substitution.c

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

bool is_valid_key(string s);  // function decl
int main(int argc, string argv[])

{
    if (argc != 2) // checkm if two arguments.
    {
        printf("Usage: ./substitution key\n");

        return 1;
    }

    if (!is_valid_key(argv[1])) // Check key validity.
    {
        puts("Key must contain 26 characters.\n");

        return 1;
    }

    string s = get_string("plaintext: ");  // User input as plaintext.

    string difference = argv[1];

    for (int i = 'A'; i <= 'Z' ; i++)
    {
        difference[i - 'A'] = toupper(difference [i - 'A']) - i ;
    }

    printf("ciphertext: ");

    for (int i = 0, len = strlen(s); i < len; i++)

    {
        if (isalpha(s[i]))  // check alphabates.

        {
            s[i] = s[i] + difference[s[i] - (isupper(s[i]) ? 'A' : 'a')];

        }
        printf("%c", s[i]);  // prints cyohertext.

    }
    printf("\n");
}


bool is_valid_key(string s)

{
    //int i = 0;
    int len = strlen(s);
    if (len != 26)

    {
       return false ;
    }

    int freq [26] = {0} ;

    for (int i = 0; i < len ; i++)
    {
        if (!isalpha(s[i]))

        {
           return false;
        }

        int index = toupper(s[i]) - 'A';
        if (freq[index] > 0)

        {
           return false;
        }

        freq[index]++;

    }

    return true;

}