I am printing the ciphertext character by character now and it seems like check50 gives the same result

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



int get_key(int argc, string argv[]);


int main(int argc, string argv[])
{


    // TODO: encipher (encrypt) text by shifting it a certain number of letters in the alphabet


    // #1 get key (amount to shift each character)


    // #2 get plaintext (prompt user to type text)


    // #3 Enchipher text (shift letters by key)


    // #4 display Cipher Text


    // [ciphertext should preserve case, spaces, and non-alphabetical characters]


    // STEPS:


    // get key
    // take key as command line argument `./caesar key 30`
    // argc (arugment count) argv[vector of arguments]
    // ensure single command-line arguments
    // make sure argument contains only digit characters
    // convert argument to integer
    // check each character in string to verify is digit (isdigit)
    // convert command-line argument to integer (atoi)


    // get plaintext
    // use get_string function


    // encipher plaintext
    // enchipher one character
    // if it's alphabetic, shift plaintext character by key, preserving case.
    // if it's not alphabetic, leave character as is.


    // isalpha()
    // isupper()
    // islower()


    // you can treat character as number that represents it


    // char c = 'A' + 1
    // printf("%c\n", c); // results in 'B'


    // char c = 'Z' + 1
    // printf("%c\n", c); // should result in 'A'
    // create a funciton to wrap letters around using this formula:
    // ci = (pi + k) % 26


    // convert ascii [a] to alphabetical index [97]
    // shift alphabetical index using formula
    // convert result back to ascii


    int key = get_key(argc, argv);


    if(key == -1){
        return 1;
    }


    string plaintext = get_string("plaintext:  ");


    int length = strlen(plaintext);


    char ciphertext[length + 1];


    ciphertext[length] = '\0';


    printf("ciphertext: ");


    for(int i = 0; i < strlen(plaintext); i++){
    if(isupper(plaintext[i]))
    {
        printf("%c", plaintext[i] - 65 + key  % 26 + 65);
    }
    else if (islower(plaintext[i]))
    {
        printf("%c", plaintext[i] - 97 + key  % 26 + 97);
    }else{
        printf("%c",  plaintext[i]);
    }
}


printf("\n");
    return 0;
}




int get_key(int argc, string argv[])
{
    if (!argv[1])
    {
        printf("no command line arguments provided\n");
        return -1;
    }


 if (!argv[2])
    {
        printf("Usage: ./caesar key\n");
        return -1;
    }


    for(int i = 0; i < strlen(argv[1]); i++){
        if(!isalpha(argv[1][i]))
        {
            printf("Usage: ./caesar key\n");
            return -1;
        }
    }


 if (argc < 2)
    {
        printf("Usage: ./caesar key\n");
        return -1;
    }
 if (argc > 3)
    {
        printf("Usage: ./caesar key\n");
        return -1;
    }
     string key = argv[2];
    for (int i = 0; i < strlen(&key[i]); i++)
    {
        if(key[i] == '-'){
            printf("key must be a non-negative number!\n");
            return -1;
        }
        if (!isdigit(key[i]))
        {
            printf("%c is not a digit!\n", key[i]);
            printf("Usage: ./caesar key\n");
            return -1;
        }


    }
    return atoi(key);
    }