what does string - '0' do (string is a char)

Question

what does this do

while(*string) {
    i = (i << 3) + (i<<1) + (*string -'0');
    string++;
}

the *string -'0'

does it remove the character value or something?

Answer 1

This subtracts from the character to which string is pointing the ASCII code of the character '0'. So, '0' - '0' gives you 0 and so on and '9' - '0' gives you 9.

The entire loop is basically calculating "manually" the numerical value of the decimal integer in the string string points to.

That's because i << 3 is equivalent to i * 8 and i << 1 is equivalent to i * 2 and (i << 3) + (i<<1) is equivalent to i * 8 + i * 2 or i * 10.