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lecture07 - CS 143 Lecture 7 1 Top-Down Par sing and I ntr...

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Unformatted text preview: CS 143 Lecture 7 1 Top-Down Par sing and I ntr o to Bottom-Up Par sing Lecture 7 CS 143 Lecture 7 2 Pr edictive Par ser s • Like recursive-descent but parser can “predict” which production to use – By looking at the next few tokens – No backtracking • Predictive parsers accept LL(k) grammars – L means “left-to-right” scan of input – L means “leftmost derivation” – k means “predict based on k tokens of lookahead” – I n practice, LL(1) is used CS 143 Lecture 7 3 LL(1) vs. Recur sive Descent • I n recursive-descent, – At each step, many choices of production to use – Backtracking used to undo bad choices • I n LL(1), – At each step, only one choice of production – That is • When a non-terminal A is leftmost in a derivation • The next input symbol is t • There is a unique production A → α to use – Or no production to use (an error state) • LL(1) is a recursive descent variant without backtracking CS 143 Lecture 7 4 Pr edictive Par sing and Left Factor ing • Recall the grammar E → T + E | T T → int | int * T | ( E ) • Hard to predict because – For T two productions start with int – For E it is not clear how to predict • We need to left-factor the grammar CS 143 Lecture 7 5 Left-Factor ing Example • Recall the grammar E → T + E | T T → int | int * T | ( E ) • Factor out common prefixes of productions E → T X X → + E | ε T → ( E ) | int Y Y → * T | ε CS 143 Lecture 7 6 LL(1) Par sing Table Example • Left-factored grammar E → T X X → + E | ε T → ( E ) | int Y Y → * T | ε • The LL(1) parsing table: int * + ( ) $ E T X T X X + E ε ε T int Y ( E ) Y * T ε ε ε leftmost non-terminal next input token rhs of production to use CS 143 Lecture 7 7 LL(1) Par sing Table Example (Cont.) • Consider the [E, int] entry – “When current non-terminal is E and next input is int , use production E → T X ” – This can generate an int in the first position • Consider the [Y,+] entry – “When current non-terminal is Y and current token is + , get rid of Y ” – Y can be followed by + only if Y → ε CS 143 Lecture 7 8 LL(1) Par sing Tables. Er r or s • Blank entries indicate error situations • Consider the [E,*] entry – “There is no way to derive a string starting with * from non-terminal E ” CS 143 Lecture 7 9 Using Par sing Tables • Method similar to recursive descent, except – For the leftmost non-terminal S – We look at the next input token a – And choose the production shown at [S,a] • A stack records frontier of parse tree – Non-terminals that have yet to be expanded – Terminals that have yet to matched against the input – Top of stack = leftmost pending terminal or non-terminal • Reject on reaching error state • Accept on end of input & empty stack CS 143 Lecture 7 10 LL(1) Par sing Algor ithm initialize stack = <S $> and next repeat case stack of <X, rest> : if T[X,*next] = Y 1 …Y...
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lecture07 - CS 143 Lecture 7 1 Top-Down Par sing and I ntr...

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