Re: Can someone help me understand this queue package?

["Liddle Feesh" <no_see_reply_address@spam.com>]

"Michal Nowak" wrote:

> Look at the line where you defined access type to node (you even marked
> it with comment).
>
> You had errors, because you did not declared a variable of this type.
> Try now:
>
> > procedure add (n:  in integer;  q:  in out queue) is
>     New_Item : Link;   --declare a variable of link type
> > begin
>     New_Item := new node;
>
>     --now set appriopriate fields in New_Item,
>     --set Head and Tail for queue, and should be OK.

No - after adding those lines the package spec no longer compiles. I don't
understand why you want to add those lines in there.

Errors generated are : "Direct Name, Link, is not visable, Ignoring future
references",  and "Parse error expecting declaration, got BEGIN,
Skipping..."

[snip]
> >How do I create a new node, and point the 'next' value to the 'value' of
> >the
> >next node? I know how this is done on paper...
>
> link is access type to node. So speaking in C language, it should point
> to element of type node. In your queue you have components Head and Tail,
> that should point to first and last element in the queue. They are of the
> same type as New_Item is. So:
>
> New_Item.Next := Tail;     --next points you to element, which is still
>                            --considered to be on the end of the queue
> Tail          := New_Item; --now the last element is the one you created.

Link is access type to node - okay
Should point to element of type node - okay (HOW is this done?)
Queue is made up of a Head, and the remainder(Queue). Tail in this case will
point to the last element, although this is not strictly correct.

New_Item.Next := Tail   -- Next is the pointer to the value section of the
next element (node). I see - you assign "Tail" to the New_Item's NEXT
pointer. Excellent. Thanks.

Tail := New_Item -- Now, this I don't understand. You are assigning a new
element to the tail?

----

What I don't get - from the original post, is what is the syntax for
manipulating this queue stuff, and where do I put it? I'm completely stuck
when it comes to these three files - 1. Test Harness, 2. Package Body, and
3. Package Spec.

Okay - so I can call procedures and functions in the spec from the from the
test harness, but what do I do? I mean - I've added the line

add;

into a main menu in the test harness - to call the procedure 'add' - which
currrently has just a 'null;' in it. It doesn't do anything!

Thanks for your help though,

Mike