title: C character values arithmetic
body: <p>I have been reading from the book "The C Programming Language" learning C, and I stumbled upon the arithmetic <code>s[i] - '0'</code>  which they said that it gives the numeric value of the character stored in s[i]. I didn't quite understand it, how could it give the value by subtraction?
  Note This is used in the atoi function, which converts a string of digits into its numeric equivalent.
  Thanks</p>