Convert PCFG to CNF?

1
votes

I have this rule in a grammar :

VP->Verb NP NP   [0.05]
VP->Verb NP PP   [0.1]

I want to convert this PCFG(Probabilistic Context Free grammar ) to CNF(Chomsky Normal Form)

To do that I know we can split the rule into two non-terminals 

VP->@V_N NP
VP->@V_N PP
@V_N->Verb NP

Which probability to set for each rule?

Thanks

1
nlpcontext-free-grammarchomsky-normal-form
The conversion will depend on what you would want the CNF to do and also on the other rules. I assume that you want the resulting CNF probabilities to sum to one. If not, @vsevolod's answer resolves the problem. But he's taking a conditional approach to the probability assignment, i.e. prob(VP->@V_N NP) = prob(@V_N) * prob(@V_N NP | @V_N) so the sum of probabilities don't sum to 1 - alvas
Well ,I'll use it to fill CKY table . - Nada Ghanem

1 Answers

3
votes
VP->@V_N NP     [0.05]
VP->@V_N PP     [0.1]
@V_N->Verb NP   [1.0]

Because you need to get the same probability for the result by applying both the original and the converted set of rules.