C - "Error: Invalid operands to binary != ..."

Question

I seem to not be able to find the problem to make the program work. C is telling me "error: Invalid operands to binary != 'grocerylist' (aka struct grocerylist) and 'int' When i try to solve this problem other bugs pop out, can anyone see other problems in this code except for the bug I posted about?

#include <stdio.h> 
#include <stdlib.h>
#define MAX 100

typedef struct grocerylist
{
    char name[MAX];
    float ammount;
    char unit[MAX];
} grocerylist;



struct grocerylist * enterItems(int arr)
{
    grocerylist * itemtolist;
    itemtolist = (grocerylist*)malloc(sizeof(grocerylist)*arr);

    if (*itemtolist != 0)
    {
        int i;

        for(i = 0; i < arr; i++)
        {
            printf("Enter item name: /n");
            scanf("%c", (itemtolist[i]).name);
            printf("Enter ammount of item: /n");
            scanf("%f", &(itemtolist[i]).ammount);
            printf("Enter unit of item: /n");
            scanf("%c", (itemtolist[i]).unit);
        }

    }
    return itemtolist;
}


void printShoppingList(grocerylist *itemtolist, int arr)
{
    int i;
    for (i = 0; i < arr; i++)
    {
        printf("%s, %f, %s", itemtolist[i].name, itemtolist[i].ammount, 
               itemtolist[i].unit);
    }
}



int main(void)
{
    int arr, number;
    grocerylist * itemtolist;

    while (number == 0)
    {
        printf("How many items would you like to add to your list? /n");
        scanf("%i", &arr);

        itemtolist = enterItems(arr);
        printShoppingList(itemtolist, arr);
        free(itemtolist);

        printf("Do you want to enter another item. 0 for yes, 1 for no");
        scanf("%i", &number);
    }
    return 0;
}

Please copy-paste (as text) the complete and full output of the compiler into your question. And then show us (with e.g. a comment) where in the [**Minimal**, Complete, and Verifiable Example](http://stackoverflow.com/help/mcve) the error is. — Some programmer dude, Mar 07 '18 at 08:00
Note that there are other problems with your code as well. For example, uninitialized local variables (like `number` in your `main` function) really *are* uninitialized. Their values are *indeterminate* and will seem random. Don't use variables without initialization. — Some programmer dude, Mar 07 '18 at 08:01
how do you solve your problem which you say produces other bugs? because obviously, you cannot perform `!=` between `int` and `struct` — Wreigh, Mar 07 '18 at 08:06
As for your problem, is the purpose of the comparison `*itemtolist != 0` to check for a null pointer returned by `malloc`? Then you should probably reread your text books about pointers and the dereference operator `*` and what it does. — Some programmer dude, Mar 07 '18 at 08:27
Oh and you might want to learn that there might be reasons [to not cast the result of `malloc`](https://stackoverflow.com/questions/605845/do-i-cast-the-result-of-malloc). — Some programmer dude, Mar 07 '18 at 08:31
It would appear that you have invalid operands to the != operator. How much clearer can a compiler message get? Look at the operands then fix them. — Lundin, Mar 07 '18 at 10:26

score 0 · Answer 1 · answered Mar 07 '18 at 08:53

You want to check that itemtolist itself (a pointer) isn't a null pointer.

But instead, your code attempts to compare *itemtolist (the pointed-at list), which isn't comparable to 0.

This fragment should be improved:

grocerylist * itemtolist;
itemtolist = (grocerylist*)malloc(sizeof(grocerylist)*arr);

if (*itemtolist != 0)

I'd write this as:

grocerylist *itemtolist = malloc((sizeof *itemtolist) * arr);

if (itemtolist)

Notes:

itemtolist != 0 is equivalent to just itemlist here in a boolean context. You can write it the long way if you want, but the short form is probably more idiomatic in C.
We don't cast the result of any of the malloc() family of functions.
Use the sizeof operator on the *itemlist so it automatically uses the correct type.
Initialize the variable as you declare it - that helps avoid accidentally using unintialized variables (but your compiler warnings should include that).

Also note that you really ought to check the return value from scanf() - I assume you removed the checking to make your example short for the question. You will need to change %c to %99s, but again, compiling with gcc -Wall or equivalent will help you spot that.

C - "Error: Invalid operands to binary != ..."

1 Answers1