Parser types Top Down Parser Bottom up Parser LALR(0) Parser SLR(0) Parser LL(0) Parser LR(!) Parser

1. Types of Parsers
Group : TY 57

2. Contents
● Parser
● Types of Parser
● Top down parser
● Bottom up parser
● Difference between top-down and bottom-up parser
● Conclusion

3. Parser
● Parser is a compiler that is used to break the data into smaller elements
coming from lexical analysis phase.
● A parser takes input in the form of sequence of tokens and produces output in
the form of parse tree.
● There are mainly 2 types of parsing in compiler design.
1. Top Down Parser
2. Bottom Up Parser

4. Role of Parser

5. Types of Parser

6. Top-Down Parser
1.

7. ● Generates parse for the given input string with the
help of grammar productions by expanding the
non-terminals.
● The process of constructing the parse tree which
starts from the root and goes down to the leaf.
● There are 2 types of top down parser.
1) Recursive descent parser. 2) LL(1) parser

8. Recursive Descent Parser
● Technique that constructs the parse tree from
the top and the input is read from left to right.
● There are two types of recursive descent parser.
■ 1) Backtracking parser
■ 2) Non-Backtracking parser.
● Given Grammer:
○ S → aAd
○ A → ab / a
● Input string:
○ W = cad

9. W = cad
i/p

10. Non Recursive Descent parser
● Also known as LL(1) parser or predictive parser or
dynamic parser.
● Special form of recursive descent parsing, where no
backtracking is required.
● So this can predict which products to use to replace
the input string.

11. Algorithm to construct LL(1) Parser
Step 1: First check for left recursion in the grammar, if there is left recursion in the
grammar remove that and go to step 2.
Step 2: Calculate First() and Follow() for all non-terminals.
Step 3: For each production A –> α. (A tends to alpha)
1. Find First(α) and for each terminal in First(α), make entry A –> α in the table.
2. If First(α) contains ε (epsilon) as terminal than, find the Follow(A) and for each
terminal in Follow(A), make entry A –> α in the table.
3. If the First(α) contains ε and Follow(A) contains $ as terminal, then make entry A
–> α in the table for the $.

12. 2. Bottom-Up Parser

13. ● Starts from the leaf nodes of a tree and works in
upward direction till it reaches the root node.
● Generates the parse tree for the given input string
with the help of grammar productions by
compressing the non-terminals.
● There are 2 types of bottom up parser.
1) LR parser
2) Operator Precedence parser.

14. LR-Parser
● In the LR parsing, "L" stands for left-to-right scanning of the input,
and “R” stands for constructing a right most derivation in reverse.
● It is used to parse the large class of grammars.
● generates the parse tree for the given string by using
unambiguous grammar.

15. LR(0) parser
● An LR(O)parser is a shift-reduce
parser that uses zero tokens of
lookahead to determine what
action to take (hence the 0).
● We need two functions –
Closure(): If I is a set of items for a
grammar G, then closure(I) is the set
of items constructed from I by the
two rules:
Goto():
Goto(I, X) => 1. Add I by moving dot
after X.
2. Apply closure to first step
SLR(1) parser
● The SLR parser is similar to LR(0)
parser except that the reduced
entry.
● SLR(1) parsers can parse a larger
number of grammars than LR(0).
● There are 2 functions in SLR(1)
parser such as goto[list of
terminals] and action[list of non-
terminals]

16. CLR(1) Parser
❏ Canonical LR parser.
❏ Large set of items called LR(1) items.
❏ It is a more powerful LR parser and makes use of lookahead symbols.
STEPS:
● Add Augment Production rule ,insert ‘.’ symbol at the first position for every production in G .
● Add the lookahead at each state.
● If variable comes then open each statement.Otherwise go to the next state.

17. CLR(1) Example

18. LALR(1) Parser
● Lookahead LR parser.
● It is the most powerful parser which
can handle large classes of grammar.
● The canonical collection
of LR(1) items is used.

19. LALR(1) Parsing Table

20. Operator Precedence Parser
● Bottom-up parser that interprets an operator-
precedence grammar.
● A simple shift-reduce parser that is capable of
parsing a subset of LR(1) grammars.
● It accepts only operator grammar
○ E→ E+E / E*E / id
● There are 3 operator precedence relation.
○ a ⋗ b
○ a ⋖ b
○ a ≐ b

21. Bottom up parser
● First looks at the lowest level of
the parse tree.
● Works up the parse tree by
using the rules of grammar.
● This parsing technique uses
Right Most Derivation.
● First looks at the highest level of
the parse tree .
● Works down the parse tree by
using the rules of grammar .
● This parsing technique uses Left
Most Derivation.
Top Down Parser
Difference between top-down and bottom-up parser

22. Conclusion
● Parser - Break data into smaller elements.
● Two types of parsers - Top down and bottom up
● Algorithms to construct different parsers.
● Differences
● Examples

23. References
● https://www.geeksforgeeks.org/types-of-parsers-in-compiler-design/
● https://www.youtube.com/watch?v=hGuXUIefVkc&t=47s
● https://www.tutorialspoint.com/difference-between-top-down-
parsing-and-bottom-up-parsing
● https://www.javatpoint.com/operator-precedence-parsing
● https://www.tutorialspoint.com/compiler_design/compiler_design_typ
es_of_parsing.htm

24. Thank
you !