How do you convert char numbers to decimal and back or convert ASCII 'A'-'Z'/'a'-'z' to letter offsets 0 for 'A'/'a' …?

Question

If you have a char that is in the range \'0\' to \'9\' how do you convert it to int values of 0 to 9

And then how do you convert it back?

Also given letters

Answer 1

If you know a character c is either a letter or number, you can just do:

int cton( char c )
{
  if( 'a' <= c ) return c-'a';
  if( 'A' <= c ) return c-'A';
  return c-'0';
}

Add whatever error checking on c is needed.

To convert an integer n back to a char, just do '0'+n if you want a digit, 'A'+n if you want an uppercase letter, and 'a'+n if you want lowercase.

Note: This works for ASCII (as the OP is tagged.) See Pete's informative comment however.