cs419 lec8 top-down parsing
DESCRIPTION
TRANSCRIPT
Compilers
WELCOME TO A JOURNEY TO
CS419 Lecture 8
Top-Down Parsing
Cairo UniversityFCI
Dr. Hussien SharafComputer Science [email protected]
TOP-DOWN PARSING
A parser is a top-down if it discovers a parse tree top to bottom.
A top-down parse corresponds to a preorder traversal of the parse tree.
A left most derivation is applied to each step.
Dr. Hussien M. Sharaf
S
dc
A ba
LL PARSING LL parsing is a technique of top-down parsing. Consists of:
Parser stack: that holds grammar symbols: non-terminals and tokens.
Parsing table: that specifies the parser actions (Match, Predict, Accept, Error).
Driver function: that interacts with parser stack, parsing table, and scanner.
Dr. Hussien M. Sharaf
ScannerParser driver
Parsing table
OutputNexttoken
Parsing stack
PROBLEMS FACING LL(1) PARSERS
1. Left recursion.2. Left factoring.Both problems prevents any LL parser from deciding deterministically which rule should be fired.
Dr. Hussien M. Sharaf
TOP-DOWN PARSER ACTIONS
Match: to match top of parser stack with next input token.
Predict: to predict a production and apply it in a derivation step.
Accept: parsed successfully. Error: failure.
Dr. Hussien M. Sharaf
EXAMPLE 1Consider the following grammar: S ( S ) | to parse (()), we follow these steps:
Dr. Hussien M. Sharaf
Parser stack Input Parser action
S (())$ Predict S (S)
(S) (())$ Match (
S) ())$ Predict S (S)
(S)) ())$ Match (
S)) ))$ Predict S
)) ))$ Match )
) )$ Match )
Empty $ Accept
EXAMPLE 2Consider the following grammar:
Dr. Hussien M. Sharaf
1.E TQ 6.R *FR2.Q +TQ 7.R /FR3.Q -TQ 8.R 4.Q 9.F (E)5.T FR 10.F id
EXAMPLE 2 (CONT.)
Dr. Hussien M. Sharaf
+ - * / ( ) id $
E 1 1
Q 2 3 4 4
T 5 5
R 8 8 6 7 8 8
F 9 10
The parsing table for id*(id+id)$ is:
EXAMPLE 2Consider the following grammar:
Dr. Hussien M. Sharaf
1.E TQ 6.R *FR2.Q +TQ 7.R /FR3.Q -TQ 8.R 4.Q 9.F (E)5.T FR 10.F id
EXAMPLE 2 (CONT.)
Dr. Hussien M. Sharaf
stack Input Parser action
E id*(id+id)$
Predict E TQ
TQ id*(id+id)$
Predict T FR
FRQ id*(id+id)$
Predict F id
id RQ id*(id+id)$
Match id
RQ *(id+id)$ Predict R*FR
*FRQ *(id+id)$ Match *
FRQ (id+id)$ Predict F(E)
(E)RQ (id+id)$ Match (
E)RQ id+id)$ Predict ETQ
TQ)RQ id+id)$ Predict T FR
FRQ)RQ id+id)$ Predict F id
id RQ)RQ id+id)$ Match id
id RQ)RQ id+id)$ Match id
RQ)RQ +id)$ Predict R
Q)RQ +id)$ Predict Q +TQ
+TQ)RQ +id)$ Match +
TQ)RQ id)$ Predict T FR
FRQ)RQ id)$ Predict Fid
id RQ)RQ id)$ Match id
RQ)RQ )$ Predict R
Q)RQ )$ Predict Q
)RQ )$ Match )
RQ $ Predict R
Q $ Predict Q
Empty $ Accept
To parse id*(id+id)$, we follow these steps:
THANK YOU
Dr. Hussien M. Sharaf