Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!uunet!munnari.oz.au!uniwa!vax7!nlewispn
From: Lewis_P@cc.curtin.edu.au (Peter Lewis)
Newsgroups: comp.sys.mac.programmer
Subject: Re: Problems with THINK Pascal
Message-ID: <6736.2792f672@cc.curtin.edu.au>
Date: 15 Jan 91 04:32:49 GMT
References: <1991Jan11.190458.7714@swift.cs.tcd.ie>
Organization: Curtin University of Technology
Lines: 88

In article <1991Jan11.190458.7714@swift.cs.tcd.ie>, david@swift.cs.tcd.ie writes:
> I have just purchased a new version of THINK Pascal to run on a Mac II si,
> and have encountered two problems when dealing with PACKED ARRAYs of BOOLEAN.
> 
>        setting A[0]:=TRUE corresponds to setting the byte value to 1,
>        ...
>        setting B[1]:=TRUE corresponds to setting the byte value to 2,

Certainly does seem that way

> ii. Assigning a boolean value based on the comparison of elements from packed
>     boolean data structures, will not always produce the correct result
>        VAR
>           RESULT: BOOLEAN;
>           LOGICAL1,LOGICAL2: PACKED ARRAY[0..7] OF BOOLEAN;
> 
>        RESULT:=LOGICAL1[3]=LOGICAL2[3];
>     will produce the following results
>        LOGICAL[1]    LOGICAL[2]    RESULT
>        ----------    ----------    ------
>              TRUE          TRUE      TRUE
>              TRUE         FALSE      TRUE    !!!
>             FALSE          TRUE     FALSE
>             FALSE         FALSE      TRUE

In fact, there is no problem with this assignment.  The problem stems from 
the fact that the following code
    bool:=false;
    logical1[3]:=bool;
does not work.  In fact it clears a bit far away from logical1.  The code
generates this -
   CLR.B      D6               ; bool:=false;
   MOVEQ      #$03,D0
   MOVE.W     D0,D1
   LSR.W      #$3,D1           ; offset in bytes from the start of logical1
   TST.B      D6
   BEQ.S      ClrIt
   ADDI.W     #-$108,D1        ; Ooops!
   BSET       D0,$00(A5,D1.W) 
   BRA.S      Finish
ClrIt
   BCLR       D0,$00(A5,D1.W)  ; where is the corresponding ADDI
Finish

   So assigning a boolean variable to logical1[3] will work fine if the
variable is true, but will fail if the boolean is false.

> This problem can be solved by using an if statement
> 
>     IF LOGICAL1[3] = LOGICAL2[3]
>        THEN RESULT:=TRUE
>        ELSE RESULT:=FALSE;

I suspect when you tested this, you put "LOGICAL1[3]:=true" and not 
   for bool:=false to true do begin
     logical3[3]:=bool;
as you probably did for the truth table.

> I would like to know if I am missing something obvious, if there are any other
> known problems in this area, etc.

The only related problem I know of, is that this code -

const
   s = 'hello';
begin
   writeln(s[2]);   { will print random stuff }
end.

will fail if range checking is on.

> David M. Abrahamson,
> Dept. of Computer Science,
> Trinity College, Dublin 2, Ireland.

I suggest you leave packed arrays of booleans alone unless you are very
careful!
Fun fun fun,
   Peter.

-- 
Disclaimer:Curtin & I have an agreement:Neither of us listen to either of us.
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