help on comparing structs

nusstu · #1 01-Apr-2004, 22:09

Hi,

I've got a problem where I can't check a slot in an array of struct to see if it's empty. I believe the following code would be legal in JAVA but I dunno why it isn't in C.

Here's a snippet of my code. Any help appreciated.

CPP / C++ / C Code:

    int found = 0;
    struct_name newP; //some struct

    //bg is an array of struct.
    while ((!found) && (i < 20)) {
      if (bg[i] == NULL) {     //the error here is: invalid operands to binary ==
	found = 1;
	bg[i] = newP;  
	return;
      }//if
      i++;
    }//while

aaroncohn · #2 01-Apr-2004, 22:19

You're getting an error because bg doesn't exist in the context that you referenced it with. You must access members of a structure using dot notation, like so:

CPP / C++ / C Code:

if (newP.bg[i] == NULL)

Since the array bg is a member of the structure newP, you must access its contents by calling the name of the structured variable, followed by a dot, followed by the name of the member you want to access. If the member is also a struct, you can just keep going with dots like this.

CPP / C++ / C Code:

#include <stdio.h>

typedef struct
{
  char *first_name;
  char *last_name;
} NAME;

typedef struct
{
  NAME name;
  long id;
} STUDENT;

int main()
{
  STUDENT aaroncohn;
  STUDENT nusstu;

  aaroncohn.name.first_name = "Aaron";
  aaroncohn.name.last_name = "Cohn";
  aaroncohn.id = 123456;
  nusstu.name.first_name = "Some";
  nusstu.name.last_name = "Person";
  nusstu.name.id = 234567;

  printf("My first name: %s", aaroncohn.name.first_name);
  printf("My last name: %s", aaroncohn.name.last_name);
  printf("My ID number: %d", aaroncohn.id);
  printf("Your first name: %s", nusstu.name.first_name);
  printf("Your last name: %s", nusstu.name.last_name);
  printf("Your ID number: %d", nusstu.id);

  return 0;
}

nusstu · #3 01-Apr-2004, 22:47

Hi aaroncohn,

hm..i don't think my code was clear enough. i'll be more specific i have a struct called proc, bg is an array of proc (proc bg[20]; ). newP is declared as a proc and I put some stuff into newP. Now, I want to find an empty slot to dump newP into array bg. So there;s no such thing as newP.bg[i]

The main problem I have is how to check if a slot in a struct array is empty (NULL)?

So the code is now:

CPP / C++ / C Code:

    int found = 0;
    proc newP; //declare new proc

    //bg is an array of struct.
    while ((!found) && (i < 20)) {
      if (bg[i] == NULL) {     //the error here is: invalid operands to binary ==
         found = 1;
         bg[i] = newP; //dump newP into empty slot 
         return;
      }//if
      i++;
    }//while

Do u get the picture now ? :+

aaroncohn · #4 02-Apr-2004, 00:58

You cannot perform aggregate comparisons on structured types. You would have to compare each member of the array element to NULL, then if they were all NULL, you dump newP, but aggregate comparison of a struct type to anything is not allowed. Also, for future reference, when you're referring to an array, you use the plural form of the word. IE... an array of structs. When you said "an array of struct" in your comment, I took that to mean that it was a member of the struct called newP.

Code:

Aggregate Operation                           Allowed on Structs?
-------------------                           -------------------
1. I/O                                        No
2. Assignment                                 Yes
3. Arithmetic                                 No
4. Comparison                                 No
5. Argument Passage                           Yes, by value or by reference
6. Return as a function's return value        Yes

nusstu · #5 03-Apr-2004, 03:22

Hi aaroncohn,

Thanks, for the tip.

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