caesar.c

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

int main(int argc, string argv[])
{
    string cipher_key;
    //check to see if argv has only one argument. [0] == prog-name hence the <=2
    //if it does assign it to var.
    if (argc == 2)
    {
        cipher_key = argv[1];
    }
    
    //check to see if arg is valid(0-9) if so do nothing, else print usage and exit.
    for (int i = 0; i < strlen(cipher_key); i++)
    {
        if (cipher_key[i] > 47 && cipher_key[i] < 58)
        {
        }
        else
        {
            printf("Usage: ./caesar key\n");
            return 1;
        }
    }
    //if we didn't exit, print succes and ask for message.
    printf("success = %s\n", cipher_key);
    string plain = get_string("plaintext: ");
    string ciphered = plain;
    
    // the conversion. check if char is a valid letter (lower or uppercase) then change it.
    int upper[] = {64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90};
    int lower[] = {97, 98, 99, 100, 101, 102, 103, 104, 105, 106, 107, 108, 109, 110, 111, 112, 113, 114, 115, 116, 117, 118, 119, 120, 121, 122};
    
    for (int i = 0; i < strlen(ciphered); i++)
    {
        if ((int)ciphered[i] > 64 && (int)ciphered[i] < 91)
        {
            ciphered[i] = upper[((ciphered[i] + atoi(cipher_key) - 64) % 26)];
        }
        else if ((int)ciphered[i] > 96 && (int)ciphered[i] < 123)
        {
            ciphered[i] = lower[((ciphered[i] + atoi(cipher_key) - 97) % 26)];
        }
    }
    
    //print encoded message
    printf("ciphertext: %s\n", ciphered);
}