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From: whiting_ms@corning.com (whiting_ms@corning.com (Matt Whiting))
Subject: Re: GNAT messages and the not operator (pitfall alert!)
Date: 1996/10/25
Message-ID: <1996Oct25.103321.1@corning.com>#1/1
X-Deja-AN: 192162716
references: <dewar.846162354@merv> <dewar.846163972@merv>
organization: Corning, Incorporated
newsgroups: comp.lang.ada
nntp-posting-user: whiting_ms
Date: 1996-10-25T00:00:00+00:00
List-Id: <comp.lang.ada>


In article <dewar.846163972@merv>, dewar@merv.cs.nyu.edu (Robert Dewar) writes:
> Here is another pitfall, pretty horrible if it hits, but fortunately rare.
> Ask an Ada expert what is the value of
> 
>     -5 mod 3
> 
> Almost anyone (the better they know Ada, the more likely they are to make
> this mistake) will strain to remember the table in the RM that talks about
> negative mods, and, if they remember it right, come up with the answer of 1.
> But Ada has VERY odd precedence rules which mean that the default
> parenthesiztion of this expression is -(5 mod 3). The only person I
> ever knew to get this right (and remember this is under conditions they
> know they are being asked to answer a trick question) was Robert Eachus
> (both Jean and Tuck got this wrong originally, and so did I :-)

I'm really confused now.  How does -(5 mod 3) yield 1?  I thought 5 mod 3 would
evaluate to 2 and then the unary minus would yield -2.  Will someone provide a
play-by-play evaluation sequence for this expression?

Matt