pop function

prototype:

Code:

struct stack pop (struct stack ** tos);

In main:

Code:

				if (tos == NULL) 
				{
					printf ("stack empty!\n");
				}else
				{
					struct stack temp = pop(&tos);
					printf("%d popped.\n", temp.sdata);
				}

function definition:

Code:

struct stack pop (struct stack ** tos) 
{
	//holds the return value (data)
	struct stack temp;
	//use to free the head node
	struct stack **freep = tos; 
	//reposition the head of the stack for post pop
	if((*tos)->next != NULL) *tos = (*tos)->next;
	
	//get data value
	temp = **freep;
	//free head node
	free(*freep);

	return temp;

 }

Where did I go wrong with this? I can't quite figure it.

What I've done is quite a bit different to what you've done, but this is a stack program which does contain push and pop that I did ages ago.

I would like to know how to do this function with a pointer to a pointer so that the stack is updated in main automatically. I already know how to do other stack function. I'm not asking how to write a stack, but instead how to use this method of updating the stack in main through a pointer to a pointer to the stack object. Thanks anyway.

Oic...well gimme a little bit and I'll see what I can come up with.

BTW here is the push function if needed:

Code:

struct stack * push(struct stack * tos, int data) 
{
	struct stack *newp = NULL;

	if(tos == NULL)
	{
		newp = (struct stack  *) malloc (sizeof (struct stack));
		newp->next = NULL;
		newp->sdata = data;
		tos = newp;
		return tos;
	}else
	{
		newp = (struct stack  *) malloc (sizeof (struct stack));
		newp->sdata = data;
		newp->next = tos;
		return newp;
	}

}

The structure is:

Code:

struct stack
{
   struct stack *next;
   int sdata;
};

But my problem is the pop function:

And in main the definition:

Code:

int main()
{
    struct stack *tos = NULL;
    ...
    return 0;
}

I don't think I'm missing much, something small, but this moment I'm forced to study systems analysis because I have a test in 6 hrs. It would be great if someone can spot the error. I tried but could not figure it out.

I have posted my code before,u should read it carefully.

[1]->[2]->[3]->NULL

what will happen after free([3])?
[2]->next still point to [3] !!!!!

if((*tos)->next != NULL) *tos = (*tos)->next;

I don't go there if it is NULL.
Yet it's true that tos is than freed. Not sure what to do at that point. Maybe allocate a new node. hmm

The only way your pop would make sense to me is if your stack was circular.

hmm, but it make the same mistake.
if the linked list has only one item.
Like
[1]->NULL

>if((*tos)->next != NULL) *tos = (*tos)->next;
>I don't go there if it is NULL.

so after free(*freep), *tos point to a unavail item.

BTW: temp=**freep; can't work
replace it with memcpy(&temp,*freep,sizeof(struct stack));

temp=**freep;

This did work. But I also have your code now. Yes, I am slightly stuck. I'll keep working on it. I might have to consult a text book or something. I want to use the pointer to pointer. That is my goal. To pass the structure like so:

struct stack *tos;

struck stack temp = pop(&tos);

I can kind of see how your code works if the stack is circular but I don't quite get it.

Code:

struct stack pop (struct stack ** tos) 
{
	//holds the return value (data)
	struct stack temp;
	//use to free the head node
	struct stack *freep = *tos; 

	//reposition the head of the stack for post pop
	if((*tos)->next == NULL) 
	{
		*tos = NULL;
		temp = *freep;
		free(freep);
	}else
	{
		*tos = (*tos)->next;
	
		//get data value
		temp = *freep;
		//memcpy(&temp,*freep,sizeof(struct stack));
		//free head node
		free(freep);
	}

	return temp;

 }

I got it!

Sorry I didn't get back sooner well it looks like you've worked it out anyway. I did do something pretty similar (albeit with some minor differences in structure), but I won't bother posting the code now. Good job Troll_King.