Question

I am reading K&R; so far I’m doing well with it, but there is something in function itoa() which I don’t understand. Here in itoa() they say they reverse the numbers themselves. For example 10 is 01 (they reverse the string):

void itoa(int n, char s[])
{
    int i, sign;
    if ((sign = n) < 0) /* record sign */
        n = -n; /* make n positive */
    i = 0;
    do { /* generate digits in reverse order */
        s[i++] = n % 10 + '0'; /* get next digit */
    } while ((n /= 10) > 0); /* delete it */
    if (sign < 0)
        s[i++] = '-';
    s[i] = '\0';
    reverse(s);
    return;
}

I don’t understand how it reversed the number. Even though we are just doing n % 10 + '0' then its the following digit which 10 then 1 gets deleted then it goes to 0 right ? Or I don’t get its logic ?

Answer 1

In the do-while loop, it is pulling the numbers off from behind (the least significant digit first). So, if you had the number -123456789, it processes the 9, then the 8, then the 7, etc.

So, when it hits the null-terminator (3rd to last line), you would have “987654321-“, which is then reversed.