Array of structs

This is a discussion on Array of structs within the C Programming forums, part of the General Programming Boards category; Hi! I have problems with an array of structs. This is an example of my problem. Code: #include <stdio.h> #include ...

  1. #1
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    Question Array of structs

    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?

  2. #2
    Registered User Tanuj_Tanmay's Avatar
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    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!!

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    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.

  4. #4
    ATH0 quzah's Avatar
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    Quote Originally Posted by Tanuj_Tanmay View Post
    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 View Post
    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.
    Hope is the first step on the road to disappointment.

  5. #5
    ATH0 quzah's Avatar
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    Quote Originally Posted by Bladactania View Post
    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.
    Hope is the first step on the road to disappointment.

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    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?

  7. #7
    CSharpener vart's Avatar
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    Quote Originally Posted by paolor View Post
    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
    The first 90% of a project takes 90% of the time,
    the last 10% takes the other 90% of the time.

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    pointre to pointer? Hwo?

  9. #9
    Kernel hacker
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    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.

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

  11. #11
    CSharpener vart's Avatar
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    Quote Originally Posted by paolor View Post
    Thanks matsp, but how? Could you edit my code with your idea. Please
    Why? you cannot edit your code yourself?
    The first 90% of a project takes 90% of the time,
    the last 10% takes the other 90% of the time.

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    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 12:51 PM.

  13. #13
    Registered User ssharish2005's Avatar
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    >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

  14. #14
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    thank you

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