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From: TRL3@psuvm.psu.edu (Tim Larson)
Newsgroups: comp.lang.modula2
Subject: Re: Clever way to deal with this?
Message-ID: <90044.135327TRL3@PSUVM.BITNET>
Date: 13 Feb 90 18:53:27 GMT
References: <"90-02-12-20:09:32.76*UK4H"@DKAUNI2.BITNET>
 <90043.154337MARK@UCF1VM.BITNET>
Organization: Penn State University
Lines: 117

In article <90043.154337MARK@UCF1VM.BITNET>, MARK@UCF1VM.BITNET (Mark Woodruff)
says:
>In C, I'd say:
>
>    struct
>    {
>         int count;
>         char c;
>    } buffer;
>
>    for (i = 0; i <= count; i++)
>    {
>         putchar(&buffer.c+i);
>    }
>
>In Modula-2, I said:
>
>    TYPE
>       BuffType =
>          RECORD
>             count:  CARDINAL;
>             text:  CHAR;
>          END;
>
>    VAR
>       buffer:  BuffType;
>       i:  CARDINAL;
>       charPtr:  POINTER TO CHAR;
>
>    BEGIN
>       FOR i := 0 TO buffer.count DO
>          charPtr := SYSTEM.ADDRESS(buffer.text)+i;
>          InOut.Write(charPtr@)
>       END;
>    END;
>
>(note:  I haven't actually tried to write this in assembly or C
>and I don't have my Modula-2 code at hand, so this is from memory)
>
>I'm not thrilled with this, but it's about the best equivalent to
>an equate or a pointer to a character I can get in Modula-2.
>
>I'm using this representation to represent buffers in an editor.
>The actual characters are fixed in length, but the length is entirely
>determined at run time.
>
Most Modula-2 systems won't like the address arithmetic, so it would be
better to write

   TYPE
      BuffType = RECORD
         count: CARDINAL;
         text: ARRAY CARDINAL OF CHAR
      END;

   VAR
      buffer: BuffType;
      i: CARDINAL;
      charPtr: POINTER TO CHAR;

   BEGIN
      FOR i := 0 TO buffer.count DO
         charPtr := SYSTEM.ADR(buffer.text[i]);
         InOut.Write(charPtr:)
      END
   END;

This is essentially what you wrote except for the use of ADR and the use
of i as an index into buffer.text, which required a change in the type
definition.  This is according to PIM 3e Modula-2.  I'm not sure what the
upcoming standard will provide but I think JPI's M2 would be a little more
succinct

   TYPE
      bufType = RECORD
         count: CARDINAL;
         text: ARRAY CARDINAL OF CHAR
      END;

   VAR
      buffer: bufType;
      i: CARDINAL;

   BEGIN
      FOR i := 0 TO buffer.count DO
         IO.WrChar (CHAR([ADR(buffer.text[i])]:))
      END
   END ...

Not a compelling reason to buy JPI's product, perhaps, but just a touch
more elegant since it eliminates the need for the charPtr variable and
probably speeds the compiled code up the tiniest fraction.  In Modula-2,
which relies on words more than symbols, you just will never get the
bulk of this type of code down to C-size.  On the other hand, it is more
difficult to do something wrong and much easier to read.

The other way to do this (if possible) is to null-terminate the string
at location buffer.count+1 and simply use WriteString.

   TYPE
      bufType = RECORD
         count: CARDINAL;
         text: ARRAY CARDINAL OF CHAR
      END;

   VAR
      buffer: bufType;

   BEGIN
      IF buffer.count<MAX(CARDINAL) THEN buffer.text[buffer.count+1] := 0C END;
      InOut.WriteString (buffer.text)
   END ...

Hope this adds something to the discussion.

-Tim Larson
trl3@psuvm.bitnet