Thursday, 14 February 2008

Fixing the grammar

The test that was failing was this one:

  def test_left_compound # "((500*20)+16)"
  tokens = [,,,,,,,,]
Why? Well, do what I did - try to build the expression ((500*20)+16) by hand using the rules. Here they are again:
E => (E) | nT T => +E | -E | * E | /E | ε
Give up yet? It can't be done! Also, for the same reason, this grammar cannot generate (1)+2. The simple fact is that I botched the job of making the grammar LL(1) compatible. So how to fix it? Well its simple really, especially when you take the (1)+2 example. Seems to me that a slight tweak to the production rules for E is necessary:
E => (E)T | nT
Great! Lets modify the lookup table accordingly:
lookup[:E][LeftParenToken] = [:lpar, :E, :rpar, :T]
The tests all pass! Now I can turn my attention to using the same algorithm for building the tree. However I have some more reading to do before I can proceed. Come on CS164 lectures notes, don't let me down!

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