PSET2 Caesar: Ordered Comparison Between Pointer and Integer Error - c

Question

I am currently taking Harvard's CS50 course (Intro to CompSci).
I have not even learned pointers yet, I am confused on this error message. The line it is facing an error on in question is this: For context, the full message is as follows:
caesar.c:48:36: error: ordered comparison between pointer and integer ('string' (aka 'char *') and 'char') [-Werror] if (plaintext[j] + key > z)
So, I am unable to understand the error message due to not yet being educated on pointers. Help50 is useless when I compiled with it. Here is my code if it is needed, I hope the comments help!
#include <cs50.h>
#include <stdio.h>
#include <ctype.h>
#include <string.h>
int main(int argc, string argv[])
{
//Creates string key
string key = argv[1];
//Checks for appropriate argument count
if (argc != 2)
{
printf("Usage: ./caesar key");
return 1;
}
//Checks if all argument characters are digits
for (int i = 0; i < strlen(key); i++)
{
if (isdigit(key[i]) == 0)
{
printf("Usage: ./caesar key");
return 1;
}
}
//Gets plaintext from user
string ciphertext = NULL;
char z;
string plaintext = get_string("plaintext: ");
//Converts plaintext to ciphertext
for (int j = 0; j < strlen(plaintext); j++)
{
//Asks if character is alphabetical
if (isalpha(plaintext[j]))
{
//Asks if character is uppercase and assigning ASCII code accordingly
if (isupper(plaintext[j]))
{
z = 90;
}
else
{
z = 122;
}
//Performs conversion operation
if (plaintext[j] + key > z)
{
ciphertext[j] = plaintext[j] + key - 25;
}
else
{
ciphertext[j] = plaintext[j] + key;
}
}
//Keeps text the same since it is not a letter, and therefore shouldn't be shifted
else
{
ciphertext[j] = plaintext[j];
}
}
printf("ciphertext: %s\n", ciphertext);
}
Thank you!

Answer 1

//Creates string key
string key = argv[1];
key is a "string", not an integer value.
if (plaintext[j] + key > z)
The code is attempting to add a char to a string, then compare that to a char.
You need to go back to the basics and review the lesson until you understand arrays of characters and strings.
string ciphertext = NULL;
Here, you initialise a string, then seem to expect it to somehow grow as you later write characters into that string.
Sit down with a paper and pencil and diagram out what you expect to be happening with memory storage...