C Rotating String Program?

Question

#include <stdio.h>


int main()
{
int rotation, i=0;
char str[80]={0}; 
printf("Enter Text: ");
scanf("%[^\n]", str); 
printf("\"");

printf("Enter Rotation: ");
scanf("%i", &rotation);

while(str[i]) 
    {
        if (str[i] >= 'a' && str[i] <= 'z')
        printf("%c\n", 'a' + (str[i] - 'a' + rotation)%26); 
        else    
        printf("%c\n", str[i]);
        i++;    
    }

return 0;
}

Have a hard time understanding this line of code (printf("%c\n", 'a' + (str[i] - 'a' + rotation)%26); )

Can anyone just write a brief explanation quickly it would help me

Answer 1

The program is taking the user's input text and on a per character basis rotating it through the alphabet based on the number entered. It works because of the ASCII table.

The line in question takes the character a user entered, offsets it by 'a' (which equals 91 in ASCII), adds in the rotation factor then performs modulo 26 on the result (how many characters are there in the alphabet again?) to make sure the result is still a lowercase character.

I bet you could find a good way to break this program :)