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
LEFT RECURSION
We have to eliminate left recursion because
top down parsing methods can not handle
left recursive grammars.
 A grammar is left recursive if it has a
nonterminal A such that there is a derivation
A  Aα for some string α
 Consider the left recursive grammar
A  A α1 | A α2 | β1 | β2

+

6. Dr. Hussien M. Sharaf
REMOVE LEFT RECURSION
1.
2.
3.
Let A  other productions of A not starting
with A (i.e. β1, β2), followed by A′ (A  β1A′|
β2A′)
Then let A′  postfix of the productions
starting with A (α1, α2) followed by A′ (A′ 
α1A′|α2A′)
Then add |ε to A′ (A′  α1A′|α2A′| ε)
+

7. Dr. Hussien M. Sharaf
ELIMINATION OF LEFT RECURSION EXAMPLE
E  E+T|T
T  T*F|F
F  (E)|id
 The equivalent non-left recursive grammar
is:
E  T E′
E′  +TE′|ε
T  FT′
T′  *FT′|ε
F  (E) | id

8. Dr. Hussien M. Sharaf
LEFT FACTORING

In left factoring it is not clear which of two
alternative productions to use to expand a
nonterminal A.
i.e. if A  αβ1 | αβ2
W
e don’t know whether to expand A to αβ1
or to αβ2
 To
remove left factoring for this grammar
replace all A productions containing α as
prefix by A  αA′ then A′  β1 | β2

9. Dr. Hussien M. Sharaf
ELIMINATION OF LEFT FACTORING EXAMPLE
S  iEtS | iEtSeS | a
Eb

The equivalent non-left factored grammar is:
S  iEtSS′ | a
S′ eS | ε
Eb

10. Dr. Hussien M. Sharaf
THANK YOU