Re: [BLACKBOX] Re [BLACKBOX] Seasons Greetings from Linuxland

From: [at]} <Chris>
Date: Tue, 28 Dec 2010 10:24:43 +1030

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> -----Original Message-----
> From: BlackBox [mailto:BLACKBOX{([at]})nowhere.xy
> Sent: Monday, 27 December 2010 9:35 PM
> To: BLACKBOX{([at]})nowhere.xy
> Subject: [BLACKBOX] Re [BLACKBOX] Seasons Greetings from Linuxland
>
>
> The code
>
> PROCEDURE IsDigit(ch: CHAR): BOOLEAN;
> BEGIN
> RETURN (ch <= '9') & (ch >= '0')
> END IsDigit;
>
> is regarded as good 'style'.
>
> Yet it suffers from the same problem as the C code above. To
> understand
> what the programmer intended the reader has to evaluate what
> follows the
> RETURN.
>
> The code
>
> PROCEDURE IsDigit(ch: CHAR): BOOLEAN;
> VAR
> b: BOOLEAN;
> BEGIN
> b := FALSE;
> IF (ch >= '0') & (ch <='9') THEN
> b := TRUE;
> END;
> RETURN b;
> END IsDigit;
>
> is regarded as bad 'style', but the intention of the programmer is
> more easily understood.
>

To understand what the programmer intended in your example he has to
evaluate what follows the IF statement so what is the difference?
Unfortunately, he then also has to analyze an additional three statements in
order to ascertain what the procedure is actually doing.

I suspect you are alluding to the fact that some programmers do not have a
good understanding of Boolean algebra.

The following hypothetical examples are intended to show what a programmer
could concoct if he has muddled thinking regarding boolean logic and is a
newcomer to languages with a boolean data type like Pascal, Modula-2 and
Oberon. The statements are all supposed to be equivalent (I guess if any
aren't I have managed to prove a point!). Draw your own conclusions ...

----------------------

IF (a # FALSE) THEN
  b := FALSE
ELSE
  IF (a = FALSE) THEN
    b := TRUE
  END
END;

----------------------

IF (a # FALSE) THEN
  b := FALSE
ELSIF (a = FALSE) THEN
  b := TRUE
END;

----------------------

IF (a = TRUE) THEN
  b := FALSE
ELSIF (a = FALSE) THEN
  b := TRUE
END;

----------------------

IF a THEN
  b := FALSE
ELSIF ~A THEN
  b := TRUE
END;

----------------------

IF a THEN
  b := FALSE
ELSE
  b := TRUE
END;

----------------------

b := TRUE;
IF a THEN b := FALSE END;

----------------------

b := ~a;

----------------------

Regards,
Chris

Chris Burrows
CFB Software
http://www.cfbsoftware.com

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Received on Tue Dec 28 2010 - 00:54:43 UTC