general question about how struct works.

I have exams tomorrow morning, so i need a answer soon(if it is possible)...i know that we will have a program about structs and files. First, if anyone knows the most important points in structs please post me and second i have a question about a program...here is the code:

obviously the next program does not compile correct. I try to understand some differences about structs. More specific about passing printing parametres in structs. My teacher told us that we should think structs like boxes. This is what i do. So, i imagine something like the following and i want to correct me if i am wrong.

There is one big box (struct Foo) which contains:
1) a box named foo
2) a box named group1
3) a box named group2

and finally each one of these 3 boxes contains x and y variable.
CORRECT? WRONG?

Code:

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

struct Foo{
	int x;
	int y;
}group1,group2;

int main(void){
	struct Foo foo;
	
	printf("x: %d\n", struct Foo.x);
	printf("y: %d\n", struct Foo.y);
	
	printf("x: %d\n", foo.x);
	printf("y: %d\n", foo.y);

	printf("x: %d\n", group1.x);
	printf("y: %d\n", group1.y);

	return 0;
}

Originally Posted by nullifyed

I have exams tomorrow morning, so i need a answer soon(if it is possible)...i know that we will have a program about structs and files. First, if anyone knows the most important points in structs please post me and second i have a question about a program...here is the code:

obviously the next program does not compile correct. I try to understand some differences about structs. More specific about passing printing parametres in structs. My teacher told us that we should think structs like boxes. This is what i do. So, i imagine something like the following and i want to correct me if i am wrong.

There is one big box (struct Foo) which contains:
1) a box named foo
2) a box named group1
3) a box named group2

and finally each one of these 3 boxes contains x and y variable.
CORRECT? WRONG?

Wrong, but close. Foo *defines* what the group1 and group2 structs will have inside them, but it doesn't CONTAIN, anything except Foo definitions.

And yes, both group1 and group2, contain members x and y: group1.x, group1.y, group2.x and group2.y.

Originally Posted by Adak

Wrong, but close. Foo *defines* what the group1 and group2 structs will have inside them, but it doesn't CONTAIN, anything except Foo definitions.

And yes, both group1 and group2, contain members x and y: group1.x, group1.y, group2.x and group2.y.

thank you...what happens with 1)?

The struct "Foo" doesn't contain the object of struct Foo called "foo"...
(So 1) is wrong)

ok...thanks....

what about accessing the x and y variables?
which of the six ways are correct and which wrong?

Originally Posted by nullifyed

what about accessing the x and y variables?
which of the six ways are correct and which wrong?

For this question, you can test it yourself. Comment all of them out except for one, each in turn, and compile. That said, before you test, make an intelligent guess with a good reason, then check to see if you were correct. If your guess is wrong, come back and tell us what you thought it should be, and what it turned out to be. If your guess is correct, congratulations, but of course you could always ask us to confirm the reason.

The last four, because struct Foo is a datatype, EXACTLY like int and char.

Code:

int group1, group2;

Here, group1 is an int, and group2 is an int. In your example, group1 was a struct Foo, and group2 was a struct Foo.

i would like to confirm that....i thought tha last four were correct.

after main() the command struct Foo foo; means that insteed struct Foo we can write foo?

struct Foo foo, creates or declares the new struct foo, which is of type struct Foo.

After it's declared, foo is just "foo", and no more "struct foo". That would be an error, because you already declared foo, once.

Originally Posted by nullifyed

after main() the command struct Foo foo; means that insteed struct Foo we can write foo?

No. That is like saying that in this case:

Code:

int number;

I can now write "number" instead of int.

So think: it does not make sense to say, what is the value of int? Instead, you would consider the value of an int.

if an exercise says: make a struct with some variables, what should i do?

1)

Code:

#include <stdio.h>
#include <stdlib.h>
struct Foo{
	int x;
	int y;
}group;
int main(void){

	printf("x: %d\n", group.x);
        printf("y: %d\n", group.y);
return 0;
}

or

2)

Code:

#include <stdio.h>
#include <stdlib.h>
struct Foo{
	int x;
	int y;
};
int main(void){
	struct Foo foo;
	foo.x=10;
	foo.y=11;
	
	printf("x: %d\n", foo.x);
	printf("y: %d\n", foo.y);
	return 0;
}

?

Either one. With the first one, "group" is global.

Originally Posted by MK27

Either one. With the first one, "group" is global.

can you be more specific please?

Both examples create a new Foo struct. In the first example, it's named group, in the second it's named foo.
In the first example, group is global. That is, it is available to all functions in your project. In the second, foo is local. That is, it's only accessible to the function main (and it's destroyed as soon as main finishes).

I usually use typedef to do that.

Code:

typedef struct{
        int a;
        int b;
}foo;

Now you can use it like this.

Code:

foo myfoo = {1, 2};