vigenere

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

int main(int argc, string argv[])
{
    (void)argc;
    if (argv[1] != NULL)
    {
        string k = (argv[1]);
        string p = get_string("plaintext: \n");
        string c = p;      //declaring "c" as a variable for the cipher
        printf("ciphertext: ");
        int j = 0;
        int m = strlen(k);
        for (int i = 0, n = strlen(p); i < n; i++)
        {
            if isalpha(p[i])    //checks if letter
            {
                if isupper(p[i]) //preserve case
                {
                    if isupper(k[m]) //checks case of k[m]
                    {
                        c[i] = (char)((p[i] + k[m] - 130) % 26 + 65);   //askii to alpha adjust to askii
                        printf("%c", p[i]); //add character to string cipher
                    }
                    else //checks case of k[m]
                    {
                        c[i] = (char)((p[i] + k[m] - 162) % 26 + 65);
                        printf("%c", p[i]);
                    }
                }
                else //preserve case
                {
                    if isupper(k[m])
                    {
                        c[i] = (char)((p[i] + k[m] - 162) % 26 + 97);   //askii to alpha adjust to askii
                        printf("%c", p[i]);    //add character to string cipher
                    }
                    else
                    {
                        c[i] = (char)((p[i] + k[m] - 194) % 26 + 97);
                        printf("%c", p[i]);
                    }
                }
                j++; //counts to next character in key
            }
            else //prints non-letters
            {
                printf("%c", p[i]);
            }
            if (m == j) //checks if all characters in k have been used
            {
                j = 0; //reset j
            }
        }
        printf("\n");
        return 0;
    }
    return 1;
}