Newsgroups: comp.lang.pascal
Path: utzoo!utgpu!watserv1!watmath!nmouawad
From: nmouawad@watmath.waterloo.edu (Naji Mouawad)
Subject: Re: need a little flexibility
Message-ID: <1991Apr26.152604.23483@watmath.waterloo.edu>
Organization: University of Waterloo
References: <1991Apr24.183315.7997@ux1.cso.uiuc.edu> <1991Apr24.235739.25115@watmath.waterloo.edu> <91115.111155IO92203@MAINE.BITNET>
Date: Fri, 26 Apr 1991 15:26:04 GMT
Lines: 76

In article <91115.111155IO92203@MAINE.BITNET> IO92203@MAINE.BITNET (Scott Maxell) writes:
>>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.
>

Hi Scott,

  While this may be true in Pascal, it is not the case in Turbo
Pascal:

 "An *optional* identifier, the tag field identifier, can be placed
  in the variant part. If a tag field identifier is present, it becomes
  the identifier of an additional fixed field - the tag field - of
  the record. The program can use the tag field's value to show 
  which variant is active at a given time. Whithout a tag field,
  the program selects a variant by another criterion."
               TP V 6.0 programmer's manual, page 31.

Cheers,

--Naji.
-- 
     -------------------------------------------------------------------
    | Naji Mouawad  |          nmouawad@watmath.waterloo.edu            |
    |  University   |---------------------------------------------------|
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