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From: IO92203@MAINE.BITNET (Scott Maxell)
Newsgroups: comp.lang.pascal
Subject: Re: need a little flexibility
Message-ID: <91115.111155IO92203@MAINE.BITNET>
Date: 25 Apr 91 15:11:55 GMT
References: <1991Apr24.183315.7997@ux1.cso.uiuc.edu>
 <1991Apr24.235739.25115@watmath.waterloo.edu>
Organization: University of Maine System
Lines: 82

In article <1991Apr24.235739.25115@watmath.waterloo.edu>,
nmouawad@watmath.waterloo.edu (Naji Mouawad) says:
>
>In article <1991Apr24.183315.7997@ux1.cso.uiuc.edu> amead@s.psych.uiuc.edu    n
>(ala
>mead) writes:
>>Hi.  I want to manipulate a data structure in the cleanest way possible
>>(after I write this, I'd like to minimize maintenance time).  I've been
>>working with something like this:
>>
>>Block = record
>>       x    : byte;
>>       y    : byte;
>>       ...
>>       data : array[1..252] of byte;
>>end;
>>
***... text deleted ...***
>>And a variant record requires an exhaustive listing of it's fields right?
>
>I don't understand your last statement, it seems that a record of
>the following form:
>
>Block = record
>   x : byte;
>   y : byte;
>   case boolean of
>    True : (data : array[1..252] of byte);
>    False: (dataw: array[1..126] of word);
>end;
>
>would do the trick. You could access just as easily data or dataw
>as follows:
>
>Var B : block;
>    w : word;
>
>Begin
>  w := 12345;
>  b.data[i] := hi(w);
>  b.dataw[i] := w;
>end.
>

   This last approach won't work as stated because a variant record can
only contain one of the possible fields declared. The field in the variant
should also have a declared variable and not just a type.

Type
  Block = RECORD
            X,
            Y : Byte;
            Case WordData : Boolean OF
              True  : ( DataW : ARRAY [1..126] OF Word );
              False : ( Data  : ARRAY [1..252] OF Byte );
          END;  (* record *)

VAR
  WordData : BOOLEAN;
  OneWord  : Word;
  OneByte  : Byte;

BEGIN
  B.WordData := True;   (* or false depending on what your data is. *)
  OneByte    := ????;   (* Your byte data.                          *)
  OneWord    := ????;   (* Your word data.                          *)
  n          := ????;   (* Array index.                             *)
  Case B.WordData OF
    True  : B.Data  [n] := OneByte;
    False : B.DataW [n] := OneWord;
  END;  (* case *)
END;

  When adding data to a variant record, there needs to be a value placed in
the tag field, or you will generally get an error.

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+---------+ Scott Maxell  -- Bitnet   ->> IO92203 @ maine
|         |               -- Internet ->> IO92203 @ maine.maine.edu
|    O    |
|    |    | "What I need is a computer that will do what I want it to
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