how to shift down alphabet characters in a cyclic way? - c

Question

i want to find the alphabet charachter that is 7 charachters before mine ,so i wrote this function to do so and it works fine :
char find7_before(char letter){
switch (letter){
case 'g':
return 'z';
break;
case 'f':
return 'y';
break;
case 'e':
return 'x';
break;
case 'd':
return 'w';
break;
case 'c':
return 'v';
break;
case 'b':
return 'u';
break;
case 'a':
return 't';
break;
default:
return (char)(((int)letter) - 7);
}
}
but i think i can do it in a smarter way without all of these cases but i just can't figure it out ! (i figured how to find 7 letters after in a cyclic way ) any help or an idea or a hint ?
thank you :)

Answer 1

Assuming ASCII with continuous ['a' , 'z']...
Simply "mod 26".
letter = ((letter - 'a' - 7) mod 26) + 'a';
Yet C does not have a Euclidean mod operator.
See What's the difference between “mod” and “remainder”?
So create a Euclidean mod function - save for later use.
int modulo_Euclidean(int a, int b) {
int m = a % b;
if (m < 0) {
// m += (b < 0) ? -b : b; // avoid this form: it is UB when b == INT_MIN
m = (b < 0) ? m - b : m + b;
}
return m;
}
letter = modulo_Euclidean(letter - 'a' - 7, 26) + 'a';
Alternately code can take advantage that 'a' has a value of 97 and not subtract so much that letter - ('a'%26) - 7 becomes negative.
letter = (letter - ('a'%26) - 7)%26 + 'a';
Pedantic code would not assume continuous ['a' , 'z'] and perform more elaborate code.

Answer 2

subtract 'a' (so now it's in 0-25), subtract 7, and mod 26. Then add 'a' again so it's back to a char.

Answer 3

In my opinion, the clearest and simplest way is to use an if statement:
char find7_before(char letter) {
char value = letter - 7;
if (value < 'a') {
value += 26;
}
return value;
}
The precondition here is the letter is between 'a' and 'z', inclusive.
This technique generalizes as well:
char findn_before(char letter, int n) {
char value = letter - n;
if (value < 'a') {
value += 26;
}
return value;
}
Precondition on letter is the same as before; n must be between 0 and 26, inclusive.