Question

I have a string of digits, e.g. ‘123456789’, and I need to extract each one of them to use them in a calculation. I can of course access each char by index, but how do I convert it into an int?

I’ve looked into atoi(), but it takes a string as argument. Hence I must convert each char into a string and then call atoi on it. Is there a better way?

Answer 1

You can utilize the fact that the character encodings for digits are all in order from 48 (for ‘0’) to 57 (for ‘9’). This holds true for ASCII, UTF-x and practically all other encodings (see comments below for more on this).

Therefore the integer value for any digit is the digit minus ‘0’ (or 48).

char c = '1'; int i = c - '0'; // i is now equal to 1, not '1' 

is synonymous to

char c = '1'; int i = c - 48; // i is now equal to 1, not '1' 

However I find the first c - '0' far more readable.