Parsing is the process of analyzing a string of symbols according to the rules of a formal grammar to determine its syntactic structure. It involves expanding a parse tree from the start symbol to the leaves using either a top-down or bottom-up approach. Top-down parsers expand the leftmost non-terminal at each step, while bottom-up parsers start at the leaves and grow the tree toward the root. Bottom-up, or LR, parsing is preferred in practice.

2. Topic:

3. What is “PARSING”

The term parsing comes from Latin
pars meaning part Parsing or syntactic
analysis is the process of analyzing a string of
symbols, either in natural language or
in computer languages according to the rules of
a formal grammar.

So Parsing means determining the syntactic
structure of an expression

4. Example of parsing

X=(A-(D*F))
A

5. Types of Parser

The task of the parser is essentially to determine if
and how the input can be derived from the start
symbol of the grammar. This can be done in
essentially two ways
Top-down
1) Top-down parser
2) Bottom-up parser
Bottom-up

6. Top-down parsers

Top-down parsing expands a parse tree from the start
symbol to the leaves
 Always expand the leftmost non-terminal
E
id
+
id
*
id

7. Top-down parsers
Apply E-> E+E
E
E
E
id
+
id
*
id

8. Top-down parsers
Apply E-> id
E
E
E
id
+
id
*
id

9. Top-down parsers
Apply E-> E*E
E
E
E
E
id
+
id
E
*
id

10. Top-down parsers
Apply E-> id
E
E
E
E
id
+
id
E
*
id

11. Top-down parsers
Apply E-> id
E
E
E
E
id
+
id
E
*
id

12. Bottom-up Parser

Start at the leaves and grow toward root




And just as efficient
Builds on ideas in top-down parsing
Preferred method in practice
Also called LR parsing


L means that tokens are read left to right
R means that it constructs a rightmost derivation -->

14. 14
int + (int) + (int)
int
+ ( int ) + ( int
)

15. Example of Bottom-up Parse
15
int + (int) + (int)
E + (int) + (int)
E
int
+ ( int ) + ( int
)

16. Example of Bottom-up Parse
16
int + (int) + (int)
E + (int) + (int)
E + (E) + (int)
E
int
E
+ ( int ) + ( int
)

17. 17
int + (int) + (int)
E + (int) + (int)
E + (E) + (int)
E + (int)
E
E
int
E
+ ( int ) + ( int
)

18. Example of Bottom-up Parse
18
int + (int) + (int)
E + (int) + (int)
E + (E) + (int)
E + (int)
E + (E)
E
E
int
E
E
+ ( int ) + ( int
)

19. Example of Bottom-up Parse
19
E
int + (int) + (int)
E + (int) + (int)
E + (E) + (int)
E + (int)
E + (E)
E
E
E
int
E
E
+ ( int ) + ( int
)