subtracting letters in c

Question

I would like to know how to "subtract letters" in C:
I mean, I have 2 letters, 'a' and 'c' and i want to execute 'c'-'a'='b' which is 3-1=2.
How is it possible to obtain the same behaviour in C?
I can conversion Letters->Numbers but how to manage the limited lenght of the alphabet? Thank you.

Answer 1

you can treat the letters as numbers and then add the letter 'a' back to normalize it

so

char c1 = 'a';
char c2 = 'c';
int diff = c2 - c1; //'c' - 'a' = 2
char ans = diff + 'a' - 1; //add 'a' and subtract 1 to normalize it

If you want the number difference just use diff from my answer (ans will give you the letter).

This will not wrap around so

'a' - 'b' 

will result in -1 (or the character before a)

If you want to handle negatives with a wrap you have to check it

int diff = c2 - c1;
char ans;
diff > 0 ? ans = diff + 'a' - 1 : 'z' + diff + 1; 

This will give:

1. 'z' for 'b'-'c'
2. 'y' for 'b'-'d'