((num == 0) && (sum > 9)) {
            num = sum;
            sum = 0;
        };
    };
    printf("the sum of digits is %d",sum);
    return 0;
};

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