Negative Logic (was: Re: an infinate loop)

[Jeffrey Carter <jrcarter@acm.org>]

Robert Dewar wrote:
> 
> Jeffrey Carter <jeffrey.carter@boeing.com> wrote in message news:<3B4F764E.DF2DB15@boeing.com>...
> 
> > Studies indicate that people understand positive logic better than
> > negative logic.
> 
> Without a reference, such an argument is simply an appeal to authority,
> a well known debating technique, but not one to be paid much attention
> to. Now *with* a reference, it would be an interesting contribution.

I agree. Sometimes, though, one doesn't have the reference available. I
recall this being mentioned in the Ada Quality and Style Guide; a quick
look at AdaIC shows

5.5.4 Positive Forms of Logic 

     guideline 

     Avoid names and constructs that rely on the use of negatives.
     Choose names of flags so they represent states that can be used in
positive form. 

     example 

     Use: 

     if Operator_Missing then

     rather than either: 

     if not Operator_Found then

     or: 

     if not Operator_Missing then

     rationale 

     Relational expressions can be more readable and understandable when
stated in a positive form.

Not exactly a reference, but perhaps a little better than proof by
blatant assertion.

On Google I also found

http://www.seas.gwu.edu/~mfeldman/cs1book/fk3-im/chap7.html

Instructor's Manual for M.B. Feldman and E.B. Koffman, Ada 95 Problem
Solving and Program Design, 3rd edition.

with the statement

The material on loops other than FOR loops is heavily revised in this
edition, to present general loops before WHILE loops. This reflects the
observation that beginners deal more naturally with "positive logic" --
stating a positive condition to exit a loop ...

Happy Bastille Day.

-- 
Jeff Carter
"Your mother was a hamster and your father smelt of elderberries."
Monty Python & the Holy Grail