CS50's Problem Set 2 - Substitution

substitution.c

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

bool isAlpha(string key);
bool hasDuplicates(string key);

int main(int argc, string argv[])
{
    if (argc != 2 || !isAlpha(argv[1]))
    {
        printf("Usage: ./caesar key\n");
        return 1;
    }
    else if (strlen(argv[1]) < 26)
    {
        printf("Key must contain 26 characters.\n");
        return 1;
    }
    else if (hasDuplicates(argv[1]))
    {
        printf("Key must not contain duplicates.\n");
        return 1;
    }
    else
    {
        string key = argv[1];
        printf("%s \n", key);
        string plaintext = get_string("Plaintext: ");
        printf("ciphertext: ");
        for (int i = 0, len = strlen(plaintext); i < len; i++)
        {
            if (isalpha(plaintext[i]))
            {
                if (isupper(plaintext[i]))
                {
                    char letter = key[ ((plaintext[i] - 65) % 26)];
                    if(islower(letter))
                    {
                        printf("%c", letter - 32);
                    }else
                    {
                        printf("%c", letter);
                    }
                }
                else
                {
                    char letter = key[((plaintext[i] - 97) % 26)];
                    if (isupper(letter))
                    {
                        printf("%c", (int)letter + 32);
                    }else
                    {
                        printf("%c", letter);
                    }

                }
            }
            else
            {
                printf("%c", plaintext[i]);
            }

        }
        printf("\n");

        return 0;

    }
}


bool isAlpha(string key)
{
    for (int i = 0; i < strlen(key); i++)
    {
        if (!isalpha(key[i]))
        {
            return false;
        }
    }
    return true;
}

bool hasDuplicates(string key)
{
    for (int i = 0, len = strlen(key); i < len; i++)
    {
        for (int j = 0; j < len; j++)
        {
            if (key[i] == key[j] && i != j)
            {
                return true;
            }
        }
    }
    return false;
}