caesar.c

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

#define UPPER_ALPHA_INDEX 65
#define LOWER_ALPHA_INDEX 97
#define ALPHABET_MAX 26

char encipher(char index, int cipher_key);

int main(int argc, string argv[])
{
    // Get the key from the user
    int cipher_key;
    // Guard against the user not entering the correct number of command line arguments
    if(argc != 2 || atoi(argv[1]) <= 0)
    {
        printf("Error: Incorrect number of arguments passed or value not a positive number!\n");
        return 1;
    }
    else
    {
        cipher_key = atoi(argv[1]);
    }
    
    // Get the plain text they want to encode
    string cipher_text = "";
    do
    {
        printf("\nNOTE: Only uppercase and lowercase letters will be encoded\n");
        cipher_text = get_string("Please enter the text to encode to continue: ");
    }
    while(strlen(cipher_text) == 0);
    
    
    // Encipher
    printf("ciphertext: ");
    
    for (int i = 0; i < strlen(cipher_text); i++)
        printf("%c", encipher(cipher_text[i], cipher_key));
        
    printf("\n");
}


// The way to encipher is by understanding the ascii indicies for each letter.
// Because lowercase letters and uppercase letters have arbitrary indicies
// we must first adjust the indicies so 'a' and 'A' start at index 0
// This is done by subtracting either UPPER_ALPHA_INDEX or LOWER_ALPHA_INDEX

// once the index is in the correct mapping we can add or subtract the cipher key
// and use the modulo operator to get the new index
// finally we re-adjust back up to the ascii index
// by adding back UPPER_ALPHA_INDEX or LOWER_ALPHA_INDEX

// Anything that isn't a character is returned back to the caller

char encipher(char index, int cipher_key)
{
        if (isupper(index))
            return ((index - UPPER_ALPHA_INDEX) + cipher_key) % ALPHABET_MAX + UPPER_ALPHA_INDEX;
        else if (islower(index) )
            return ((index - LOWER_ALPHA_INDEX) + cipher_key) % ALPHABET_MAX + LOWER_ALPHA_INDEX;
        else
            return index;
}