Access to linked list variables inside a linked list

Question

I have a linked list with another linked list in it and I want to integrate data in them but I couldn't.

Here is my code:

Structures declaration:

typedef struct BigStructure {
    UINT x;
    UINT y;

    struct SmallStructure* smallStructure;

    struct BigStructure* next;
} BigStructure;

typedef struct SmallStructure {
    UINT x;
    UINT y;

    struct SmallStructure* next;
} SmallStructure;

Structures manipulation functions:

BigStructure* addLinkedListElement(BigStructure* linkedList)
{
    if(linkedList-> next == NULL)
    {
        return NULL;
    }

    BigStructure* newLinkedList = malloc(sizeof(linkedList));
    newLinkedList->next = linkedList;
    return newLinkedList;
}

BigStructure* removeLinkedListElement(BigStructure* linkedList)
{
    //If the list is empty, we return NULL
    if(linkedList == NULL)
        return NULL;

    //If the list contains one element
    if(linkedList->next == NULL)
    {
        free(linkedList);
        return NULL;
    }

    //if the list contains at least 2 elements
    BigStructure* tmp = linkedList;
    BigStructure* ptmp = linkedList;

    /* Tant qu'on n'est pas au dernier élément */
    while(tmp->next != NULL)
    {
        //ptmp stores the address of tmp
        ptmp = tmp;
        //We move tmp (but pmpt keeps the old value of tmp)
        tmp = tmp->next;
    }

    ptmp->next = NULL;
    free(tmp);
    return linkedList;
}

BigStructure* getLinkedListElement(BigStructure* linkedList, int id)
{
    int i = 0;

    for(i=0; i<id && linkedList != NULL; i++)
    {
        linkedList = linkedList->next;
    }

    if(linkedList == NULL)
    {
        return NULL;
    }
    else
    {
        return linkedList;
    }
}

I tried the above code to access a SmallStructure variable, but I get a big number (looks like an address):

BigStructure* bigStructure = NULL;

void addBigStructure(UINT x, UINT y) {

        if(bigStructureNb == 1)
        {
            bigStructure->x = x;
            bigStructure->y = y;
        }
        else
        {
            BigStructure* newBigStructure;
            newBigStructure = (BigStructure*)addLinkedListElement((BigStructure*)&bigStructure);
            newBigStructure->x = x;
            newBigStructure->y = y;
        }
}

void addSmallStucture(UINT x, UINT y) {

    if(smallStructuresNb == 1)
    {
        bigStructure->startTrigger = malloc(sizeof(BigStructure*));
        bigStructure->startTrigger->x = x;
        bigStructure->startTrigger->y = y;
    }
    else
    {
        BigStructure* tmpBigStructure = NULL;
        tmpBigStructure = (BigStructure*)getLinkedListElement(&bigStructure, rowID); //Table row ID
        g_print("%d", tmpBigStructure->id); //Here I get a false value !!!!
        //Here I want to set the value of the tmpBigStructure->smallStructure->x/y
}
}

How come when you're adding a linked list element, you return if the next element is NULL? Shouldn't you be walking down the list until the next value is NULL, and then put a new linked list element where the next pointer points to? — KrisSodroski
– KrisSodroski, Commented May 31, 2013 at 20:16
I corrected it, thanks ! I still have a problem now with the "child" linked-list. — MHDaouas
– MHDaouas, Commented May 31, 2013 at 21:00
Just a debug comment: Did you try & dereference this address-looking number to see if it is in fact containing the small structure value? Might get you closer to your answer. — Ryhan
– Ryhan, Commented Jun 1, 2013 at 8:13
Thanks Ryan. Can you be more precise ? You mean I don't put "&" before the parameter "bigStructure" ? — MHDaouas
– MHDaouas, Commented Jun 1, 2013 at 9:09

VAT69 · Accepted Answer · 2013-06-01 12:32:06Z

1

Seems to me that the problem is with getLinkedListElement(). Here's some code suggestion :

  BigStructure* getLinkedListElement(BigStructure** linkedList, int id)
    {
        int i;
        if( linkedList == NULL || *linkedList == NULL)
        return NULL ; 

     //We cannot update HEAD(linkedList), therfore using local pointer.
        BigStructure* linkWalk = * linkedList;

    /*I am asuming ids are mapped to linked list nodes as below.
     id 0 -> first node
     id 1 -> second node
     ......
     id n -> n-1 node
    */

//starting from second node since linkWalk is already pointing to first above.
        for(i=1; i<id && linkWalk != NULL; i++)  
                linkWalk = linkWalk->next;

   // At this point , either id = 0 OR id = i OR the link has been traversed.

       return linkWalk ;

    }

Lastly , before calling g_print("%d", tmpBigStructure->id) , Please check tmpBigStructure != NULL.

answered Jun 1, 2013 at 12:32

VAT69

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MHDaouas Over a year ago

Yesss it works !! Thank you very much :) You should become a member of Stackoverflow to help people :)

VAT69 · Accepted Answer · 2013-06-01 07:49:18Z

0

Seems to me that the global : "BigStructure* bigStructure = NULL" is your head pointer and you passed its address.You can add nodes at the head.Therefore , you don't need to walk down the list to add a link node.

Suggestion : you are passing the address of bigStructure( i.e a double pointer) and operating on it as a single pointer in structure manipulation functions. This needs to be changed in all your functions.

As an example, your function to add nodes could be like(Assuming linkedList is HEAD) :

BigStructure* addLinkedListElement(BigStructure** linkedList)
{
    BigStructure* newLinkedList = malloc(sizeof(BigStructure));
    if (newLinkedList == NULL)
      return NULL ;    // Just to handle malloc failure

    newLinkedList->next = *linkedList;
    *linkedList = newLinkedList;
    return newLinkedList;
}

answered Jun 1, 2013 at 7:49

VAT69

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1 Comment

MHDaouas Over a year ago

Thank you user2442730. Well this code works for the big structure but when I want to add a small structure I can't have right values and the program crashes. Can you give me the right implementation of " void addSmallStucture(UINT x, UINT y)" ? Thank you very much.

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