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| | #REDIRECT [[ist:Top-Down Parsing/Exercise 3]] |
| = Problem =
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| Consider the following grammar, where '''<tt>G</tt>''' is the initial symbol and '''<tt>{a,b,c,d,e}</tt>''' is the set of terminal symbols:
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| O -> a
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| G -> F c | O c d | (eps)
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| F -> G b | O c F e
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| # Examine the grammar and rewrite it so that an LL(1) predictive parser can be built for the corresponding language.
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| # Compute the FIRST and FOLLOW sets for all non-terminal symbols in the new grammar and build the parse table.
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| # Show the analysis table (stack, input, and actions) for the parsing process of the '''<tt>acbec</tt>''' input sequence. | |
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| = Solution =
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| Something to keep in mind at all times: '''never eliminate the rules corresponding to the initial symbol'''
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| There are two ways of handling the mutual recursion: either expand F in G or G in F.
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| == Expanding G in F ==
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| Initial grammar:
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| O -> a
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| G -> F c | O c d | (eps)
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| F -> G b | O c F e
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| Eliminating the singularity (O->a):
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| G -> F c | a c d | (eps)
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| F -> G b | a c F e
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| Expanding G in F:
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| G -> F c | a c d | (eps)
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| F -> F c b | a c d b | b | a c F e
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| Eliminate left recursion in F:
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| G -> F c | a c d | (eps)
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| F -> a c d b F' | b F' | a c F e F'
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| F' -> c b F' | (eps)
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| Factor F prefixes and expand it in G:
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| G -> a c F" c | b F' c | a c d | (eps)
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| F -> a c F" | b F'
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| F' -> c b F' | (eps)
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| F" -> d b F' | F e F'
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| Note that we still have common prefixes to factor in G and a non-terminal left-corner in F''. The next step will handle those cases:
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| G -> a c G' | b F' c | (eps)
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| G' -> F" c | d
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| F' -> c b F' | (eps)
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| F" -> d b F' | a c F" e F' | b F' e F'
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| F was eliminated because it became unreachable from the start symbol. The grammar still has a non-terminal left corner in G', so we need to eliminate it.
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| G -> a c G' | b F' c | (eps)
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| G' -> d b F' c | a c F" e F' c | b F' e F' c | d
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| F' -> c b F' | (eps)
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| F" -> d b F' | a c F" e F' | b F' e F'
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| Factoring prefixes in G', we get to the final version:
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| G -> a c G' | b F' c | (eps)
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| G' -> d G" | a c F" e F' c | b F' e F' c
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| G" -> b F' c | (eps)
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| F' -> c b F' | (eps)
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| F" -> d b F' | a c F" e F' | b F' e F'
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| == Expanding F in G ==
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| Initial grammar:
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| O -> a
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| G -> F c | O c d | (eps)
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| F -> G b | O c F e
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| Eliminating the singularity (O->a):
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| G -> F c | a c d | (eps)
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| F -> G b | a c F e
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| Expanding F in G:
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| G -> G b c | a c F e c | a c d | (eps)
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| F -> G b | a c F e
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| Eliminating left recursion in G:
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| G -> a c F e c G' | a c d G' | G'
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| G' -> b c G' | (eps)
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| F -> G b | a c F e
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| Factoring G prefixes:
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| G -> a c G" | b c G' | (eps)
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| G' -> b c G' | (eps)
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| G" -> F e c G' | d G'
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| F -> a c G" b | b c G' b | b | a c F e
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| Factoring F prefixes:
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| G -> a c G" | b c G' | (eps)
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| G' -> b c G' | (eps)
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| G" -> F e c G' | d G'
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| F -> a c F' | b F"
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| F' -> G" b | F e
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| F" -> c G' b | (eps)
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| Eliminating non-terminal left corners (note that F becomes unreachable):
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| G -> a c G" | b c G' | (eps)
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| G' -> b c G' | (eps)
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| G" -> a c F' e c G' | b F" e c G' | d G'
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| F' -> a c F' e c G' b | b F" e c G' b | d G' b | a c F' e | b F" e
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| F" -> c G' b | (eps)
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| Factoring F' prefixes:
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| G -> a c G" | b c G' | (eps)
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| G' -> b c G' | (eps)
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| G" -> a c F' e c G' | b F" e c G' | d G'
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| F' -> a c F' e F" | b F" e F"
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| F" -> c G' b | (eps)
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| [[category:Compilers]] | |
| [[category:Teaching]]
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