CS50x 2020 Problem Set 2 Substitution

substitution.c

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

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

    if (argc < 2)
    {
        printf("Usage: ./substitution key\n");
        return 1;
    }
    string key = argv[1];
    //char plain_text[] = "Hello, world";


    if (strlen(key) != 26)
    {
        printf("Key must contain 26 characters.\n");
        return 1;
    }
    //iterate through the string, check if there's non alphabetic char isalpha, convert key into lowercase (tolower())
    for (int i = 0; i < strlen(key); i++)
    {
        if (isalpha(key[i]) == 0)
        {
            printf("Key must only contain alphabets.\n");
            return 1;
        }
        //loop the string again to check if there's repeated char
        for (int j = 0; j < strlen(key); j++)
        {
            if (tolower(key[i]) == tolower(key[j]) && i != j)
            {
                printf("Each character can only appear once.\n");
                return 1;
            }
            else
            {
                key[i] = tolower(key[i]);
            }
        }
    }

    // get user input
    string plain_text = get_string("plaintext: ");

    printf("ciphertext: ");
    for (int i = 0; i < strlen(plain_text); i++)
    {
        if (plain_text[i] >= 97 && plain_text[i] <= 122)
        {
            //lower case
            printf("%c", key[plain_text[i] - 'a']);
        }
        else if (plain_text[i] >= 65 && plain_text[i] <= 90)
        {
            //upper case
            printf("%c", key[plain_text[i] - 'A'] - 32);
        }
        else
        {
            // do nothing, just print the character
            printf("%c", plain_text[i]);
        }
    }
    printf("\n");
    return 0;
}