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From: Chris.Holt@newcastle.ac.uk (Chris Holt)
Newsgroups: comp.software-eng
Subject: Re: Effect of execution-speed on reliability/testing
Message-ID: <1991Jan8.224440.20624@newcastle.ac.uk>
Date: 8 Jan 91 22:44:40 GMT
References: <1990Dec19.102005.11830@engin.umich.edu> <470@eiffel.UUCP> <1047@gistdev.gist.com> <15809.278a0d04@levels.sait.edu.au> <12212@pucc.Princeton.EDU>
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Organization: Computing Laboratory, U of Newcastle upon Tyne, UK NE1 7RU.
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In article <12212@pucc.Princeton.EDU> EGNILGES@pucc.Princeton.EDU writes:
>In article <15809.278a0d04@levels.sait.edu.au>, xtbjh@levels.sait.edu.au writes:
>>
>>I agree strongly that software is an engineering discipline.  Any program
>>is a trade off between many competing forces, including correctness,
>>reliability, reusability, maintainability as well as speed, memory, disk etc.
>
>Hmph?  I am not sure that you can "trade off" various levels of
>correctness.  A software system is either correct, or else it isn't.

Not quite right.  Case 1: the results of a system are not critical,
or the time between production of results and their use is long; and
it is feasible to determine by other means whether or not they are
right, once they are produced.  Then, you chug away, and every time
you check and find one wrong, you sigh and use some different longer
algorithm instead.

Case 2: a real time system has a choice of different algorithms
that yield different levels of accuracy, or an iterative procedure
is used that converges on the "right" answer.  Then as soon as
you need an answer of some sort, you take the most accurate one
available; here you trade off time and accuracy.

You might argue that in the latter case, the time/accuracy
tradeoff is part of the specification, and so the program as
a whole is correct; but the real world isn't very good at
formal specifications that incorporate several attributes of
this kind, much less specifications of tradeoffs.
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 Chris.Holt@newcastle.ac.uk      Computing Lab, U of Newcastle upon Tyne, UK
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