Ada: Why does my queue only hold it's value value in the loop?

Question

So I'm compiling Ada with the Gnat compiler. I have a queue that is holding a record and one of the records variables is an Unbounded_String called name. That is what I'm testing right now although all the records variables should be accessible outside of the loop. The code I'm having issues with is

with ada.text_io, stacks, queues, ada.Strings.Unbounded, ada.Strings.Unbounded.Text_IO, ada.Integer_Text_IO;
use ada.Text_IO, ada.Strings.Unbounded, ada.Strings.Unbounded.Text_IO, ada.Integer_Text_IO;
procedure eliminate is
   type person is record
    index: Positive;
    age: Positive;
      skill: natural;
      wins: natural;
      loss: natural;
      name: Unbounded_String;
   end record;

   package stkPkg is new stacks(person);
   use stkPkg;
   output: Stack;

   package quePkg is new Queues(person);
   use quePkg;
   noLossQ : Queue;

   count : Positive := 1;
   input : Person;       
begin
   while not end_of_file loop
      input.name := get_line;   
      get(input.skill);
      get(input.age);
      skip_line;

      input.index := count;
      count := count + 1;
      input.wins := 0;
      input.loss := 0;
      enqueue(input, noLossQ);
      put_line(front(noLossQ).name);
   end loop;
   put_line(front(noLossQ).name);
end eliminate;

If you run it you'll notice that inside the loop put_line(front(noLossQ).name) works just fine, but once outside the loop it does nothing or prints a blank line. The end of the loop where the put_line worked, the loop ended, and then put_line doesn't work; that code follows.

   enqueue(input, noLossQ);
   put_line(front(noLossQ).name);
end loop;
put_line(front(noLossQ).name);

Here is my Queue implementation

with Unchecked_Deallocation, ada.Text_IO;
use ada.Text_IO;
package body queues is
    function is_Empty (Q: Queue) return Boolean is
    begin
        return ((Q.front = null) and (Q.back = null));
    end is_Empty;

    procedure Dequeue (Q : in out Queue) is
    begin
       if is_Empty(Q) then
          raise Queue_Empty;
       else
          Q.Back := Q.Back.Next;
          if Q.Back = null then
             Q.Front := null;
          end if;

       end if;
    end Dequeue;

    procedure Enqueue (Item: ItemType; Q: in out Queue) is
       tmp : QueueNodePointer := new QueueNode'(Item, null);
    begin
        if is_Full(q) then
            raise Queue_Full;
        else
          if Q.Back = Null then
             Q.Back := tmp;
          end if;
          if Q.Front /= null then
             q.front.next := tmp;
          end if;
          q.Front := tmp;
        end if;
    end Enqueue;

    function Front (Q: Queue) return ItemType is
    begin
        if is_Empty(Q) then
           raise Queue_Empty;
        else
           return Q.back.data;
        end if;
    end Front;
end queues;

redirected file I'm using:

Person one
            500 50 
Person 2 
            400 50 
Person 3
            300 50 
Person 4
            200 50 
Person 5 
            100 50

I can't figure out why it only works in the loop, any help is appreciated. Thanks in advance!!

Where are you getting the input from? A file, or the console? — ajb
Make sure your program isn't raising an End_Error exception. — ajb
Can’t run it because you haven’t included queues.ads. Also, Front returns Q.back.data which is at the very least confusing! — Simon Wright
I'm sorry for the late response, but I'm redirecting from a text file I'll add that file to the original post above. — Eric Ashenden
You really need to include queues.ads. And the implementation of Is_Full. — Simon Wright

Jacob Sparre Andersen Jacob Sparre Andersen · Accepted Answer · 2015-05-01T07:00:32

It seems like the problem must be in your queue implementation. You should test it properly, to ensure that it holds the properties you expect of it.

Ada: Why does my queue only hold it's value value in the loop?

1 Answers