caesar attempt

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

bool only_digits(string s);
int rotate(string text, int k);
int atoi(string t);

int main(int argc, string argv[])
{
    //Command line gets a user input of an int, error code if other input given than a single arg
    if (argc != 2)
    {
        printf("Usage: ./caesar key\n");
        return 1;
    }
    //Check if digits
    if (only_digits(argv[1]) == false)
    {
        printf("Usage: ./caesar key\n");
        return 1;
    }
    //else
    //{
        //printf("Works! \n");
    //}
    int key;
    key = atoi(argv[1]);

    //Program prompts user for text input
    string ptext = get_string("Plaintext: \n");

    //Run function that uses text input as array and replaces letters by a number of positions (int from command line) in alphabet array
    rotate(ptext, key);

    //print new coded message
}

bool only_digits(string s)
{
    int digits = 0;
    int n = strlen(s);
    for (int i = 0; i < n; i++)
    {
        if (isdigit(s[i]))
        {
            digits++;
        }
    }
    if (digits == n)
        {
            return true;
        }
    else
        {
            return false;
        }
}

int rotate(string text, int k)
{
    printf("ciphertext: ");
    for (int i = 0, n = strlen(text); i < n; i++)
    {
        if (!isalpha(text[i]))
        {
            printf("%c", text[i]);
        }
        else if (isupper(text[i]))
        {
            printf("%c", ((((text[i] - 65) + k) % 26) + 65));
        }
        else if (islower(text[i]))
        {
            printf("%c", ((((text[i] - 97) + k) % 26) + 97));
        }
        else
        {
            printf("Something went wrong.");
        }
    }
    printf("\n");
    return 0;
}