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From: johnson@m.cs.uiuc.edu
Newsgroups: comp.software-eng
Subject: Re: recap so far
Message-ID: <39400111@m.cs.uiuc.edu>
Date: 8 Jul 90 03:09:00 GMT
References: <4235@<268462df>
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Nf-ID: #R:<268462df:4235:m.cs.uiuc.edu:39400111:000:1660
Nf-From: m.cs.uiuc.edu!johnson    Jul  7 22:09:00 1990


kandt@ai-jupiter.JPL.NASA.GOV said:
>Automatic data structure selection was a hot topic in the 70s because
>people realized that there were tremendous pay-offs (in terms of
>efficiency) by optimizing data storage and manipulation algorithms for
>the expected data.

Although automatic data structure selection would be nice, it is not
really all that important.  I program in Smalltalk using sets, sequences,
and mappings (i.e. Sets, OrderedCollections, and Dictionaries) and
almost never use arrays directly.  Once a program works, I profile its
performance.  Sometimes I find that using a too-general data structure
is causing a bottleneck, and I will then rewrite a few classes.  However,
very few of the uses of the general collection classes are bottlenecks,
so it doesn't matter when they are used when a more specialized one could
be used.

To generalize, arguments against software reuse because it will result
in systems that are too slow are wrong.  Only the bottlenecks need to
be optimized.  I save a lot of time reusing components and am happy to
spend some of it rewriting working code to make it faster.

Another interesting fact is that the standard Collection classes are
really fairly efficient, considering what they do.  The average programmer
who would try to write a linked list routine in C is likely to write
something that is slower, and is likely to make a mistake or two.  There
are a few people who are really good at making programs fast.  Once you
have a library of reusable components, you should have one of them take
a look at it and share their skills with others.

Ralph Johnson -- University of Illinois at Urbana-Champaign