PSET 2 - Substitution - incompatible integer to pointer conversion using strstr function

Question

I am doing PSET 2 - Substitution and am trying to implement a check for every character of the key to be unique. For that I create a helper array key_check, fill it with letters from the key one by one and check if the later I am trying to add to the helper array is already in there. For that, I am trying to use strstr function.

Here is my code:

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

#define _GNU_SOURCE
#include <string.h>

int main(int argc, string argv[])
{
    // use a variable not to have to deal with argv[] array all the time
    string key = (argv[1]);

    // capitalize everything
    for (int i = 0; i < strlen(key); i++)
    {
        key[i] = toupper(key[i]);
    }

    if (argc != 2)
    {
        printf("Usage: ./substitution key\n");
        return 1;
    }
    else if (strlen(key) != 26)
    {
        printf("Key must contain 26 characters.\n");
        return 1;
    }
    else
    {
        string key_check = malloc(strlen(key)+1);
        for (int i = 0; i < strlen(key); i++)
        {
            // I compare if the same letter was already used before
            if (!isalpha(key[i]) || strstr(key_check, key[i]) != NULL)
            {
                return 1;
            }
            // here I fill the letters to helper array one by one
            key_check[i] = key[i];
        }
    }

    return 0;
}

When I try to compile it, I get this error:

test.c:36:55: error: incompatible integer to pointer conversion passing 'char' to parameter of type 'const char *'; take the address with & [-Werror,-Wint-conversion]
            if (!isalpha(key[i]) || strstr(key_check, key[i]) != NULL)
                                                      ^~~~~~
                                                      &
/usr/include/string.h:350:58: note: passing argument to parameter '__needle' here
extern char *strstr (const char *__haystack, const char *__needle)

I don't understand why are integers involved in here and what it is trying to convert. key[i] is supposed to return a single character from the key string, and if I replace key[i] with a hardcoded letter, like 'A', it works. Trying to reduce variables and use argv[1][i] instead also did not do it.

I've read the documentation on strstr multiple times and tried to find a similar problem here, but did not find one about strstr specifically.

I am only on Week 2 right now, maybe there's something about pointers I don't know or understand right now, but I am completely clueless. Do you have any ideas what it is it doesn't like?

Answer 1

The compiler doesn't differentiate between int and char. char is essentially a subset of integers. I will venture a guess that 'A' does not in fact work (single-quotes makes it a char), whereas "A" does work (double quotes makes it a char * (aka string)). char * string pointers are covered in section 4 if memory serves. Until then, you might be interested in the string function strchr.