# C Program to find the sum of the digits of a number untill the sum is reduced to a single digit

Last updated on July 27, 2020

The following is a C program to find the sum of the digits till the sum is reduced to a single digit.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 | ```
/**************************************************************************************************
* Program to find the sum of the digits of a number till the sum is reduced to a single digit
***************************************************************************************************/
#include<stdio.h> // include stdio.h
int main()
{
long int num;
int sum = 0, rem;
printf("Enter a number: ");
scanf("%ld", &num);
while(num / 10 != 0)
{
sum = 0;
while(num != 0)
{
rem = num % 10; // get the last digit of num
sum += rem; // add rem to sum
num = num / 10; // remove the last digit from num
}
num = sum;
}
printf("%d", sum);
return 0;
}
``` |

**Expected Output:** 1st run:

1 2 | ```
Enter a number: 12345
6
``` |

2nd run:

1 2 | ```
Enter a number: 749
2
``` |

## How it works #

The following table demonstrates what happens a each iteration of the inner while loop, assuming `num = 12345`

:

Iteration | num | sum | rem |
---|---|---|---|

After 1st iteration | `remainder = 12345%10 = 5` |
`sum = 0+5 = 5` |
`num = 12345/10 = 1234` |

After 2nd iteration | `remainder = 1234%10 = 4` |
`sum = 5+4 = 9` |
`num = 1234/10 = 123` |

After 3rd iteration | `remainder = 123%10 = 3` |
`sum = 9+3 = 12` |
`num = 123/10 = 12` |

After 4th iteration | `remainder = 12%10 = 2` |
`sum = 12+2 = 14` |
`num = 12/10 = 1` |

After 5th iteration | `remainder = 1%10 = 1` |
`sum = 14+1 = 15` |
`num = 1/10 = 0` |

This completes the first iteration of the outer while loop. Since the num is still not reduced to single digit, the condition `num / 10 != 0`

evaluates to true (`1`

) and the body of inner while loop is executed again.

Iteration | num | sum | rem |
---|---|---|---|

After 1st iteration | `remainder = 15%10 = 5` |
`sum = 0+5 = 5` |
`num = 15/10 = 1` |

After 2nd iteration | `remainder = 1%10 = 1` |
`sum = 5+1 = 6` |
`num = 1/10 = 0` |

**Recommended Reading:**

- C Program to find the sum of digits of a number
- C Program to find the factorial of a number
- C Program to find Armstrong numbers
- C Program to find Prime Numbers
- C Program to generate Fibonacci sequence
- C Program to count the number of digits in a number.

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