vigenere

#include <stdio.h>
#include <cs50.h>
#include <string.h> 
#include <stdlib.h> 
#include <ctype.h>
 
// Prototyping alpabetical check function.
int checkalpha(string k);
 
int main(int argc, string argv[])
{ 
    // Check if there is an argument. 
    if (argc == 1)
    {
        printf("You should provide an argument!\n");
        return 1;
    }
     
    // Store a keyword as an argument.
    string wordkey = argv[1];
     
    /* Initialising integer array to store numeric values
     * of characters inside keyword. 
     */
      
    int numkey[strlen(wordkey)];
     
    /* Checking if there's no more then one argument
     * with alphabetical-only characters
     */
      
    if (argc == 2 && checkalpha(wordkey) == 0)
    {   
        // Converting lowercase characters in the key to numerical values.   
        for (int i = 0; i < strlen(wordkey); i++)
        {
            numkey[i] = (islower(wordkey[i])) ? 
            (wordkey[i]) - 97 : (wordkey[i]) - 65;         
        }
         
        // Ask user for a message to encode. 
        string msg = GetString(); 
        char msgc[strlen(msg)];
     
        int i = 0;
        int j = 0;
     
        // Iterating through plain message.
        while (msg[i] != '\0')
        {          
         
            // Routine for lowercase. 
            if (islower(msg[i]))
            {
                msgc[i] = msg[i] + numkey[j] > 122 ? 
                msg[i] - 26 + numkey[j] : msg[i] + numkey[j];
             
                printf("%c", msgc[i]);
                j++;
             
                if (j > strlen(wordkey) - 1)
                {
                    j = 0;
                }          
            }   
            // Routine for uppercase. 
            else if (isupper(msg[i]))
            {
                msgc[i] = msg[i] + numkey[j] > 90 ? 
                msg[i] - 26 + numkey[j] : msg[i] + numkey[j];
             
                printf("%c", msgc[i]);
                j++;
             
                if (j > strlen(wordkey) - 1)
                {
                    j = 0;
                }
            } 
            // Do nothing if non-alphabetical. 
            else
            {
                msgc[i] = msg[i];
                printf("%c", msg[i]);
            }
     
            i++;
               
    }  
     
        printf("\n");      
        return 0;
    }
     
    // Fall through if argument conditions are not respected. 
    else
    {
        printf("You need to provide exactly one argument ");
        printf("to code your message.\n");
        printf("Alphabetic characters only!\n");
        return 1;
    }
}
// Function to check if all characters in string array are alphabetical.
int checkalpha(string k)
{
    int n = 0;
    for (int i = 0; i < strlen(k); i++)
    {
        if (isalpha(k[i]))
        {
            n = 0;
        }
        else
        {
            n = 1;
        }
    }
    return n;
}