caesar1.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;
    }

    int offset=0;
    offset = atoi(argv[1]);
    offset = offset % 26;       //remove redundant offset multiples of 26
    //printf("The shift is %i\n", offset);

    string plaintext;
    plaintext = get_string("plaintext: ");

    int shift=0;
    char newLetter;

    printf("ciphertext: ");

    //process each letter.
    for (int i=0; i<strlen(plaintext); i++)
    {
        if (isupper(plaintext[i]) != 0)
        {
            shift = 64;
            plaintext[i] = plaintext[i] - shift;    //scale letters back to to 1-26
            plaintext[i] = plaintext[i] + offset;   //add offset cipher
            plaintext[i] = plaintext[i] % 26;       //adjust if over 26
            plaintext[i] = plaintext[i] + shift;    //scale letters back to to original range
            printf("%c", plaintext[i]);
        }
        else if (islower(plaintext[i]) != 0)
        {
            shift = 96;
            plaintext[i] = plaintext[i] - shift;    //scale letters back to to 1-26
            plaintext[i] = plaintext[i] + offset;   //add offset cipher
            plaintext[i] = plaintext[i] % 26;       //adjust if over 26
            plaintext[i] = plaintext[i] + shift;    //scale letters back to to original range
            printf("%c", plaintext[i]);
        }
        else
        {
            printf("%c", plaintext[i]);
        }
    }
    printf("\n");
}