My implementation of cs50 pset2 (2020) "Caesar"

caesar.c

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



int main(int argc, string argv[])
{
    // Accept only one command-line argument
    
    if (argc == 2)
    {
        // Check that they all chars are digits
        int j = 0;
        string z = argv[1];
        for (int i = 0, n = strlen(z); i < n; i++)
        {
            if (isdigit(z[i]))
            {
                j++;
            }
        }
        // Convert according to the key
        if (j == strlen(z))
        {
            
            int k = atoi(argv[1]);
            string text = get_string("plaintext: ");
            printf("ciphertext: ");
            for (int l = 0, m = strlen(text); l < m; l++)
            {
                // Convert
                if islower(text[l])
                {
                    printf("%c", (((text[l] - 'a') + k) % 26) + 'a'); 
                }
                else if isupper(text[l])
                {
                    printf("%c", (((text[l] - 'A') + k) % 26) + 'A'); 
                }
                else 
                {
                    printf("%c", text[l]);
                }
            }
            printf("\n");
            return 0;
        }
        else
        {
            printf("Usage: ./caesar key\n");
            return 1;
        }
    }
    else
    {
        printf("Please provide the key.\n");
        return 1;
    }
}