while loop - multiple condition

Rustom Mody rustompmody at gmail.com
Mon Oct 13 19:12:13 CEST 2014


On Monday, October 13, 2014 10:13:20 PM UTC+5:30, Rob Gaddi wrote:
> On Mon, 13 Oct 2014 09:26:57 -0700 (PDT)
> Rustom Mody  wrote:

> > On Monday, October 13, 2014 9:43:03 PM UTC+5:30, Rob Gaddi wrote:
> > > On Mon, 13 Oct 2014 09:56:02 +1100
> > > Steven D'Aprano  wrote:
> > > > When you have multiple clauses in the condition, it's easier to reason about
> > > > them if you write the clauses as positive statements rather than negative
> > > > statements, that is, "something is true" rather than "something is not
> > > > true", and then use `not` to reverse it if you want to loop *until* the
> > > > overall condition is true.
> > > I was just explaining this concept to a young pup the other day.  De
> > > Morgan's lets you say that (not (p and q)) == ((not p) or (not q)), but
> > > the positive logic flavor is substantially less error-prone.  People
> > > are fundamentally not as good at thinking about inverted logic.
> > Curious: Which of
> > - (not (p and q))
> > - ((not p) or (not q))
> > is more positive (less negative)??

> The first is asking you to compare positive conditions (p and q) and
> negate the entire thing (NAND).  The second asks you to think about
> the combination of two different "not true" pieces of logic
> (OR of two inverted inputs).  The first is pretty straightforward, and
> I usually see people get it right.  The second gets screwed up as often
> as not.

> And of course, any combination of ands and ors should be broken into
> multiple statements with a descriptive variable name in the middle or
> all hope is lost.

Yeah I guess 2 nots is one more than one!

However (to my eyes)
while i < N  and  a[i] != X:

looks less negative than

while not (i==N or a[i] == X):

[Of course i < N is not identical to i != N ]



More information about the Python-list mailing list