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Date: 4 Jan 93 09:31:55 GMT
From: agate!spool.mu.edu!enterpoop.mit.edu!eru.mt.luth.se!lunic!sunic!nobeltech
!leeu@ucbvax.Berkeley.EDU  (Leif Euren)
Subject: Re: Ada Readability, Book Reference
Message-ID: <1993Jan4.093155.11894@nobeltech.se>
List-Id: <comp.lang.ada>

In article <9212281507.AA20416@ajpo.sei.cmu.edu> SAHARBAUGH@ROO.FIT.EDU writes:
>>I received one request for a pointer to Bryan & Mendal's
>>book so I may as well post it (same amount of keystrokes).
>>
>[stuff deleted]
>>
>>For many examples the book asks, "What will this
>>program print?"  For example 7.3.12 the answer is
>>"The answer to the question is indeterminate."
>>
>>So I ask "Is Ada readable if you can express an indeterminate
>>program using it?".

I think the answer should have been "The answer to the question is
compiler dependant.", and therefore: Yes, Ada is readable, although I
can express compiler dependant programs using it.

And in article <1992Dec30.035143.10291@seas.gwu.edu> mfeldman@seas.gwu.edu (Mic
hael Feldman) responded:
>The particular example depends on the evaluation order of the sides of
>the expression ... floating-point arithmetic has indeterminate results
>sometimes, so that (A*B)/C may yield a different result than A*(B/C).
>
>In the specific case, (6.0 * 1.0)/3.0 can yield a different result from 
>6.0 * (1.0 / 3.0) because 1.0/3.0 cannot be exactly represented.
>Therefore an expression like
>  6.0 * (1.0 / 3.0) = (6.0 * 1.0) / 3.0
>may not evaluate to True. Traditional floating-point gotcha.

Er... This is not a very good example. The constants used are of
universal_real type, and according to LRM 4.10(4) "... if a universal
expression is a static expression, then the evaluation must be exact."

But if I write

	B := 1.0;

	if 6.0 * (B / 3.0) = (6.0 * B) / 3.0 then

	else

	end if;

your arguments hold true.
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