substitution

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

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

    // check for 26 characters in key, check to make sure there are right amount of cmd line arguments
    if (argc != 2)
    {
        printf("Usage: ./substitution (key)\n");
        return 1;
    }

    // is it a letter?
    string cipherkey = argv[1];
    int len = strlen(cipherkey);

    for (int i = 0; i < len; i++)
    {
        if (!isalpha(cipherkey[i]))
        {
            printf("Usage: ./substitution (key)\n");
            return 1;
        }

        // is it 26 characters?
        if (len != 26)
        {
            printf("Key must be 26 characters!\n");
            return 1;
        }

        // has to be 26 diff alphabet
        for (int first = 0; first < len; first++)
        {
            for (int second = first + 1; second < len; second++)
            {
                if (toupper(cipherkey[first]) == toupper(cipherkey[second]))
                {
                    printf("Usage: ./substitution (key)\n");
                    return 1;
                }
            }
        }
    }

    // run actual cipher decoding part
    string text = get_string("plaintext: ");

    for (int i = 0; i < len; i++)
    {
        if (islower(cipherkey[i]))
        {
            cipherkey[i] = cipherkey[i] - 32; // difference between upper and lower case letter equivalent in ASCII ('A' - 'a' = 32 for example)
        }
    }

    // final result, look for if the index is upper or lower case
    printf("ciphertext: ");

    for (int i = 0; i < len; i++)
    {
        if (isupper(text[i]))
        {
            int upperpos = text[i] - 65;
            printf("%c", cipherkey[upperpos]); // print result of math, make it an index of cipher key to print later
        }

        else if (islower(text[i]))
        {
            int lowerpos = text[i] - 97;
            printf("%c", cipherkey[lowerpos] + 32);
        }
        else printf("%c", text[i]);
    }
    printf("\n");
}