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

2. Dr. Hussien M. Sharaf
LR PARSING

LR(1) parsing is a technique of bottom-up parsing.




L: scan input string from left to right.
R: uses right most derivation.
1: uses one token ahead to decide which rule to use for
reduction.
Consists of:



Parser stack: that holds grammar symbols: non-terminals and
tokens.
Parsing table: that specifies the parser actions (Shift,
Reduce, Accept, Error).
Driver function: that interacts with parser stack, parsing table, and
scanner.
Output
Scanner
Next
token
Parser
driver
Parsing
table
Parsing stack

3. Dr. Hussien M. Sharaf
BOTTOM-UP PARSER ACTIONS




Shift: parser shifts the next token on the
parser stack.
Reduce: parser reduces the RHS of a
production to its LHS.
Accept: parsed successfully.
Error: failure.

4. Dr. Hussien M. Sharaf
BOTTOM-UP PARSING
A parser is a bottom-up if it traverse a parse
tree bottom up.
 A right most derivation is applied to each
step.
 Bottom-up parsers are preferred in practice.
 They do not need left factoring.
 It reduces the input string to the starting
symbol by using the reverse of production
rules.


5. Dr. Hussien M. Sharaf
BOTTOM-UP (CONT.)
Consider the following grammar:
E→T+E|T
T → id * T | id | (E)
; Input string: id + id * id

Input string
id * id + id
id * T + id
T + id
T+T
T+E
E
Rules
T → id
T → id * T
T → id
E→T
E→T+E
Is there another parsing for the same input string
using bottom up algorithm?

6. Dr. Hussien M. Sharaf
BOTTOM-UP (CONT.)
Input string
id * id + id
id * id + T
id * T + T
T+T
T+E
E


Rules
T → id
T → id
T → id * T
E→T
E→T+E
Is there another parsing for the same input
string using bottom up algorithm?
What happens if “T + T” was reduced to “E +
T”? Would the parsing still continue?

7. Dr. Hussien M. Sharaf
PARSING TREE
E
Input string
id * id + id
id * id + T
id * T + T
T+T
T+E
E
E
T
T
T
i
d
*
i
d
+
i
d

8. Dr. Hussien M. Sharaf
PARSING TREE
Consider the following grammar:
E→T+E|T
T → id * T | id | (E)
; Input string: id + id * id

But what if we start with reducing the first id to T as
follows:
Input string
id * id + id
T * id + id

Rules
T → id
We would be stuck with “T * id” or “T * T” or “T * E”

9. Dr. Hussien M. Sharaf
CONFUSING POINTS

How does the parser choose whether to shift
or reduce?
 General
strategy is to reduce whenever it is
possible otherwise shift.

How does the parser handles the situations
when there is more than one choice for
reducing?
 Even
non-ambigious grammars can cause this
situation. The parser uses back-tracking to
decide which choice is correct.

10. Dr. Hussien M. Sharaf
Example 1
Consider the following grammar….
A(A) | a
Stack
Input
Action
$
((a))$
Shift
$(
(a))$
Shift
$((
a))$
Shift
$((a
))$
Reduce A a
$((A
))$
Shift
$((A)
)$
Reduce A (A)
$(A
)$
Shift
$(A)
$
Reduce A (A)
$A
$
Accept

11. Dr. Hussien M. Sharaf
LR Parsing Example 2
Stack
Input
Action
$
$ id
$T
$E
$E+
$E+(
$ E + ( id
$E+(T
$E+(E
$E+(E+
$ E + ( E + id
$E+(E+T
$E+(E
$E+(E)
$E+T
$E
id + ( id + id ) $
+ ( id + id ) $
+ ( id + id ) $
+ ( id + id ) $
( id + id ) $
id + id ) $
+ id ) $
+ id ) $
+ id ) $
id ) $
)$
)$
)$
$
$
$
S
R, G3
R, G2
S
S
S
R, G3
R, G2
S
S
R, G3
R, G1
S
R, G4
R, G1
A
G1:
G2:
G3:
G4:
E
E
T
T




E+T
T
id
(E)

12. Dr. Hussien M. Sharaf
Example 3

13. Dr. Hussien M. Sharaf
THANK YOU