vigenere cipher

vigenere.c

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

int shift(char c);

int main(int argc, string argv[])
{
    string keyw = argv[1];
    if(argc == 2) // argument count validator
    {
        for (int i = 0; keyw[i] != '\0'; i++) // key validator
        {
            if (isalpha(keyw[i]) == 0)
            {
                printf("Usage: ./vigenere keyword\n");
                return 1;
            } 
        }
        int key;
        int j = 0; // new iterator
        string keyword = argv[1];
        string p = get_string("plaintext: "); // ask for text
        for (int i = 0; i < strlen(p); i++) // ciphering
        {
            j = j % strlen(keyword);
            if (isalpha(p[i]) && isupper(p[i]))
            {
                key = shift(keyword[j]);
                p[i] -= 65;
                p[i] = (p[i] + key) % 26;
                p[i] += 65;
            }
            else if(isalpha(p[i]) && islower(p[i]))
            {
                key = shift(keyword[j]);
                p[i] -= 97;
                p[i] = (p[i] + key) % 26; 
                p[i] += 97;
            }
            if (isalpha(p[i]))
            {
                j++;
            }
        }
        printf("ciphertext: %s\n", p); // returns ciphered text 
       
    }
    else 
    {
        printf("Usage: ./vigenere keyword\n");
        return 1;
    } 
}

int shift(char c)
{
    int k = (int) c;
    if (isupper(c))
    {
       return k -= 65;
    }
    else if (islower(c))
    {
       return k -= 97;
    }
    else {
        return 1;
    }
}