From: Daniel Baird <danielbaird@...>
Date: 2006-06-05T21:36:57+09:00
Subject: Re: parsing a boolean expression

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On 6/5/06, Matthew Smillie <M.B.Smillie@sms.ed.ac.uk> wrote:
>
> On Jun 5, 2006, at 7:32, Daniel Baird wrote:
>
> > Hi all,
> >
> > I need to take a string typed by dirty users' fingers, that should
> > contain a
> > boolean expression, like this:
> >  one and (two or three or (four and five) or six) and (seven or eight)
>
> If your input is always going to be as simple as the above (space-
> delimited tokens and parens), and you don't know anything about yacc,
> then there's the distinct possibility that you'll lose more time
> fiddling with it than you will writing your own parser.
>
> Ferinstance, you could pretty easily add the 'missing' parens (after
> each operator to the end of the expression) to make it explicit, then
> change it to an s-expression by switching the order of the first
> token and operator in each sub-expression.  And an s-expression is
> basically a tree.  It would depend on what you want to do with it
> when you're done.
>
> matthew smillie.


Hmm.. I haven't done anything with yacc before, and my expressions _are_
going to be pretty simple, so I guess your advice is to write it myself :)

I'd kinda fancied returning a tree made up of nodes that are either:

- a token
- a list of nodes with a common operator

eg "A and ( B or C or D) and E" would be:

node0: AND(A, node1, E)
node1: OR(B, C, D)

It's not a binary tree but it would be somewhat convenient for the
processing / storing / re-displaying I need to do.. I just hoped I'd save a
day or two standing on someone else's shoulders.

Thanks Matthew.

;D

-- 
Daniel Baird
http://danielbaird.com (TiddlyW;nks! :: Whiteboard Koala :: Blog :: Things
That Suck)

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