Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!uwm.edu!bionet!agate!ucbvax!isidev.UUCP!nowlin
From: nowlin@isidev.UUCP
Newsgroups: comp.lang.icon
Subject: Re: RE: terrible code
Message-ID: <9103190245.AA11982@uunet.uu.net>
Date: 19 Mar 91 02:45:15 GMT
Sender: daemon@ucbvax.BERKELEY.EDU
Distribution: inet
Organization: The Internet
Lines: 49

>From article <1991Mar18.171946.28280@midway.uchicago.edu> (Richard L. Goerwitz)
> In article <9103181558.AA20389@uunet.uu.net> nowlin@isidev.UUCP writes:
> >
> >    m := 0
> >    while *(p := (s ? =!l)) > m do m := *p
> >    return m + 1
>
> Very clever.  Try matching each member of l, keeping a record of the
> length of the match.  The longest match wins.
> 
> This sort of code does exactly what my code does.  Here's the problem.
> Solution of the type exemplified above involve mindless interation
> through the entire list, l.  I decided that it would be sensible to
> ...
> There must be a way to do the kinds of things we're talking about here
> in Icon, and do it with somewhat greater speed than the =!l approach.

I've included the source from a subsequent reposting of the piece of code
being discussed since it was modified to follow the original program more
closely:

   m := 0
   while *(p := (s[i:j] ?  =!l)) > m do m := *p
   if /p then fail
   else return i + m

The key to this is that it's not a "mindless iteration through the entire
list".  It's an iteration, but in any language but Icon you'd have to
explicitly do a lot more to control this iteration than in the simple
expression above.  Simple is in the eyes of the beholder :-)

Any expression that follows the 'while' control structure must try to
succeed due to goal directed evaluation.  That means if the expression
contains a generator, in this case the !l, results are generated until the
generator is exhausted or the expression succeeds in the context of one of
the results.  If the expression succeeds the result is used to modify the
expression.  Eventually none of the generator's results will cause the
expression to succeed and the loop is exited.

This Icon stuff is pretty slick.  I fail to see why a few of dozen lines of
admittedly "terrible" code make a better solution than these four lines.
I could do a matching table in C that would blow this away in terms of
speed but this is Icon.  It should be done Iconishly.

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|   | S |  Iconic Software, Inc.  -  Jerry Nowlin  - uunet!isidev!nowlin  |
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