rot.c


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#include <stdio.h>
#include <string.h>
#include <ctype.h>
#include <stdlib.h>

#define mod(A) ((A)%26)

int main(int argc, char **argv)
{
    if(argc!=3)
        exit(1);
    char *string = *(argv+1);
    int i, rot = (int)strtol(*(argv+2), NULL, 10);
    for(i=0; i<strlen(string); i++)
        if(isalpha(string[i])){
            if(string[i]-'A'<26){ // Simple way to differentiate lower and upper case if all letters are known to be alpha.
                putchar(mod(string[i]-'A'+rot)+'A');
            }else{
                putchar(mod(string[i]-'a'+rot)+'a');
            }
        }else{
            putchar(string[i]);
        }
    putchar('\n');
    return 0;
}