substitution_v1.c

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

int main(int argc, string argv[])
{

    //must provide key
    if (argc != 2)
    {
        printf("USAGE: ./substitution 26KEY\n");
        return 1;
    }

    //store key in a new variable
    string key = argv[1];

    //check if key is 26 long
    if (strlen(key) != 26)
    {
        printf("Key must contain 26 characters.\n");
        return 1;
    }

    //check if all 26 characters are acctually letters
    for (int i=0; i<strlen(key); i++)
    {
        if (isalpha(key[i]) == 0)
        {
            printf("Key must contain letters only.\n");
            return 1;
        }
    }

    //check if any letters are repeated
    //check each letter and keep track in an array called "used"
    //eg:  When you see the letter A in the key, you set position 1 as true
    //if its already set to true, that means you already had that letter in the key
    bool used[26] = {};
    int pos=0;
    for (int i=0; i<strlen(key); i++)
    {
        pos=toupper(key[i])-65;
        if (used[pos] == true)
        {
            printf("Key can't repeat letters.\n");
            return 1;
        }
        else
        {
            used[pos] = true;
        }
    }

    string plaintext="";

    //get word to encrypt
    plaintext = get_string("plaintext: ");

    //how to substitute each letter using the 26KEY?
    //loop through the entire word...
    //get position of each letter (eg: a=1, b=2, etc)
    //then look up that position in the 26KEY and print that letter
    int position=0;
    char oldChar;
    char newChar;

    printf("ciphertext: ");

    for (int i=0; i<strlen(plaintext); i++)
    {
        oldChar = plaintext[i];

        //only translate letters
        if (isalpha(oldChar) == 0)
        {
            printf("%c", oldChar);
        }
        else
        {
            //translate a-z to 0-25 position to look up in the key
            position = toupper(oldChar)-65;
            newChar = key[position];

            //preserve the case of the original letter, regardless of case in the key
            //ie:  lower stays lower, upper stays upper
            if (islower(oldChar))
            {
                newChar=tolower(newChar);
            }
            else
            {
                newChar=toupper(newChar);
            }
            printf("%c",newChar);
        }
    }
    printf("\n");
}