Re: Precedence Rules (was Re: Software vendors not using Ada but C)

[cis.ohio-state.edu!math.ohio-state.edu!sdd.hp.com!network.ucsd.edu!munnar i.oz.au!goanna!ok@ucbvax.Berkeley.EDU (Richard A. O'Keefe)]

In article <1993Jun25.205022.29499@seas.gwu.edu>, mfeldman@seas.gwu.edu (Michae
l Feldman) writes:
> I agree. This has always been a tough one for me to get across to
> students who are used to Pascal's MOD semantics, which of course has
> the semantics of Ada's REM.

The Pascal standard (IOS/EIC 7185:1990) says that
(i mod j) is an error if j <= 0.  That surprised me, I hadn't realised
that it was defined for i < 0.  It goes on to point out explicitly
"Only for i >= 0 and j > 0 does the relation (i div j)*j + i mod j = i hold."
_That_ stunned me.  i div j and i mod j both defined for negative i,
but not compatibly?

Students who have been taught well about Pascal use the rule
    "Never use i div j or i mod j except when (i >= 0) and (j > 0)."
I use the same rule in Ada.

-- 
Richard A. O'Keefe; ok@goanna.cs.rmit.oz.au; RMIT, Melbourne, Australia.