CS50's Problem Set 2 - Caesar Cipher

caesar.c

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

bool isNumber(char number[]);

int main(int argc, string argv[])
{
    if (argc != 2 || !isNumber(argv[1]))
    {
        printf("Usage: ./caesar key\n");
        return 1;
    }
    else
    {
        int k = atoi(argv[1]);
        printf("%i", k);
        string plaintext = get_string("Plaintext: ");
        printf("ciphertext: ");
        for (int i = 0, len = strlen(plaintext); i < len; i++)
        {
            if (isalpha(plaintext[i]))
            {
                if (isupper(plaintext[i]))
                {
                    printf("%c", (((plaintext[i] - 65) + k) % 26) + 65);
                }
                else
                {
                    printf("%c", (((plaintext[i] - 97) + k) % 26) + 97);
                }
            }
            else
            {
                printf("%c", plaintext[i]);
            }

        }
        printf("\n");

        return 0;

    }


}

bool isNumber(char number[])
{
    int i = 0;

    if (number[0] == '-')
        i = 1;
    for (; number[i] != 0; i++)
    {
        if (!isdigit(number[i]))
            return false;
    }
    return true;
}