1. The document discusses the phases of a compiler including lexical analysis, syntax analysis, and semantic analysis. 2. Lexical analysis breaks input into tokens which are passed to the parser, syntax analysis builds an abstract syntax tree by applying grammar rules to check structure, and semantic analysis ensures correct meaning. 3. Key aspects covered include context-free grammars, symbol tables for storing token information, and error detection and reporting across compiler phases.

1. COMPILER
ENGINEERING
LAB # 4: SYNTAX ANALYSIS (PARSING)

2. LEXICAL ANALYSIS SUMMARY
1. Start New Token
2. Read 1st character to start recognizing its type
according to the algorithm specified in 3. Slide 3
3. Pass its Token (Lexeme Type) and Value Attribute
 send to Parser
4. Repeat Steps (1-3)
5. Repeat Until End
Department of Computer Science -
10-14/3/12 2
Compiler Engineering Lab

3. Start New TOKEN
Read 1st Character
If is Digit? TOKEN = NUM
If is Letter?
Read Following Characters
If any is digit or _? TOKEN = ID
Is a
If all letters? Keyword
?
Is RELOP? Is 2nd TOKEN=
TOKEN=KEYWOR
>, <, !, = Char (=)? RELOP
D
Is AROP?
+, -. /, *, = TOEKN = AROP
Department of Computer Science -
10-14/3/12 3
Compiler Engineering Lab

4. SYNTAX ANALYSIS (PARSING)
• is the process of analyzing a text, made of a
sequence of tokens
• to determine its grammatical structure with respect
to a given (more or less) formal grammar.
• Builds Abstract Syntax Tree (AST)
• Part from an Interpreter or a Compiler
• Creates some form of Internal Representation (IR)
• Programming Languages tend to be written in
Context-free grammar Efficient + fast Parsers can
be written for them
Department of Computer Science -
10-14/3/12 4
Compiler Engineering Lab

5. PHASE 2 : SYNTAX ANALYSIS
• also called sometimes Syntax Checking
• Ensures that:
• the code is valid grammatically (without worrying about the
meaning)
• and will sequence into an executable program.
• The syntactical analyzer applies rules to the code;
For example:
• checking to make sure that each opening brace has a
corresponding closing brace,
• and that each declaration has a type,
• and that the type exists .. etc
Department of Computer Science -
10-14/3/12 5
Compiler Engineering Lab

6. CONTEXT-FREE GRAMMAR
• Defines the components that forms an expression +
defines the order they must appear in
• A context-free grammar is a set of rules specifying
how syntactic elements in some language can be
formed from simpler ones
• The grammar specifies allowable ways to combine
tokens(called terminals), into higher-level syntactic
elements (called non-terminal)
Department of Computer Science -
10-14/3/12 6
Compiler Engineering Lab

7. CONTEXT-FREE GRAMMAR
• Ex.:
• Any ID is an expression (Preferred to say TOKEN)
• Any Number is an expression (Preferred to say TOKEN)
• If Expr1 and Expr2 are expressions then:
• Expr1+ Expr2  are expressions
• Expr1* Expr2  are expressions
• If id1 and Expr2 are expressions then:
• Id1 = Expr2  is a statement
• If Expr1and Statement 2 then
• While (Expr1) do Statement 2,
• If (Expr1) then Statement 2
are statements
Department of Computer Science -
10-14/3/12 7
Compiler Engineering Lab

8. GRAMMAR & AST
TOKEN (terminals) = Leaf
Expressions, Statements (Non-Terminals) = Nodes
Lexical Stream of TOKENs
Stream of Characters
Analysis
Syntax
Stream of TOKENs Abstract Syntax Tree (AST)
Analysis
Department of Computer Science -
10-14/3/12 8
Compiler Engineering Lab

9. PHASE 2 : SYNTAX ANALYSIS
Department of Computer Science -
10-14/3/12 9
Compiler Engineering Lab

10. PHASE 2 : SYNTAX ANALYSIS
Token Syntax
Token Analyzer
Tokens (Parser)
Department of Computer Science -
10-14/3/12 10
Compiler Engineering Lab

11. SYMBOL TABLE
• A Symbol Table is a data structure containing a
record for each identifier with fields for the
attributes of an ID
• Tokens formed are recorded in the ST
• Purpose:
• To analyze expressionsstatements, that is a hierarchal or
nesting structure is required
• Data structure allows us to: find, retrieve, store a record for
an ID quickly.
• For example: in Semantic Analysis Phase + Code Generation
phase  retrieve ID Type to Type Check and Implementation
purposes
Department of Computer Science -
10-14/3/12 11
Compiler Engineering Lab

12. SYMBOL TABLE MANAGEMENT
• The Symbol Table may contain any of the following
information:
• For an ID:
• The storage allocated for an ID,
• its TYPE,
• Its Scope (Where it’s valid in the program)
• For a function also:
• Number of Arguments
• Types of Arguments
• Passing Method (By Reference or By Value)
• Return Type
• Identifiers will be added if they don’t already exist
Department of Computer Science -
10-14/3/12 12
Compiler Engineering Lab

13. SYMBOL TABLE MANAGEMENT
• Not all attributes can always be determined by a
lexical analyzer  because of its linear nature
• E.g. dim a, x as integer
• In this example the analyzer at the time when seeing the IDs
has still unreached the type keyword
• So, following phases will complete filling IDs
attributes and using them as well
• For example: the storage location attribute is assigned by
the code generator phase
Department of Computer Science -
10-14/3/12 13
Compiler Engineering Lab

14. ERROR DETECTION & REPORTING
• In order the Compilation process proceed
correctly, Each phase must:
• Detect any error
• Deal with detected error(s)
• Errors detection:
• Most in Syntax + Semantic Analysis
• In Lexical Analysis: if characters aren’t legal for token
formation
• In Syntax Analysis: violates structure rules
• In Semantic Analysis: correct structure but wrong invalid
meaning (e.g. ID = Array Name + Function Name)
Department of Computer Science -
10-14/3/12 14
Compiler Engineering Lab

15. COMPILER PHASES
Department of Computer Science -
10-14/3/12 15
Compiler Engineering Lab

16. LEXICAL ANALYZER &
Lexical Analyzer
SYMBOL TABLE
Token Token Token Location
ID Type Value
Id1 ID position
expr1 AROP ASS
1d2 ID Initial
Expr2 AROP SUM
Id3 ID Rate
Expr3 AROP MUL
N1 Num 60
Department of Computer Science -
10-14/3/12 16
Compiler Engineering Lab

17. SYNTAX ANALYZER &
SYMBOL TABLE
Department of Computer Science -
10-14/3/12 17
Compiler Engineering Lab

18. SYNTAX ANALYZER &
SYMBOL TABLE
A LEAF is a record with two or more fields
One to identify the TOKEN and others to identify info
attributes
Token Token Token
Location
ID Type Value
Id1 ID position
expr1 AROP ASS
1d2 ID Initial
Expr2 AROP SUM
Id3 ID Rate
Expr3 AROP MUL
N1 NUM 60
Department of Computer Science -
10-14/3/12 18
Compiler Engineering Lab

19. SYNTAX ANALYZER &
SYMBOL TABLE
An interior NODE is a record with a field for the operator
and two fields of pointers to the left and right children
Left Child Right Child
Operator
(Pointer) (Pointer)
Expr1 id1 Expr2
Expr2 id2 Expr3
Expr3 id3 N1
Department of Computer Science -
10-14/3/12 19
Compiler Engineering Lab

20. TASK 1: THINK AS A COMPILER!
• Analyze the following program syntactically:
int main()
{
std::cout << "hello world" << std::endl;
return 0;
}
Department of Computer Science -
10-14/3/12 20
Compiler Engineering Lab

21. LEXICAL ANALYZER OUTPUT
• 1 = string "int”
• 2 = string "main”
• 3 = opening parenthesis
• 4 = closing parenthesis
• 5 = opening brace
• 6 = string "std”
• 7 = namespace operator
8 = string "cout”
• 9 = << operator
• 10 = string ""hello world"”
• 11 = string "endl”
• 12 = semicolon
• 13 = string "return”
• 14 = number 0
• 15 = closing brace
Department of Computer Science -
10-14/3/12 21
Compiler Engineering Lab

22. TASK 2: A STATEMENT AST
• Create an abstract syntax tree for the following
code for the Euclidean algorithm:
while b ≠ 0
if a > b
a := a − b
else
b := b − a
return a
Department of Computer Science -
10-14/3/12 22
Compiler Engineering Lab

23. TASK 2: A STATEMENT AST
Department of Computer Science -
10-14/3/12 23
Compiler Engineering Lab

24. LAB ASSIGNMENT
Write the Syntax Analyzer Components and Ensure
fulfilling the following :
• Create a Symbol Table (for all types including IDs,
Functions, .. Etc)
• Fill the Symbol Table with Tokens extracted from the
Lexical Analysis phase
•Differentiate between Node and Leaf
• Applying grammar rules (tokens, expressions,
statements)
Department of Computer Science -
10-14/3/12 24
Compiler Engineering Lab

25. QUESTIONS?
Thank you for listening 
Department of Computer Science -
10-14/3/12 25
Compiler Engineering Lab