stack problem

[anu]

hi friends

i am stuck in a stack program in which pop function is not ok.

its removing element first time only, then showing error.

dont know why free is not working

i am attaching my program. any help would be appreciated.

[Troll_King]

Give me a few minutes and I'll post an answer if this stinking website hold up.

[winbill]

ur pop function should be:

struct stack *pop (struct stack * tos) {
struct stack *temp=tos;
struct stack *pre=NULL;

while (tos->next != NULL) {
pre=tos;
tos = tos -> next;
}
printf ("element removed is %d\n",tos -> sdata);
free(tos);

if(pre==NULL)temp=NULL;
else pre->next=NULL;

return temp;
}

[Troll_King]

This is a stack, that's not a pop function. It's part of a push function. I'm stuck on one thing. Need some more time to figure it out.

[winbill]

It is a stack.
It is a linked list (but not a queue) stack.

[Troll_King]

So why did you traverse the list to pop an item?

[winbill]

while (tos->next != NULL) {
pre=tos;
tos = tos -> next;
}

[Troll_King]

Okay, bad mornig. You add nodes to the top, yeah that's right, but I don't understand why you traversed the list.

[winbill]

struct stack * push(struct stack * tos, int data) {

struct stack *ptr = tos;
if (tos != NULL) {
while (tos -> next != NULL)
tos = tos -> next;
/* ~~~~~~~~~~~~~~~~~~~*/
tos -> next = (struct stack *) malloc (sizeof (struct stack));
tos = tos -> next;
tos -> next = NULL;
tos -> sdata = data;
return ptr;
}
else {
tos = (struct stack *) malloc (sizeof (struct stack));
tos -> next = NULL;
tos -> sdata = data;
return tos;
}
}

[Troll_King]

I may just be having a bad morning. Sorry if I didn't read the code correctly. I guess I just need more time this morning.

[Troll_King]

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;
	}

}

Okay, yes this is the right push. I confused myself. Actually I'm still a little lost with your pop function. I'm looking into it.

[Troll_King]

pop function called as:

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

Defined like:

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;

 }

Took a long time to figure out.

[anu]

finally i could get an errorfree program with ur efforts.

thnax guys ,