Re: Bottom-up versus Top-down

Jack Olsen <jacko@post8.tele.dk> writes:


      Could anyone enlighten me about bottom-up versus top-down parsing?


Hmmm, let me anthropomorphise a bit:


A bottom up parser says things like
- hmm, I have tokens like "i := i + 1;"
- That's an Expression!
- hmm, I have tokens like "begin Expression ; Expression ; end"
- That's a Block!
- hmm, I have tokens like "procedure foo Block"
- That's a ProcDef!
- hmm, I have tokens like "program bar Procdef Procdef Block"
- That's a Program!


whereas a top down parser says things like:
- I'm compiling a Program, so I need
"program bar Procdef Procdef Block"
- I'm compiling a Procdef so I need
"procedure foo Block"
- I'm compiling a Block so I need
"begin Expression ; Expression ; end"
- I'm compiling an Expression so I need
"i := i + 1;"
- Got it, so this must be a valid Program.


Hope that helps more than it hinders!
--
Gregory Bond ITG Australia Ltd, Melbourne, Australia
<mailto:gnb@itga.com.au> <http://www.bby.com.au/~gnb>
--

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