The document discusses different types of parsing techniques: - Parsing is the process of analyzing a string of tokens based on the rules of a formal grammar. It involves constructing a parse tree that represents the syntactic structure of the string based on the grammar. - The main types of parsing are top-down parsing and bottom-up parsing. Top-down parsing constructs the parse tree from the root node down, while bottom-up parsing constructs it from the leaf nodes up. - Predictive and recursive descent parsing are forms of top-down parsing, while shift-reduce parsing is a common bottom-up technique. Each method has advantages and limitations regarding efficiency and the type of grammar they can handle.

1. G R O U P M E M B E R S :
H I R A S H A H Z A D
J A V E R I A K H A L I D
T A N Z E E L A H U S S A I N
P R E S E N T E D T O :
M S . S A N I A B A T O O L

2. COMPILER
CONSTRUCTION

3. PARSING
• The term parsing comes from Latin pars meaning “part”.
• Parsing is a process that constructs a syntactic structure (i.e. parse tree) from the
stream of tokens.
• Parsing is the process of determining if a string of tokens can be generated by a
grammar.
• For any context-free grammar there is a parser that takes at most Ο(n3) time to parse a
string of n tokens.
• Parsing a string with a CFG:
– Finding a derivation of the string consistent with the grammar
– The derivation gives us a PARSE TREE

4. PARSING TECHNIQUES
• Syntax analyzers follow production rules defined by means of context-free
grammar. The way the production rules are implemented (derivation) divides
parsing into two types :
Top-down parsing and
Bottom-up parsing.

5. TYPES OF PARSING
Parsing
Bottom-up
parsing
Top-Down
Parsing
Predictive
parsing
Recursive decent
parsing
Recursive
predictive parsing
Non-Recursive
predictive
parsing
Shift reduce
parsing
LALR parsing
Canonical
parsing
SLR parsing
Operator
Precedence parsing

6. TOP DOWN PARSING
• Top-down parsers build parse trees from the top (root) to the bottom (leaves).
• A top-down parse corresponds to a preorder traversal of the parse tree
• A leftmost derivation is applied at each derivation step
• Top-Down Parsing may need to backtracking
• Two top-down parsing are further sub-divided into the following categories:
 Predictive Parsing.
 Recursive Descent Parsing

7. EXAMPLE:
Consider the following Grammar:
<program>  begin <stmts> end $
<stmts>  SimpleStmt ; <stmts>
<stmts>  begin <stmts> end ; <stmts>
<stmts>  €
Input: begin SimpleStmt; SimpleStmt; end $

10. RECURSIVE DECENT PARSER
• This parsing technique recursively parses the input to make a parse tree
• A recursive-descent parser consists of several small functions, one for each
nonterminal in the grammar
• A procedure is associated with each nonterminal of a grammar..
• Recursive descent parsing involves backtracking.
• For an input string: read
S → rXd
X → oa
X → ea

11. PREDICTIVE PARSING
• Predictive parser, has the capability to predict which production is to be used to
replace the input string.
• The predictive parser does not suffer from backtracking.
• The predictive parser uses a look-ahead pointer, which points to the next input
symbols.
• To make the parser back-tracking free, the predictive parser puts some constraints on
the grammar.
• It accepts only a class of grammar known as LL(k) grammar.
• Hence, Predictive Parser is also known as LL(1) Parser.

12. LL(1) PARSER
• LL(1) Parser accepts LL(1) grammar.
• LL(1) grammar is a subset of context-free grammar but with some restrictions to get
the simplified version
• In LL(1) parser, the first L in LL(1) is parsing the input from left to right, the second L
in LL(1) stands for left-most derivation and the 1 means one input symbol of look
ahead.

13. CONSTRUCTING PREDICTIVE PARSER
Following are the steps for constructing predictive parser.
o Removing unreachable productions.
o Removing ambiguity from the Grammar.
o Eliminating left recursion.
o Left Factoring of a grammar.
o First and Follow
o Constructing a parse table

14. REMOVING UNREACHABLE PRODUCTIONS
An unreachable production is one that cannot possibly appear in the parse tree rooted at
the start symbol.
For example, in the following grammar :
S A (1)
A a (2)
B b (3)
Production (3) is unreachable because the non-terminal B does not appear on the right
side of any production.
A non-terminal can be unreachable either it appears on the right side of any production.
if it is on the right side of unreachable non-terminal.

15. Data Structures:
• A stack
• A List for Reachable Non-Terminals
Method:
Initially both the stack and list are Empty.
Step 1:
Start symbol to the list of reachable non-
terminal also push onto the stack.
Step 2:
While (The stack is not Empty)
{
P= POP one Item of the stack
for (Each non-terminal X on right hand side
are P)
{
If (X is not in the list of reachable non -
terminals)
{
Push X;
Add X to the list of Reachable non-
terminal;
}
}
}
Step 3:
Remove all the productions from the
grammar where L-H-S is not in the list of
reachable non –terminals.
Algorithmto remove unreachable production

16. • Grammer:
S aB | bA
A a | bAA | aS
B b | aBB | bS
C aD | bS | €
D bD | €
After Removing Unreachable Productions we have :
S aB | bA
A a | bAA | aS
B b | aBB | bS

17. ELIMINATING AMBIGUITY
A grammar that produces more than one parse tree for some sentence (input string) is
said to be ambiguous.
Ambiguity can be remove only by constructing a new grammar.
Note:
 For left associative, replace right non-terminal.
 For right associative, replace left no-terminal.
 If a grammar contains more than one operators, ambiguity will be removed first from
the production involving the operator having the lowest precedence.

18. EXAMPLE:
• S  S+S | S-S | S*S | S/S | NUM
The operators with lower precedence will deal first:
S  S + S ’ | S - S ’ | S ’
S ’  S | S * S | S / S | N U M
R e p l a c e S b y S ’ f r o m R - H - S
S ’  S ’ | S ’ * S ’ | S ’ / S ’ | N U M
After Eliminating the redundant productions i-e S’  S’ , We will get :
S ’  S ’ * S ’ | S ’ / S ’ | N U M
 S ’  S ’ * S ’ ’ | S ’ / S ’ ’ | S ’ ’
S ’ ’  S ’ | N U M
Replace again S’ by S’’ from R-H-S
S ’ ’  S ’ ’ | N U M
After Eliminating the redundant productions i-e S’’  S’’ , We will get :
S’ ’  N U M

19. Unambiguous Grammar:
S  S +S ’ | S - S ’ | S ’
S ’  S ’* S ’’ | S ’/S ’’ | S ’’
S ’’ NUM

20. TRANSITION DIAGRAMS
• Transition diagrams can describe predictive parsers, just like they can describe lexical
analyzers, but the diagrams are slightly different.
For Predictive Parser For Lexical Analyzer
There is one diagram foe Every
Non-terminal
There is one Diagram for the
Entire language construct.
The labels of edge was terminals
and Non-terminals.
The label of edges are only
terminals.

21. CONSTRUCTION
1. Eliminate left recursion from G
2. Left factor G
3. For each non-terminal A, do
Create an initial and final (return) state
For each production A -> X1 X2 … Xn, create a path from the initial to the final
state with edges X1 X2 … Xn.
4. Simplify the transition Diagram, if possible.

22. EXAMPLE OF TRANSITION DIAGRAMS
• An expression grammar with left
recursion and ambiguity removed:
• E -> T E’
• E’ -> + T E’ | ε
• T -> F T’
• T’ -> * F T’ | ε
• F -> ( E ) | id
• Corresponding transition diagrams

23. TYPES OF PREDICTIVE PARSING
• Following are the two types of Predictive Parsing:
 Recursive Predictive Parsing
 Non-Recursive Predictive Parsing
Here, we discuss in Detail Non-Recursive Predictive Parsing Technique.

24. NON-RECURSIVE PREDICTIVE PARSING
• A non-recursive predictive parser is an efficient way of implementing by handling
the stack of activation records explicitly.

25. CONT..
• The predictive parser has an input, a stack, a parsing table, and an output.
• The input contains the string to be parsed, followed by $, the right end marker.
• The stack contains a sequence of grammar symbols, preceded by $, the bottom-of-
stack marker.
• Initially the stack contains the start symbol of the grammar preceded by $.
• The parsing table is a two dimensional array M[A ,a], where A is a nonterminal, and
a is a terminal or the symbol $.
• The parser is controlled by a program that behaves as follows:
The program determines X, the symbol on top of the stack, and a, the current input
symbol.
These two symbols determine the action of the parser.

26. CONT..
There are three possibilities:
o If X = a = $, the parser halts and announces successful completion of parsing.
o If X = a ≠ $, the parser pops X off the stack and advances the input pointer to
the next input symbol.
o If X is a nonterminal, the program consults entry M[X, a] of the parsing table
M. This entry will be either an X-production of the grammar or an error entry.
• § If M[X, a] = {X → UVW}, the parser replaces X on top of the stack
by WVU (with U on top).
• § If M[X, a] = error, the parser calls an error recovery routine.

27. PREDICTIVE PARSING ALGORITHM
repeat
begin
let X be the top stack symbol and a
the next input symbol;
if X is a terminal or $ then
if X = a then
pop X from the stack and
remove a from the input
else
ERROR( )
else /* X is a nonterminal */
if M[X, a] = X → Y1, Y2, … , Yk then
begin
pop X from the stack;
push Yk, Yk-1, … ,Y1 onto
the stack, Y1 on top
end
else
ERROR( )
end
until
X = $ /* stack has emptied */

28. EXAMPLE:
• Use the table-driven predictive parser to parse
id + id * id
• Assuming parsing table
• Initial stack is $E
• Initial input is id + id * id $
Grammar:
E  TE’
E’  +TE’ | €
T  FT’
T’  *FT’ | €
F  (E) | id

30. ERROR RECOVERY IN PREDICTIVE
PARSING
An error is detected during predictive parsing when the terminal on top of the stack does
not match input symbol or when nonterminal A is on top of the stack, a is the next input
symbol, and the parsing table entry M[A, a] is empty.
Following two Error Recovery Routines are handled in predictive parser.
Panic-mode error recovery:
It is based on the idea of skipping symbols on the input until a token in a selected
set of synchronizing tokens appears.
Its effectiveness depends on the choice of synchronizing set.
The sets should chosen so that the parser recovers quickly from errors that are
likely to occur in practice.

31. CONT..
Phrase=level recovery:
§ It is implemented by filling in the blank entries in the predictive parsing table with
pointers to error routines.
§ These routines may change, insert, or delete symbols on the input and issue
appropriate error messages.
§ They may also pop from the stack.
§ In any event, sure that there is no possibility of an infinite loop.
§ Checking that any recovery action eventually results in an input symbol being
consumed is a good way to protect against such loops.

32. DIFFERENCE BETWEEN PREDICTIVE PARSER AND
RECURSIVE DECENT PARSER
Predictive Parsing Recursive-Descent Parsing
Its Non-Recursive (Table Driven)
Predictive Parser, which is also known
as LL(1) parser
Its has a set of recursive procedures to
process the input.
No backtracking is Needed. Backtracking is needed.
Needs a special form of grammars
(LL(1) grammars) and Widely used
It is a general parsing technique, but
not widely used.
Its an efficient technique. Its not an efficient Technique.
Predictive parsers operate in linear
time
It operates in exponential time

33. BOTTOM-UP PARSING
Bottom-up parsing starts with the input symbols and tries to construct the parse
tree up to the start symbol.
Example:
Input string : a + b * c
Production rules:
S → E
E → E + T
E → E * T
E → T
T → id

34. EXAMPLE
Let us start bottom-up parsing
a + b * c
Read the input and check if any production matches with the input:
a + b * c
T + b * c
E + b * c
E + T * c
E * c
E * T
E
S