Re: [Pyparsing] parse algebra like expressions

[Paul M. <pt...@au...>]

Rita -

There are really two parts to your question - how to parse the expression
you've given, and then how to construct a useful data structure from it.
>From your example, I haven't gone too far into creating a true parent-child
graph, I'm just implementing the problem as you have stated it.  Here is a
little annotated script to try:


from pyparsing import Suppress, Word, alphas, alphanums, Literal, Group,
delimitedList

# define basic punctuation, operators, and our parentage item
LPAR,RPAR = map(Suppress, '()')
parentOp = Literal("->")
item = Word(alphas, alphanums)

# define a group of possible items, as in '(b|c)'
altItems = Group(LPAR + delimitedList(item,'|') + RPAR)

# define a hierarchy expression as a list of items delimited by parentOps
hierarchyExpr = delimitedList(item | altItems, parentOp)


# parse the test string
testStr = "a -> (b|c) -> d"
gens = hierarchyExpr.parseString(testStr)

# use zip to convert the list of hierarchy entries into parent-child pairs
for parent,child in zip(gens,gens[1:]):
    print 'parent', ' '.join(parent), 'child', ' '.join(child)
    

See if that gets you further along.

-- Paul



-----Original Message-----
From: Rita [mailto:rmo...@gm...] 
Sent: Wednesday, August 03, 2011 6:18 PM
To: pyp...@li...
Subject: [Pyparsing] parse algebra like expressions

Hello,


I would like to parse a string into something like this

str = "a -> (b|c) -> d"

This should generate
parent a child b c
parent b c child d


Any ideas how I can do this with pyparse?




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