Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Path: utzoo!mnetor!uunet!husc6!cmcl2!rutgers!mcnc!rti!xyzzy!throopw
From: throopw@xyzzy.UUCP (Wayne A. Throop)
Newsgroups: comp.lang.c
Subject: Re: what "cast" means
Message-ID: <190@xyzzy.UUCP>
Date: Sun, 9-Aug-87 19:38:05 EDT
Article-I.D.: xyzzy.190
Posted: Sun Aug  9 19:38:05 1987
Date-Received: Mon, 10-Aug-87 00:13:48 EDT
References: <263@auvax.UUCP> <1987Jul9.162103.1701@sq.uucp> <243@wrs.UUCP> <2817@psuvax1.psu.edu> <3671@ecsvax.UUCP>
Organization: Data General, RTP NC.
Lines: 57

> uccjcm@ecsvax.UUCP (Jonathon C. McLendon IV)
>> okunewck@psuvax1.psu.edu (Philip E. OKunewick)

>>>> [not looking like Pascal is not a language deficiency]
>>>does this mean that "looking like Pascal is a language deficiency!" :-)

>> (  ! = logical not;  ] = logical implication;  ^ = logical or  )
>> We have two basic postulates here:
>> 1: (looking like pascal)  (l.l.p)
>> 2: (a language deficiency)  (a l.d.)
>> The first statement can be written as:
>> !(!(looking like pascal) ] (a language deficency))
> This is wrong! This statement can be simplified to:
>   (looking like pascal) ] !(a language deficiency)

Actually, Philip is correct and Jonathon is quite wrong.  Reading "is
not" as "does not imply" as Philip does makes far more sense than
reading "is not" as "implies the negation of" as Jonathon does.  Or to
put it another way, using a simple prefix notation,
(not (implies (not A) B)) cannot be simplified to (implies A (not B)) as
Jonathon seems to think.  Or does Jonathon really think that "it is not
the case that Fred not being a poor driver implies Fred will have an
accident" is equivalent to "Fred being a poor driver implies Fred will
not have an accident"?

In particular, (not (implies (not A) B)) reduces to (and (not A) (not B)),
while (implies A (not B)) reduces to (or (not A) B).

Of course both Philip and Jonathan translate the original two claims
into somewhat more formal terms incorrectly.  More correctly, the two
claims "not looking like Pascal is not a language deficiency" and
"looking like Pascal is a language deficiency" reduce to

        (not (forall l (implies (not (pascallike l) (deficient l)))))
and
        (forall l (implies (pascallike l) (deficient l)))

respectively, and the two are not equivalent.  Reducing these claims,
the first one becomes
        (exists l (not (implies (not (pascalllike l) (deficient l)))))
which becomes
        (exists l (and (not (pascalllike l)) (not (deficient l))))

Whereas the second becomes
        (forall l (or (not (pascallike l)) (deficient l)))
or perhaps
        (not (exists l (and (pascallike l) (not (deficient l)))))

The two claims are, therefore not equivalent.  And further, the first
does not imply the second.

--
"What does self-denial mean to you, Roderick?"
"Gee Father, I don't know, is it like the cretin who says all cretins
 are liars?"
                                --- from "Roderick" by John Sladek
-- 
Wayne Throop      <the-known-world>!mcnc!rti!xyzzy!throopw