C Programming - Design

Question

I'm writing a program that when you subtract 3 from char 'a' you get the answer of char 'x'

What's the most practical way of doing this?

#include <stdio.h>

int main(void)
{
   char letter = 'a'
   int subtract_from_letter = 3;

   /*Not sure what to do here...

    char letter should now equal 'x'. If char letter was equal to 'd', it      
    would now equal 'a'.        

   */

   return 0; 
}

Thanks everyone.

Welcome to Stack Overflow! You could take the [tour](http://stackoverflow.com/tour) to have a global idea of how this site works, and read the [advice on how to ask](http://stackoverflow.com/help/asking) in the help center to see what questions are welcomed here and which are not. — Eregrith, May 19 '15 at 08:12
i.e. What have you tried? Where did you fail? What's the real problem? — Eregrith, May 19 '15 at 08:12
I'm just going through a book of C exercises by myself, trying to learn. This was one of the problems, just little brain teasers. — Zach Smithers, May 19 '15 at 08:14
I changed the wording of the question, I apologise. If char letter was 'd', it would now equal 'a', because d - 3 is 3 letters back. — Zach Smithers, May 19 '15 at 08:17
It's unclear what you want, give more examples (b,c,d,e,...x,y,z). — Jabberwocky, May 19 '15 at 08:20
Search for "caesar cipher" and you'll find plenty of solutions. — Paul Hankin, May 19 '15 at 08:22
Check this answer (use `shift = -3`): http://stackoverflow.com/a/16374718/646887 — Klas Lindbäck, May 19 '15 at 08:27
"The most practical way" seems to solicit opinions. If you have working code, submit it to [codereview.se]. If you have non-working code, show it. If you have no code and no idea, try something. — Jongware, May 19 '15 at 08:28

score 1 · Answer 1 · answered May 19 '15 at 08:26

I'd do

convert from the letter to a respective number from 0..25,
add/subtract
do a modulo 26 operation
convert back to a letter.

So the result is

int letter_num = letter - 'a';
int result = (letter_num + 26 - subtract_from_letter) % 26; // ensures we stay positive
chat result_letter = result + 'a';

For example, with 'a' and 3, you'll get

letter_num = 0
result = 23
result_letter = 'x'

'd', however, will give you

letter_num = 3
result = (29 - 3) % 26 = 0
result_letter = 'a'

Sourav Ghosh · Answer 2 · 2015-05-19T08:32:13.493

0

I think , youre trying to go for a circular print for small case alphabets, right?

Try this logic:

check if the letter is less than d (decimal 100) or not before subtraction.
If yes, do (letter + 26 - subtract_from_letter)
otherwise, do (letter - subtract_from_letter)
print the result using %c.

edited May 19 '15 at 08:32

answered May 19 '15 at 08:16

Sourav Ghosh

133,132
16
183
261

2

Surely (`letter + 26 - subtract_from_letter)`? – Weather Vane May 19 '15 at 08:19
@WeatherVane is `(letter+122-subtract_from_letter)` – LPs May 19 '15 at 08:23
@LPs no! Start from `letter = 'c'`, so `letter = 99` then `99+26-3 = 122` which is `'z'`. – Weather Vane May 19 '15 at 08:25
@WeatherVane I misunderstood your comment, you right, but @Sourav Ghosh wrote `(letter + 'z' - subtract_from_letter)` that is `(letter+122-subtract_from_letter)` or I'm missing something? – LPs May 19 '15 at 08:27
@LPs I could have edited the incorrect answer but to be polite I queried it instead. – Weather Vane May 19 '15 at 08:30
@WeatherVane I really appriciate your patience. However, this kind of error is commonly due to silly mistakes anf you are always welcome to make the change in the answer body itself. No worries – Sourav Ghosh May 19 '15 at 08:34

score 0 · Answer 3 · answered May 19 '15 at 08:22

a according to ASCII table is 97 dec , so when you write :

char letter = 'a';
int subtract_from_letter = 3;
int result = letter - subtract_from_letter; // 97 - 3 = 94

94 in decimal, from ASCII table, is ^ in character.

x in decimal is 120, so if you want to get x from this calculation, you should do this:

char letter = 'a';
int subtract_from_letter = 3;
int result = letter - subtract_from_letter + 26; // 97 - 3 + 26 = 120

but, it's weird !

score 0 · Answer 4 · answered May 19 '15 at 13:38

0

The simplest (but safe) way is:

   char letter = 'a';
   int substract_from_letter = 3;

   letter = 'a' + ((letter + 'z' - 'a' + 1 - substract_from_letter - 'a') 
                   % ('z' - 'a' + 1));

answered May 19 '15 at 13:38

Toni Homedes i Saun

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C Programming - Design

4 Answers4