c-PSET2 Caesar:指针和整数错误之间的有序比较

我目前正在参加哈佛大学的CS50课程(CompSci简介(。我甚至还没有学会指针，我对这个错误消息感到困惑。它面临错误的行是这样的：对于上下文，完整的消息如下：

caesar.c:48:36:错误：指针和整数("字符串"(又名"char*"(和"char'"(之间的有序比较[-Weror]if(明文[j]+key>z(

因此，由于尚未接受指针方面的教育，我无法理解错误消息。当我用Help50编译时，它是无用的。如果需要的话，这是我的代码，我希望评论能有所帮助！

#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: %sn", ciphertext);
}

谢谢！