Please help! CONSTRAINT_ERROR: access check failed - This has me stumped!

Please help! CONSTRAINT_ERROR: access check failed - This has me stumped!

[tgwaltz]

Hello all,

I have a small problem with a program that's supposed to represent a
doubly-linked list using pointers.  I have most of the
functions/procedures programmed, at least as well as I can without
being able to run the program and test them!  What happens is I get a
"access check failed" error whenever one of the functions is called
(i.e. "GetAtIndex, Remove, Contain, etc")  I figure it must have
something to do with a value being set to null or something, but I've
stared at my laptop long enough and I can't figure it out myself.  If I
can get this error fixed, then I can work through the rest of the
program and test my functions and so on.

The error occurs where I have a call to "anker.next" in the code.

I apologize that the comments in the program are in German - I am
studying abroad in Stuttgart this year.  The function names and so on
are still English though - everything should be clear.  "Anker" means
"anchor."

Thanks for you help!
TW


with Ada.Text_IO; use Ada.Text_IO;
with Ada.Integer_Text_IO; use Ada.Integer_Text_IO;

procedure aufgabe1 is

   procedure liste is
      type zelle;
      type ref_zelle is access zelle;
      type zelle is record
         v : integer;
         prev : ref_zelle;
         next : ref_zelle;
      end record;

      anker,a : ref_zelle;
      max : natural := 10;

      b : ref_zelle;

      --Fuer das Leeren der Liste
      procedure Empty is
      begin
         anker.next := null;
      end;

      --Fuer das Abfragen, ob die Liste leer ist
      function IsEmpty return String is
      begin
	 if anker.next = null then
            return "true";
	 else
	    return "false";
         end if;
      end IsEmpty;

      --Fuer das Abfragen der Laenge der Liste
      function Length return Integer is
         i : Integer := 0;
      begin
         while anker.next /= null loop
	    i := i + 1;
	    anker := anker.next;
	 end loop;
	 return i;
      end Length;

      --Fuer die Ausgabe der Liste
      procedure Put is
      begin
	 while anker /= null loop
            put(anker.v);
            anker := anker.next;
         end loop;
         a := anker;
      end;

      --Fuer das Abfragen des Elements an der gegebenen Position in der
Liste.
      --Beachten Sie, dass die Position einen gueltigen Wert haben muss
      function GetAtIndex(i : integer) return Integer is
      begin
	 for q in 0..i loop
	    if anker.next /= null then
  	       anker := anker.next;
	    else
	       --nicht gefunden/Liste nicht lang genug
	       return 58008;
	    end if;
	 end loop;
	 return anker.v;
      end GetAtIndex;

      --Fuer das Hinzufuegen eines Elements am Ende der Liste
      procedure InsertAtEnd(val : Integer) is
      begin
	 while anker.next /= null loop
	    anker := anker.next;
	 end loop;

	 anker.next := new zelle;
	 anker.next.v := val;
	 anker.next.prev := anker;
	 anker.next.next := null;
      end InsertAtEnd;

      --Fuer das Hinzufuegen eines Elements am Ende der Liste, falls
die Liste das Element nicht bereits enthaelt
      procedure InsertAtEndIfNotExists(val : Integer) is
	 enthaelt : Boolean := False;
      begin
	 while anker.next /= null loop
	    anker := anker.next;
	    if anker.v = val then
	       enthaelt := true;
	    end if;
	 end loop;

	 if not enthaelt then
 	    anker.next := new zelle;
	    anker.next.v := val;
	    anker.next.prev := anker;
	    anker.next.next := null;
	 end if;
      end InsertAtEndIfNotExists;

      --Fuer das Hinzufuegen eines Elements an der
      --Position einen gueltigen Wert haben muss
      procedure InsertAtIndex(i : Integer; val : Integer) is
      begin
         b := new zelle;
         for q in 1..i loop
	    if anker.next /= null then
  	       anker := anker.next;
	    else
	       --nicht gefunden/Liste nicht lang genug
	       return;
	    end if;
	 end loop;
      b.prev := anker;
      b.next := anker.next;
      b.v := val;
      end InsertAtIndex;

      --Fuer das Entfernen eines Elements, falls es in der Liste
vorhanden ist
      procedure Remove(i : Integer) is
      begin
	  for q in 0..i-1 loop
	    if anker.next /= null then
  	       anker := anker.next;
	    else
	       --Liste nicht lang genug
	       return;
	    end if;
	 end loop;

         if anker.next.next /= null then
	    anker.next := anker.next.next;
	    anker.next.next.prev := anker;
	    anker := anker.next.next;
	 end if;
      end Remove;

      --Fuer das Pruefen ob ein Element in der Liste enthalten ist
      function Contain(val : Integer) return String is
      begin
	 while anker.next /= null loop
	    anker := anker.next;
	    if anker.v = val then
	       return "true";
	    end if;
	 end loop;
      return "false";
      end Contain;

      --Fuer das Abfragen, an welcher Position in der Liste ein Element
enthalten ist
      function IndexOf(val : Integer) return Integer is
	 ind : Integer := 0;
      begin
	 while anker.next /= null loop
	    if anker.v = val then
	       --nur das erste Ereignis wird ausgegeben
	       return ind;
	    end if;
	    anker := anker.next;
	    ind := ind + 1;
	 end loop;
      return -1;
      end IndexOf;

   begin
      anker := new zelle;
      a := anker;
      a.prev := null;
      for i in 1..max-1 loop
         a.v := i;
         a.next := new zelle;
	 a.next.prev := a;
         a := a.next;
      end loop;

      a.v := max;
      a.next := null;
      a := anker;

      put("Normale Liste: "); --funktioniert
      Put;
      new_line;

      put("Empty(): "); --funktioniert
      --Empty;
      --Put;
      new_line;

      put("IsEmpty(): "); --funktioniert
      --put(IsEmpty);
      --Put;
      new_line;

      put("Length(): ");
      --put(Length);
      new_line;

      put("GetAtIndex(5): ");
      --put(GetAtIndex(5));
      new_line;

      put("InsertAtEnd(11): ");
      --InsertAtEnd(11);
      Put;
      new_line;

      --passt
      put("InsertAtEndIfNotExists(54): ");
      --InsertAtEndIfNotExists(54);
      Put;
      new_line;

      --passt nicht
      put("InsertAtEndIfNotExists(2): ");
      --InsertAtEndIfNotExists(2);
      Put;
      new_line;

      put("InsertAtIndex(2,123): ");
      --InsertAtIndex(2,123);
      Put;
      new_line;

      put("Remove(2): ");
      --Remove(2);
      Put;
      new_line;

      put("Contain(8): ");
      --Put(Contain(8));
      Put;
      new_line;

      put("IndexOf(1): ");
      Put(IndexOf(1));
      Put;
      new_line;

   end liste;

begin
   liste;
end aufgabe1;







Here's the output of the program:
[tgwaltz@tgwaltz Aufgabenblatt 9]$ ./aufgabe1
Normale Liste:           1          2          3          4          5
        6          7          8          9         10
Empty():
IsEmpty():
Length():
GetAtIndex(5):
InsertAtEnd(11):
InsertAtEndIfNotExists(54):
InsertAtEndIfNotExists(2):
InsertAtIndex(2,123):
Remove(2):
Contain(8):
IndexOf(1):

raised CONSTRAINT_ERROR : aufgabe1.adb:157 access check failed

Re: Please help! CONSTRAINT_ERROR: access check failed - This has me stumped!

[Jean-Pierre Rosen]

> The error occurs where I have a call to "anker.next" in the code.
> 
Of course. If Anker is null, you can't access anker.next

Re: Please help! CONSTRAINT_ERROR: access check failed - This has me stumped!

[Gautier]

tgwaltz:

> raised CONSTRAINT_ERROR : aufgabe1.adb:157 access check failed

I get the error on the line with the arrow.
A little help: anker.next is a shorter expression for anker.all.next.
Alles klar, Herr Kommissar ?...

       --Fuer das Abfragen, an welcher Position in der Liste ein Element
       --enthalten ist
       function IndexOf(val : Integer) return Integer is
         ind : Integer := 0;
       begin
  -->    while anker.next /= null loop
             if anker.v = val then
                --nur das erste Ereignis wird ausgegeben
                return ind;
             end if;
             anker := anker.next;
             ind := ind + 1;
         end loop;
         return -1;
       end IndexOf;

HTH, Gautier
______________________________________________________________
Ada programming -- http://www.mysunrise.ch/users/gdm/gsoft.htm

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Re: Please help! CONSTRAINT_ERROR: access check failed - This has me stumped!

[tgwaltz]

Problem solved.  Thanks for your help!

TW

On 22 Dez., 14:12, Jean-Pierre Rosen <r...@adalog.fr> wrote:
> > The error occurs where I have a call to "anker.next" in the code.Of course. If Anker is null, you can't access anker.next