Array of structs

paolor · 04-22-2009, 12:33 PM

Hi! I have problems with an array of structs. This is an example of my problem.

Code:

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

int size = 0;

struct a{
	int x;
};

void add(struct a *as){
	as = realloc(as, size++ * sizeof(struct a));
	as[size - 1].x = size;

	printf("%d\n", as[size -1].x);
}

int main(){
	struct a *as = NULL;
	add(as);
	
	printf("%d\n", as[size - 1].x);
	

return 0;
}

Segmetation fault is the error. Why?

Tanuj_Tanmay · 04-22-2009, 12:59 PM

many errors....
i)after u made ur data type named 'a',why r u using struct always....
remember its like others ..int,float....why complicate it....
ii)how will main() know that u changed something in add()...if u keep add() as void

here's mine..

Code:

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

int size=0;

typedef struct
{
	int x;
}a;

a *add(a *as)
{
	as = (a*)malloc(++size * sizeof(a));
	as[size - 1].x = size;

	printf("%d\n", as[size -1].x);
	return as;
}

int main()
{
	a *as;
	as =add(as);
	
	printf("%d\n", as[size-1].x);
	

	return 0;
}

I don't use realloc....malloc rocks!!

Bladactania · 04-22-2009, 01:16 PM

the add(as); in the original post is results in the same thing as the same line in your code. In the original, he passes a pointer to the structure, so any changes to as within the function are reflected in as in main. So as = add(as); is redundant.

quzah · 04-22-2009, 02:54 PM

Quote:

Originally Posted by Tanuj_Tanmay

many errors....
i)after u made ur data type named 'a',why r u using struct always....
remember its like others ..int,float....why complicate it....

No it's not. This isn't C++. You cannot omit the word struct if you have not created a new type with typedef.

Quote:

Originally Posted by Tanuj_Tanmay

ii)how will main() know that u changed something in add()...if u keep add() as void

Well, considering they're changing size, they could compare the value before and after the call. However, unless they use a pointer to a pointer, their pointer won't be keeping track of the newly allocated memory. Returning it as you have shown will get around that limitation as well.

Your program is wrong for what they're trying to do though. They're trying to have the ability to reallocate the array bigger and bigger each time. The actual problem is that the size is being incremented AFTER it's being allocated. So what's the problem? What's zero * something?

Quzah.

quzah · 04-22-2009, 02:57 PM

Quote:

Originally Posted by Bladactania

the add(as); in the original post is results in the same thing as the same line in your code. In the original, he passes a pointer to the structure, so any changes to as within the function are reflected in as in main. So as = add(as); is redundant.

That's wrong. You cannot change what the pointer points at, unless you actually have the address of that pointer. A pointer will let you change the value of what is pointed at. Not what is pointed at.

Quzah.

paolor · 04-23-2009, 03:45 AM

I dont want that the add function returns a struct pointer. This works, but .... How can i save a int in x and to have the same value int the main?

vart · 04-23-2009, 04:59 AM

Quote:

Originally Posted by paolor

I dont want that the add function returns a struct pointer. This works, but .... How can i save a int in x and to have the same value int the main?

you could pass pointer to pointer

paolor · 04-23-2009, 09:17 AM

pointre to pointer? Hwo?

matsp · 04-23-2009, 09:42 AM

pass in add(&as), make the function take add(a **as), perhaps?

--
Mats

paolor · 04-23-2009, 10:00 AM

Thanks matsp, but how? Could you edit my code with your idea. Please

vart · 04-23-2009, 10:46 AM

Quote:

Originally Posted by paolor

Thanks matsp, but how? Could you edit my code with your idea. Please

Why? you cannot edit your code yourself?

paolor · 04-23-2009, 11:21 AM

Yes I can do it, but I cannot what to do.
Please, could you post the right code.

ssharish2005 · 04-23-2009, 04:28 PM

>but I cannot what to do.
haha i like that. lol

Well, people here won't write the code for you here. We can just hint you but not write the code or give the code.

Here is a peice of hint

Code:

void add(struct a **as){  <-- This is how the function prototype should look like

and in the main

Code:

struct a *as = NULL;
add(&as);

As you can see your sending the address of the pointer 'as' to the function. But since you are passing the pointer address and function 'add' should refereing to pointer to a pointer struct a.

-ssharish

paolor · 04-24-2009, 05:38 AM

thank you

04-22-2009, 12:59 PM	#2
Tanuj_Tanmay Registered User Join Date: Feb 2009 Location: &CProgramming Posts: 12	many errors.... i)after u made ur data type named 'a',why r u using struct always.... remember its like others ..int,float....why complicate it.... ii)how will main() know that u changed something in add()...if u keep add() as void here's mine.. Code: #include <stdio.h> #include <stdlib.h> int size=0; typedef struct { int x; }a; a add(a as) { as = (a)malloc(++size sizeof(a)); as[size - 1].x = size; printf("%d\n", as[size -1].x); return as; } int main() { a *as; as =add(as); printf("%d\n", as[size-1].x); return 0; } I don't use realloc....malloc rocks!!
Tanuj_Tanmay is offline

04-22-2009, 01:16 PM	#3
Bladactania Registered User Join Date: Feb 2009 Posts: 278	the add(as); in the original post is results in the same thing as the same line in your code. In the original, he passes a pointer to the structure, so any changes to as within the function are reflected in as in main. So as = add(as); is redundant.
Bladactania is offline

04-23-2009, 03:45 AM	#6
paolor Registered User Join Date: Apr 2009 Posts: 6	I dont want that the add function returns a struct pointer. This works, but .... How can i save a int in x and to have the same value int the main?
paolor is offline

04-23-2009, 09:17 AM	#8
paolor Registered User Join Date: Apr 2009 Posts: 6	pointre to pointer? Hwo?
paolor is offline

04-23-2009, 09:42 AM	#9
matsp Kernel hacker Join Date: Jul 2007 Location: Farncombe, Surrey, England Posts: 15,686	pass in add(&as), make the function take add(a **as), perhaps? -- Mats __________________ Compilers can produce warnings - make the compiler programmers happy: Use them! Please don't PM me for help - and no, I don't do help over instant messengers.
matsp is offline

04-23-2009, 10:00 AM	#10
paolor Registered User Join Date: Apr 2009 Posts: 6	Thanks matsp, but how? Could you edit my code with your idea. Please
paolor is offline

04-23-2009, 11:21 AM	#12
paolor Registered User Join Date: Apr 2009 Posts: 6	Yes I can do it, but I cannot what to do. Please, could you post the right code. Last edited by paolor; 04-23-2009 at 11:51 AM.
paolor is offline

04-23-2009, 04:28 PM	#13
ssharish2005 Protocol Test Engineer Join Date: Sep 2005 Location: fseek(UK) Posts: 1,316	>but I cannot what to do. haha i like that. lol Well, people here won't write the code for you here. We can just hint you but not write the code or give the code. Here is a peice of hint Code: void add(struct a *as){ <-- This is how the function prototype should look like and in the main Code: struct a as = NULL; add(&as); As you can see your sending the address of the pointer 'as' to the function. But since you are passing the pointer address and function 'add' should refereing to pointer to a pointer struct a. -ssharish __________________ Life is like riding a bicycle. To keep your balance you must keep moving - Einstein
ssharish2005 is offline

04-24-2009, 05:38 AM	#14
paolor Registered User Join Date: Apr 2009 Posts: 6	thank you
paolor is offline

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