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csubtractionletters-and-numbers

subtracting letters in c

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.

Solution

  • 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'