substitution.c

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

int main(int argc, string argv[])
{
    // Validate key length
    if (argc == 2)
    {
        string key = argv[1];
        int key_len = strlen(key);

        // Check key is 26 characters, and alphabetic
        for (int i = 0, n = key_len; i < key_len; i++)
        {
            if (key_len < 26 && isalpha(key[i]))
            {
                printf("Key must contain 26 characters.\n");
                return 1;
            }
            if (key_len > 26 && isalpha(key[i]))
            {
                printf("Key must contain 26 characters.\n");
                return 1;
            }
        }


        // Reject digits
        int j = 0;
        if (isdigit(key[j]))
        {
            printf("Usage: ./substitution key\n");
            return 1;
        }

        // Check for duplicate letters
        for (int q = 0; q < key_len; q++)
        {
            for (int w = q + 1; w < key_len; w++)
            {
                if (key[q] == key[w])
                {
                    printf("Key must not contain repeated characters.\n");
                    return 1;
                }
            }
        }


        // Collect plaintext
        string s = get_string("plaintext: ");

        // Output encrypted text
        printf("ciphertext: ");
        for (int o = 0, m = strlen(s); o < m; o++)
        {
            if (isalpha(s[o]) && isupper(s[o]))
            {
                int upper = s[o] - 'A';
                printf("%c", toupper(key[upper]));
            }
            else if (isalpha(s[o]) && islower(s[o]))
            {
                int lower = s[o] - 'a';
                printf("%c", tolower(key[lower]));
            }
            else
            {
                printf("%c", s[o]);
            }
        }
        printf("\n");
        return 0;

    }

    // User inputs more than a single key
    if (argc != 2 || isdigit(argv[1]))
    {
        printf("Usage: ./substitution key\n");
        return 1;
    }
}