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From: kevq@banana.demon.co.uk (Kevin F. Quinn)
Subject: Re: raised Constraint_Error?
Date: 1995/04/04
Message-ID: <19950404.003251.35@banana.demon.co.uk>#1/1
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sender: news@demon.co.uk (Usenet Administration)
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references: <Pine.SUN.3.91.950403111430.12054B-100000@babbage>
organization: The Banana Arc
reply-to: kevq@banana.demon.co.uk
newsgroups: comp.lang.ada
Date: 1995-04-04T00:00:00+00:00
List-Id: <comp.lang.ada>

In article <Pine.SUN.3.91.950403111430.12054B-100000@babbage>,
          winchest@csee.usf.edu (Jeff Winchester) wrote:

> Could anyone tell me why the following code would cause a constraint 
> error?  I've been pulling out hairs for a couple of days now and am 
> obviously overlooking something simple.  Thanks.

Indeed you are - dereferencing null pointers you naughty boy :)

>      current := head;
>      while current /= null loop

OK, assume here that current.next = NULL (which it will be, eventually,
assuming the list isn't looped)...

> 	if (current.name (current.nl)) = (sname (snl)) then
>         current := current.next;

So now current=NULL

> 	   if (current.class (current.cl)) = (sclass (scl)) then

Crash and burn.

>            current := current.next;
>              if (current.number = snumber) then
>                 current := current.next;
>                 pay := current.cost;

Ditto here, of course (if current.next.next on entry to loop is NULL).

>        [...deletia...]
>                      text_io.put ("Thank you.  Your change is $");
>                      flt_io.put (item => pcost, aft => 2, exp => 0);
>                    else current := current.next;  \
>                    end if;                         | Phew!  Scratch
>              else current := current.next;         | this lot for a
>              end if;                               \ start!  Put the
> 	   else current := current.next;             / c := c.next
> 	   end if;                                   | outside all the ifs.
> 	else current := current.next;                |
> 	end if;                                     /
>      exit when current = null;   <--- This is redundant, BTW
>      end loop;
>    end price;

I'm not doing both for you :)

A tip for the above, I would suggest flattening stuff a bit.
Inside the loop, it's best only to consider current.*, and not
any deeper than that.  Only traverse the list at the end of the
loop, i.e. immediately before the check "current /= NULL":

   while current /= NULL loop
     [Lots of wibble about current.<whatever> here - don't
      change current]
     current := current.next;
   end loop;


BTW Why not ask your course lecturer?  That's what they're for...

-- 
Kevin F. Quinn           * "No-one ever made any money out of good
kevq@banana.demon.co.uk  *  looks and charm."
kevq@cix.compulink.co.uk * "You obviously haven't met Lady Hamilton..."
Compu$erve: 100025,1525  *                               Blackadder III