help with queue plz

[eager04]

ok im trying to change this from link list to a queue.bt whenever i run it it sill shows from last to first which is the opposite from what i want.here it is, i hope somebody can try to see whats wrong with it.

#include
#include
struct queue
{
int data;
queue* next;
};
class queueL
{
protected:
queue* last;
queue* first;
public:
queueL()
{last = NULL;}
void additem(int d);
void display();
};
void queueL::additem(int d)
{
queue* newqueue = new queue;
newqueue->data=d;
newqueue->next=last;
last = newqueue;
}
void queueL::display()
{
queue* current = last;
while(current!=NULL)
{
cout<data;
current = current->next;
}
}
void main()
{
queueL ql;
clrscr();
ql.additem(25);
ql.additem(36);
ql.additem(49);
ql.additem(64);
ql.display();
}

[kc0arf]

Hi,

I am not a well - versed programmer, but yes, you do have a linked list going there in one direction. How about trying a dual linked list (a list that has pointers going in both directions so that if you had

A B C

and the "now" is on B, that you have the pointer information for both A and C, so that you can move forwards or backwards.

Then, for queue mode, go to the end of the list, and work it backwards using the reverse pointers.

Christian

[infamous]

your add_item method should traverse to the END of the list, and add the element there. the remove_item method should remove the first element in the list.

[kc0arf]

Hi,

I think infamous is right! Might have mis-read what you are trying to do.

I do know that in my programming days, I did make a lot of linked lists both ways, just for flexibility. I also kept another copy of the head pointer around in another variable in the event I "lost" it during coding.

Christian

[abu_sager]

Hello
Why you are using a c-style struct why you are not using a c++ class
here you are a good example

[gusano79]

Hello Why you are using a c-style struct why you are not using a c++ class here you are a good example

There isnothing wrong with using a struct for the queue nodes here. If all you're storing are a few pieces of information, there's no need for an overgrown class to handle the same data.

The queue template class posted, while it is an example (I'm assuming it works as advertised), I wouldn't call it a good example--it's a bloated gob of code handed down from on high. It looks like Abu here is more interested in saying "ooh, look at all the features i'm using! classes! enumerations! pointers! const methods! templates!" ...all of which are useful things, but not really necessary for the problem at hand.

If you haven't run across templates yet, have a look at http://babbage.cs.qc.edu/STL_Docs/templates.htm for a start.

Now, on to your question--the solution provided by 'infamous' will work, as will doubly-linking the list and running through the display backward. Another approach is to alter the display method:

void queueL::display()
{
    display_recursive(last);
}

void queueL::display_recursive(queue *n)
{
    if(n == NULL) return;
    display_recursive(n->next);
    cout << endl << n->data;
}

Recursion can be a handy tool when dealing with linked lists. If it's not clear,display_recursive will continue to call itself, passing each successive queue node until it hits the end of the list, i.e. it hits a null next pointer. Then it will display them in reverse order as each recursive call terminates.

This is a little more complicated than the other two solutions; I'm just offering it as an alternative. Now you have at least three ways to get your display method to show the nodes in the desired order.

--sg