CODEWITHSUNDEEP
cstructlinked-listscopedoubly-linked-list
fmatt
fmatt

Reputation: 494

using c struct to implement linked list

I'm trying to implement a linked list in c. I get input from user, put it in a struct called Box, and save the order of the inputs using a linked list. here is the struct :

struct Box
{
   struct Box *previous;
   struct Box *next;
   int amount;
};

and here is the implementation :

void main()
{
   struct Box firstBox;
   scanf("%d", &(firstBox.amount));

   struct Box *addressKeeper;
   addressKeeper = &firstBox;

   for (int i = 0; i < 3; i++)
   {
       struct Box newBox;
       scanf("%d", &(newBox.amount));
       newBox.previous = addressKeeper;
       addressKeeper->next = &newBox;
       addressKeeper = &newBox;
   }
}

but when I print the next boxes' addresses in this way, all of them are the same?

struct Box *ptr = &firstBox;
for (int i = 0; i < 3; i++)
{
   printf("%p \n", ptr->next);
   ptr = ptr->next;
}

am I doing something wrong?

Upvotes: 0

Views: 70

Answers (2)

Outrageous Bacon
Outrageous Bacon

Reputation: 562

You are not creating the new Box elements correctly in the loop. You have one struct Box that goes out of scope every pass through the loop. You would need to allocate each one dynamically via malloc() or else allocate an array that you draw from. Something like this:

   struct Box listOfBoxes[3];
   struct Box *addressKeeper;
   addressKeeper = &listOfBoxes[0];

   for (int i = 1; i < 3; i++)
   {
       scanf("%d", &(listOfBoxes[i].amount));
       listOfBoxes[i].previous = addressKeeper;
       addressKeeper->next = &listOfBoxes[i];
       addressKeeper = &listOfBoxes[i];
   }

However, you need to examine the next and previous pointer assignments carefully. There is still something wrong there that I didn't change.

Upvotes: 0

Vlad from Moscow
Vlad from Moscow

Reputation: 310930

You are using local object newBox in this loop

for (int i = 0; i < 3; i++)
{
    struct Box newBox;
    scanf("%d", &(newBox.amount));
    newBox.previous = addressKeeper;
    addressKeeper->next = &newBox;
    addressKeeper = &newBox;
}

After the loop accessing this object invokes undefined behavior because it is not alive anymore.

It seems your program outputs the same address of this local object.

Either you need to allocate dynamically nodes or use an array of nodes declared before the loop.

Upvotes: 1

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