Question

I have a question about a part of the getint program.

When we got this part:

for(*pn = 0; isdigit(c); c = getch())
    *pn = 10 * *pn + (c - '0');

First it converts c to its real numeric value, then it multiply the data inside pn with 10.
Why does it multiply 10 with the data inside pn?

Regards,

Ken

Answer 1

It is starting from the left, and multiplying by 10 for each successive digit it encounters as it moves to the right.

Take the string “234” for example:

1. On the first iteration, take the 2. Multiply the 0 (*pn) by 10, add 2, you get 2.
2. On the second iteration, take the 3. Multiply the 2 (*pn) by 10, add 3, you get 23.
3. On the third iteration, take the 4. Multiply the 23 (*pn) by 10, add 4, you get 234.