1. WELCOME TO A
JOURNEY TO
CS419
Dr. Hussien Sharaf
Computer Science Department
dr.sharaf@from-masr.com

2. Dr. Hussien M. Sharaf
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.


3. Dr. Hussien M. Sharaf
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.
Scanner
Next
token
Parser
driver
Parsing
table
Output
Parsing stack

4. Dr. Hussien M. Sharaf
PROBLEMS FACING LL(1) PARSERS
1.
2.
Left recursion.
Left factoring.
Both problems prevents any LL parser from
deciding deterministically which rule should be
fired.

5. 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.

6. Dr. Hussien M. Sharaf
EXAMPLE 1
Consider the following grammar: S  ( S ) |
to parse (()), we follow these steps:
Parser stack
Input
Parser action
S
(())$
Predict S (S)
(S)
(())$
Match (
S)
())$
Predict S (S)
(S))
())$
Match (
S))
))$
Predict S
))
))$
Match )
)
)$
Match )
Empty
$
Accept

7. Dr. Hussien M. Sharaf
EXAMPLE 2
Consider the following grammar:
1.E
2.Q
3.Q
4.Q
5.T
TQ
+TQ
-TQ
FR
6.R
7.R
8.R
9.F
10.F
*FR
/FR
(E)
id

8. Dr. Hussien M. Sharaf
EXAMPLE 2 (CONT.)
The parsing table for id*(id+id)$ is:
+
E
Q
T
R
F
-
2
*
/
(
1
3
)
4
5
8
8
id
1
6
7
4
5
8
9
$
8
10

9. Dr. Hussien M. Sharaf
EXAMPLE 2
Consider the following grammar:
1.E
2.Q
3.Q
4.Q
5.T
TQ
+TQ
-TQ
FR
6.R
7.R
8.R
9.F
10.F
*FR
/FR
(E)
id

10. Dr. Hussien M. Sharaf
EXAMPLE 2 (CONT.)
To parse id*(id+id)$, we follow these steps:
stack
Input
Parser action id RQ)RQ
id+id)$
E
id*(id+id)$
Predict E TQ
RQ)RQ
+id)$
Predict R
TQ
id*(id+id)$
Predict T FR
Q)RQ
+id)$
Predict Q +TQ
FRQ
id*(id+id)$
Predict F id
+TQ)RQ
+id)$
Match +
id RQ
id*(id+id)$
Match id
TQ)RQ
id)$
Predict T FR
RQ
*(id+id)$
Predict R*FR
FRQ)RQ
id)$
Predict Fid
*FRQ
*(id+id)$
Match *
id RQ)RQ
id)$
Match id
FRQ
(id+id)$
Predict F(E)
RQ)RQ
)$
Predict R
(E)RQ
(id+id)$
Match (
Q)RQ
)$
Predict Q
E)RQ
id+id)$
Predict ETQ
)RQ
)$
Match )
TQ)RQ
id+id)$
Predict T FR
RQ
$
Predict R
FRQ)RQ
id+id)$
Predict F id
Q
$
Predict Q
id RQ)RQ
id+id)$
Match id
Empty
$
Accept
Match id

11. Dr. Hussien M. Sharaf
THANK YOU