反转输入的C语言数字

Question

I just want to ask on why is there a need to write sum = sum*10+digit instead of just sum = 0+digit ? since the result of sum*10 is just 0. This is a code in C language for reversing an entered number. Thanks!

#include <stdio.h>

int main()
{
    int number, x = 0, digit, temporary, div = 10, sum = 0;
    printf("Enter numbers\n");
    scanf("%d", &number);
    temporary = number;

START:

    digit = number%div;
    sum = sum*10+digit;
    number = number/div;

    if(number>0)
        goto START;

    printf("Reversed Number = %d\n", temporary);
    printf("Reversed Number = %d\n", sum);
    return 0;
}

Answer 1

In a situation like this it is a very good thing to examine what is happening with simple printouts. If you insert printf("sum: %d digit: %d number: %d\\n", sum, digit, number); before the if statement, it will become obvious what's going on.

$ ./a.out 
Enter numbers
2345
sum: 5 digit: 5 number: 234
sum: 54 digit: 4 number: 23
sum: 543 digit: 3 number: 2
sum: 5432 digit: 2 number: 0
Reversed Number = 2345
Reversed Number = 5432

Sidenote: There are "approved" ways of using goto. This is not one of them. You should switch

START:
...
if(number>0)
    goto START;

to

do {
...
} while(number>0)