caesar.c

#include <stdio.h>
#include <cs50.h>
#include <stdlib.h>
#include <string.h>
#include <ctype.h>

int main(int argc, string argv[])
{
    if (argc != 2)
    {
        printf("USAGE: ./caesar NUM\n");
        return 1;
    }

    //get offset value from cmdline args
    int offset=0;
    offset = atoi(argv[1]);

    for (int i=0; i<strlen(argv[1]); i++)
    {
        if (isdigit(argv[1][i]) == 0)
        {
            printf("USAGE: ./caesar NUM\n");
            return 1;
        }
    }

    //adjust for offset greater than 26 (redundant cycles)
    //eg: offset of 27 is just offset of 1.  Eliminate lap of 26.
    offset = offset % 26;

    //get text to encode
    string plaintext;
    plaintext = get_string("plaintext: ");

    printf("ciphertext: ");

    int shift=0;
    //loop thru and encode each letter
    for (int i=0; i<strlen(plaintext); i++)
    {
        if (isupper(plaintext[i]) != 0)
        {
            shift = 64;
            plaintext[i] = plaintext[i] - shift;    //shift letters down to 1-26
            plaintext[i] = plaintext[i] + offset;   //add the caesar encryption offset
            plaintext[i] = plaintext[i] % 26;       //Rotate around z to a (eg: 27 becomes 1)
            plaintext[i] = plaintext[i] + shift;    //shift letters back up
            printf("%c", plaintext[i]);
        }
        else if (islower(plaintext[i]) != 0)
        {
            shift = 96;
            plaintext[i] = plaintext[i] - shift;    //shift letters down to 1-26
            plaintext[i] = plaintext[i] + offset;   //add the caesar encryption offset
            plaintext[i] = plaintext[i] % 26;       //Rotate around z to a (eg: 27 becomes 1)
            plaintext[i] = plaintext[i] + shift;    //shift letters back up
            printf("%c", plaintext[i]);
        }
        else
        {
            printf("%c", plaintext[i]);
        }
    }
    printf("\n");
}