The document discusses lexical analysis, which is the first phase of compilation. It involves reading the source code and grouping characters into meaningful sequences called lexemes. Each lexeme is mapped to a token that is passed to the subsequent parsing phase. Regular expressions are used to specify patterns for tokens. A lexical analyzer uses finite automata to recognize tokens based on these patterns. Lexical analyzers may also perform tasks like removing comments and whitespace from the source code.
The document discusses the differences between compilers and interpreters. It states that a compiler translates an entire program into machine code in one pass, while an interpreter translates and executes code line by line. A compiler is generally faster than an interpreter, but is more complex. The document also provides an overview of the lexical analysis phase of compiling, including how it breaks source code into tokens, creates a symbol table, and identifies patterns in lexemes.
System software module 4 presentation filejithujithin657
The document discusses the various phases of a compiler:
1. Lexical analysis scans source code and transforms it into tokens.
2. Syntax analysis validates the structure and checks for syntax errors.
3. Semantic analysis ensures declarations and statements follow language guidelines.
4. Intermediate code generation develops three-address codes as an intermediate representation.
5. Code generation translates the optimized intermediate code into machine code.
The document provides an overview of compilers and interpreters. It discusses that a compiler translates source code into machine code that can be executed, while an interpreter executes source code directly without compilation. The document then covers the typical phases of a compiler in more detail, including the front-end (lexical analysis, syntax analysis, semantic analysis), middle-end/optimizer, and back-end (code generation). It also discusses interpreters, intermediate code representation, symbol tables, and compiler construction tools.
The document discusses lexical analysis in compilers. It begins with an overview of lexical analysis and its role as the first phase of a compiler. It describes how a lexical analyzer works by reading the source program as a stream of characters and grouping them into lexemes (tokens). Regular expressions are used to specify patterns for tokens. The document then discusses specific topics like lexical errors, input buffering techniques, specification of tokens using regular expressions and grammars, recognition of tokens using transition diagrams, and the transition diagram for identifiers and keywords.
The document discusses lexical analysis in compilers. It defines lexical analysis as the first phase of compilation that reads the source code characters and groups them into meaningful tokens. It describes how a lexical analyzer works by generating tokens in the form of <token name, attribute value> from the source code lexemes. Examples of tokens generated for a sample program are provided. Methods for handling lexical errors, buffering input, specifying tokens with regular expressions and recognizing tokens using transition diagrams are also summarized.
This document provides an introduction to compilers and their construction. It defines a compiler as a program that translates a source program into target machine code. The compilation process involves several phases including lexical analysis, syntax analysis, semantic analysis, code optimization, and code generation. An interpreter directly executes source code without compilation. The document also discusses compiler tools and intermediate representations used in the compilation process.
The document discusses lexical analysis, which is the first phase of compilation. It involves reading the source code and grouping characters into meaningful sequences called lexemes. Each lexeme is mapped to a token that is passed to the subsequent parsing phase. Regular expressions are used to specify patterns for tokens. A lexical analyzer uses finite automata to recognize tokens based on these patterns. Lexical analyzers may also perform tasks like removing comments and whitespace from the source code.
The document discusses the differences between compilers and interpreters. It states that a compiler translates an entire program into machine code in one pass, while an interpreter translates and executes code line by line. A compiler is generally faster than an interpreter, but is more complex. The document also provides an overview of the lexical analysis phase of compiling, including how it breaks source code into tokens, creates a symbol table, and identifies patterns in lexemes.
System software module 4 presentation filejithujithin657
The document discusses the various phases of a compiler:
1. Lexical analysis scans source code and transforms it into tokens.
2. Syntax analysis validates the structure and checks for syntax errors.
3. Semantic analysis ensures declarations and statements follow language guidelines.
4. Intermediate code generation develops three-address codes as an intermediate representation.
5. Code generation translates the optimized intermediate code into machine code.
The document provides an overview of compilers and interpreters. It discusses that a compiler translates source code into machine code that can be executed, while an interpreter executes source code directly without compilation. The document then covers the typical phases of a compiler in more detail, including the front-end (lexical analysis, syntax analysis, semantic analysis), middle-end/optimizer, and back-end (code generation). It also discusses interpreters, intermediate code representation, symbol tables, and compiler construction tools.
The document discusses lexical analysis in compilers. It begins with an overview of lexical analysis and its role as the first phase of a compiler. It describes how a lexical analyzer works by reading the source program as a stream of characters and grouping them into lexemes (tokens). Regular expressions are used to specify patterns for tokens. The document then discusses specific topics like lexical errors, input buffering techniques, specification of tokens using regular expressions and grammars, recognition of tokens using transition diagrams, and the transition diagram for identifiers and keywords.
The document discusses lexical analysis in compilers. It defines lexical analysis as the first phase of compilation that reads the source code characters and groups them into meaningful tokens. It describes how a lexical analyzer works by generating tokens in the form of <token name, attribute value> from the source code lexemes. Examples of tokens generated for a sample program are provided. Methods for handling lexical errors, buffering input, specifying tokens with regular expressions and recognizing tokens using transition diagrams are also summarized.
This document provides an introduction to compilers and their construction. It defines a compiler as a program that translates a source program into target machine code. The compilation process involves several phases including lexical analysis, syntax analysis, semantic analysis, code optimization, and code generation. An interpreter directly executes source code without compilation. The document also discusses compiler tools and intermediate representations used in the compilation process.
The document summarizes the key phases of a compiler:
1. The compiler takes source code as input and goes through several phases including lexical analysis, syntax analysis, semantic analysis, code optimization, and code generation to produce machine code as output.
2. Lexical analysis converts the source code into tokens, syntax analysis checks the grammar and produces a parse tree, and semantic analysis validates meanings.
3. Code optimization improves the intermediate code before code generation translates it into machine instructions.
The document discusses the roles of compilers and interpreters. It explains that a compiler translates an entire program into machine code in one pass, while an interpreter translates and executes code line-by-line. The document also covers the basics of lexical analysis, including how it breaks source code into tokens by removing whitespace and comments. It provides an example of tokens identified in a code snippet and discusses how the lexical analyzer works with the symbol table and syntax analyzer.
Lexical analysis is the process of converting a sequence of characters from a source program into a sequence of tokens. It involves reading the source program, scanning characters, grouping them into lexemes and producing tokens as output. The lexical analyzer also enters tokens into a symbol table, strips whitespace and comments, correlates error messages with line numbers, and expands macros. Lexical analysis produces tokens through scanning and tokenization and helps simplify compiler design and improve efficiency. It identifies tokens like keywords, constants, identifiers, numbers, operators and punctuation through patterns and deals with issues like lookahead and ambiguities.
We have learnt that any computer system is made of hardware and software.
The hardware understands a language, which humans cannot understand. So we write programs in high-level language, which is easier for us to understand and remember.
These programs are then fed into a series of tools and OS components to get the desired code that can be used by the machine.
This is known as Language Processing System.
The lexical analyzer is the first phase of a compiler. It takes source code as input and breaks it down into tokens by removing whitespace and comments. It identifies valid tokens by using patterns and regular expressions. The lexical analyzer generates a sequence of tokens that is passed to the subsequent syntax analysis phase. It helps locate errors by providing line and column numbers.
The phases of a compiler are:
1. Lexical analysis breaks the source code into tokens
2. Syntax analysis checks the token order and builds a parse tree
3. Semantic analysis checks for type errors and builds symbol tables
4. Code generation converts the parse tree into target code
The document discusses the phases of a compiler and their functions. It describes:
1) Lexical analysis converts the source code to tokens by recognizing patterns in the input. It identifies tokens like identifiers, keywords, and numbers.
2) Syntax analysis/parsing checks that tokens are arranged according to grammar rules by constructing a parse tree.
3) Semantic analysis validates the program semantics and performs type checking using the parse tree and symbol table.
The document discusses the phases of a compiler:
1) Lexical analysis scans the source code and converts it to tokens which are passed to the syntax analyzer.
2) Syntax analysis/parsing checks the token arrangements against the language grammar and generates a parse tree.
3) Semantic analysis checks that the parse tree follows the language rules by using the syntax tree and symbol table, performing type checking.
4) Intermediate code generation represents the program for an abstract machine in a machine-independent form like 3-address code.
The compiler is software that converts source code written in a high-level language into machine code. It works in two major phases - analysis and synthesis. The analysis phase performs lexical analysis, syntax analysis, and semantic analysis to generate an intermediate representation from the source code. The synthesis phase performs code optimization and code generation to create the target machine code from the intermediate representation. The compiler uses various components like a symbol table, parser, and code generator to perform this translation.
This document provides information about the CS213 Programming Languages Concepts course taught by Prof. Taymoor Mohamed Nazmy in the computer science department at Ain Shams University in Cairo, Egypt. It describes the syntax and semantics of programming languages, discusses different programming language paradigms like imperative, functional, and object-oriented, and explains concepts like lexical analysis, parsing, semantic analysis, symbol tables, intermediate code generation, optimization, and code generation which are parts of the compiler design process.
The document provides an overview of the compilation process and the different phases involved in compiler construction. It can be summarized as follows:
1. A compiler translates a program written in a source language into an equivalent program in a target language. It performs analysis, synthesis and error checking during this translation process.
2. The major phases of a compiler include lexical analysis, syntax analysis, semantic analysis, intermediate code generation, code optimization, code generation and linking. Tools like Lex and Yacc are commonly used to generate lexical and syntax analyzers.
3. Regular expressions are used to specify patterns for tokens during lexical analysis. A lexical analyzer reads the source program and generates a sequence of tokens by matching character sequences to patterns
This document summarizes key concepts about context-free grammars and parsing from Chapter 4 of a compiler textbook. It defines context-free grammars and their components: terminals, nonterminals, a start symbol, and productions. It describes the roles of lexical analysis and parsing in a compiler. Common parsing methods like LL, LR, top-down and bottom-up are introduced. The document also discusses representing language syntax with grammars, handling syntax errors, and strategies for error recovery.
This document provides an overview of a compiler design course, including prerequisites, textbook, course outline, and introductions to key compiler concepts. The course outline covers topics such as lexical analysis, syntax analysis, parsing techniques, semantic analysis, intermediate code generation, code optimization, and code generation. Compiler design involves translating a program from a source language to a target language. Key phases of compilation include lexical analysis, syntax analysis, semantic analysis, intermediate code generation, code optimization, and code generation. Parsing techniques can be top-down or bottom-up.
The document provides an overview of compiler design and the different phases involved in compiling a program. It discusses:
1) What compilers do by translating source code into machine code while hiding machine-dependent details. Compilers may generate pure machine code, augmented machine code, or virtual machine code.
2) The typical structure of a compiler which includes lexical analysis, syntactic analysis, semantic analysis, code generation, and optimization phases.
3) Lexical analysis involves scanning the source code and grouping characters into tokens. Regular expressions are used to specify patterns for tokens. Scanner generators like Lex and Flex can generate scanners from regular expression definitions.
This document provides an introduction to compilers. It discusses how compilers bridge the gap between high-level programming languages that are easier for humans to write in and machine languages that computers can actually execute. It describes the various phases of compilation like lexical analysis, syntax analysis, semantic analysis, code generation, and optimization. It also compares compilers to interpreters and discusses different types of translators like compilers, interpreters, and assemblers.
The document discusses language translation using lex and yacc tools. It begins with an introduction to compilers and interpreters. It then provides details on the phases of a compiler including lexical analysis, syntax analysis, semantic analysis, code generation, and optimization. The document also provides an overview of the lex and yacc specifications including their basic structure and how they are used together. Lex is used for lexical analysis by generating a lexical analyzer from regular expressions. Yacc is used for syntax analysis by generating a parser from a context-free grammar. These two tools work together where lex recognizes tokens that are passed to the yacc generated parser.
The document discusses syntax analysis in compiler design. It defines syntax analysis as the process of analyzing a string of symbols according to the rules of a formal grammar. This involves checking the syntax against a context-free grammar, which is more powerful than regular expressions and can check balancing of tokens. The output of syntax analysis is a parse tree. It separates lexical analysis and parsing for simplicity and efficiency. Lexical analysis breaks the source code into tokens, while parsing analyzes token streams against production rules to detect errors and generate the parse tree.
This document describes the different phases of a compiler: lexical analysis, syntax analysis, semantic analysis, intermediate code generation, code optimization, and code generation. It provides examples of each phase. Lexical analysis scans the source code and groups characters into lexemes and tokens. Syntax analysis builds a parse tree from the tokens. Semantic analysis checks for semantic errors. Intermediate code generation outputs an abstract representation. Code optimization improves the intermediate code. Code generation selects memory locations and translates to machine instructions. The document also defines lexemes, tokens, and patterns used in lexical analysis. It describes the role of the lexical analyzer and different buffer schemes (one buffer vs two buffer) used in lexical analysis.
The document summarizes the key phases of a compiler:
1. The compiler takes source code as input and goes through several phases including lexical analysis, syntax analysis, semantic analysis, code optimization, and code generation to produce machine code as output.
2. Lexical analysis converts the source code into tokens, syntax analysis checks the grammar and produces a parse tree, and semantic analysis validates meanings.
3. Code optimization improves the intermediate code before code generation translates it into machine instructions.
The document discusses the roles of compilers and interpreters. It explains that a compiler translates an entire program into machine code in one pass, while an interpreter translates and executes code line-by-line. The document also covers the basics of lexical analysis, including how it breaks source code into tokens by removing whitespace and comments. It provides an example of tokens identified in a code snippet and discusses how the lexical analyzer works with the symbol table and syntax analyzer.
Lexical analysis is the process of converting a sequence of characters from a source program into a sequence of tokens. It involves reading the source program, scanning characters, grouping them into lexemes and producing tokens as output. The lexical analyzer also enters tokens into a symbol table, strips whitespace and comments, correlates error messages with line numbers, and expands macros. Lexical analysis produces tokens through scanning and tokenization and helps simplify compiler design and improve efficiency. It identifies tokens like keywords, constants, identifiers, numbers, operators and punctuation through patterns and deals with issues like lookahead and ambiguities.
We have learnt that any computer system is made of hardware and software.
The hardware understands a language, which humans cannot understand. So we write programs in high-level language, which is easier for us to understand and remember.
These programs are then fed into a series of tools and OS components to get the desired code that can be used by the machine.
This is known as Language Processing System.
The lexical analyzer is the first phase of a compiler. It takes source code as input and breaks it down into tokens by removing whitespace and comments. It identifies valid tokens by using patterns and regular expressions. The lexical analyzer generates a sequence of tokens that is passed to the subsequent syntax analysis phase. It helps locate errors by providing line and column numbers.
The phases of a compiler are:
1. Lexical analysis breaks the source code into tokens
2. Syntax analysis checks the token order and builds a parse tree
3. Semantic analysis checks for type errors and builds symbol tables
4. Code generation converts the parse tree into target code
The document discusses the phases of a compiler and their functions. It describes:
1) Lexical analysis converts the source code to tokens by recognizing patterns in the input. It identifies tokens like identifiers, keywords, and numbers.
2) Syntax analysis/parsing checks that tokens are arranged according to grammar rules by constructing a parse tree.
3) Semantic analysis validates the program semantics and performs type checking using the parse tree and symbol table.
The document discusses the phases of a compiler:
1) Lexical analysis scans the source code and converts it to tokens which are passed to the syntax analyzer.
2) Syntax analysis/parsing checks the token arrangements against the language grammar and generates a parse tree.
3) Semantic analysis checks that the parse tree follows the language rules by using the syntax tree and symbol table, performing type checking.
4) Intermediate code generation represents the program for an abstract machine in a machine-independent form like 3-address code.
The compiler is software that converts source code written in a high-level language into machine code. It works in two major phases - analysis and synthesis. The analysis phase performs lexical analysis, syntax analysis, and semantic analysis to generate an intermediate representation from the source code. The synthesis phase performs code optimization and code generation to create the target machine code from the intermediate representation. The compiler uses various components like a symbol table, parser, and code generator to perform this translation.
This document provides information about the CS213 Programming Languages Concepts course taught by Prof. Taymoor Mohamed Nazmy in the computer science department at Ain Shams University in Cairo, Egypt. It describes the syntax and semantics of programming languages, discusses different programming language paradigms like imperative, functional, and object-oriented, and explains concepts like lexical analysis, parsing, semantic analysis, symbol tables, intermediate code generation, optimization, and code generation which are parts of the compiler design process.
The document provides an overview of the compilation process and the different phases involved in compiler construction. It can be summarized as follows:
1. A compiler translates a program written in a source language into an equivalent program in a target language. It performs analysis, synthesis and error checking during this translation process.
2. The major phases of a compiler include lexical analysis, syntax analysis, semantic analysis, intermediate code generation, code optimization, code generation and linking. Tools like Lex and Yacc are commonly used to generate lexical and syntax analyzers.
3. Regular expressions are used to specify patterns for tokens during lexical analysis. A lexical analyzer reads the source program and generates a sequence of tokens by matching character sequences to patterns
This document summarizes key concepts about context-free grammars and parsing from Chapter 4 of a compiler textbook. It defines context-free grammars and their components: terminals, nonterminals, a start symbol, and productions. It describes the roles of lexical analysis and parsing in a compiler. Common parsing methods like LL, LR, top-down and bottom-up are introduced. The document also discusses representing language syntax with grammars, handling syntax errors, and strategies for error recovery.
This document provides an overview of a compiler design course, including prerequisites, textbook, course outline, and introductions to key compiler concepts. The course outline covers topics such as lexical analysis, syntax analysis, parsing techniques, semantic analysis, intermediate code generation, code optimization, and code generation. Compiler design involves translating a program from a source language to a target language. Key phases of compilation include lexical analysis, syntax analysis, semantic analysis, intermediate code generation, code optimization, and code generation. Parsing techniques can be top-down or bottom-up.
The document provides an overview of compiler design and the different phases involved in compiling a program. It discusses:
1) What compilers do by translating source code into machine code while hiding machine-dependent details. Compilers may generate pure machine code, augmented machine code, or virtual machine code.
2) The typical structure of a compiler which includes lexical analysis, syntactic analysis, semantic analysis, code generation, and optimization phases.
3) Lexical analysis involves scanning the source code and grouping characters into tokens. Regular expressions are used to specify patterns for tokens. Scanner generators like Lex and Flex can generate scanners from regular expression definitions.
This document provides an introduction to compilers. It discusses how compilers bridge the gap between high-level programming languages that are easier for humans to write in and machine languages that computers can actually execute. It describes the various phases of compilation like lexical analysis, syntax analysis, semantic analysis, code generation, and optimization. It also compares compilers to interpreters and discusses different types of translators like compilers, interpreters, and assemblers.
The document discusses language translation using lex and yacc tools. It begins with an introduction to compilers and interpreters. It then provides details on the phases of a compiler including lexical analysis, syntax analysis, semantic analysis, code generation, and optimization. The document also provides an overview of the lex and yacc specifications including their basic structure and how they are used together. Lex is used for lexical analysis by generating a lexical analyzer from regular expressions. Yacc is used for syntax analysis by generating a parser from a context-free grammar. These two tools work together where lex recognizes tokens that are passed to the yacc generated parser.
The document discusses syntax analysis in compiler design. It defines syntax analysis as the process of analyzing a string of symbols according to the rules of a formal grammar. This involves checking the syntax against a context-free grammar, which is more powerful than regular expressions and can check balancing of tokens. The output of syntax analysis is a parse tree. It separates lexical analysis and parsing for simplicity and efficiency. Lexical analysis breaks the source code into tokens, while parsing analyzes token streams against production rules to detect errors and generate the parse tree.
This document describes the different phases of a compiler: lexical analysis, syntax analysis, semantic analysis, intermediate code generation, code optimization, and code generation. It provides examples of each phase. Lexical analysis scans the source code and groups characters into lexemes and tokens. Syntax analysis builds a parse tree from the tokens. Semantic analysis checks for semantic errors. Intermediate code generation outputs an abstract representation. Code optimization improves the intermediate code. Code generation selects memory locations and translates to machine instructions. The document also defines lexemes, tokens, and patterns used in lexical analysis. It describes the role of the lexical analyzer and different buffer schemes (one buffer vs two buffer) used in lexical analysis.
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1. 20IT011-COMPILER DESIGN
Text Book :
Alfred V Aho, Monica S Lam, Ravi Sethi and Jeffrey D Ullman, “Compilers - Principles,
Techniques and Tools”, Pearson Education, 2nd Edition, 2018.
2. COURSE OUTCOME
CO1: Ability to understand the functionalities of the different phases of compiler.
CO2: Ability to understand syntax-directed translation and run-time environment.
CO3: Ability to implement code optimization techniques.
CO4: Ability to apply different parsing algorithms to develop the parsers for a
given grammar.
CO5: Ability to design a scanner and a parser using LEX and YACC tools.
4. UNIT-I SYLLABUS
Structure of a compiler – Lexical Analysis – Role
of Lexical Analyzer -Input Buffering –
Specification of Tokens – Recognition of Tokens-
Lex – Finite Automata-Regular Expressions to
Automata – Minimizing DFA.
5. STRUCTURE OF A COMPILER
• Definition: A program that accept as input a program text in a certain language
and produces as output a program text in another language (Grune et all)
• Compiler is a translator program that reads a program written in one language -
the source language- and translates it into an equivalent program in another
language-the target language.
8. Lexical Analysis
• In a compiler, linear analysis is called lexical analysis or scanning.
• It takes source code as input
• It reads one character at a time and convert into lexemes( represent in the
form of tokens)
Example: position = initial + rate * 60
✓ The identifier position.
✓ The assignment symbol =.
✓ The identifier initial.
✓ The plus sign.
✓ The identifier rate.
✓ The multiplication sign.
✓ The number 60.
<Id,Position> <=,> <Id,initial> <+,> <Id,rate><*,> <num,60>
9. Contd..
• Needs to record each id attribute: keep a symbol table
• Lexical Analysis eliminates white spaces, etc..
10. Syntax Analysis
• It is called as parsing or Syntax Analysis
• It takes token as input and generate parse tree as output.
• The parser checks that the expression made by tokens is
syntactically correct or not.
• Context free grammars formalize the rules and guide syntax analysis.
11. Semantic Analysis
• It checks the parse tree follows the rules of language
• It keeps tracks of identifiers, types and expressions
• Output is annotated as tree syntax
12. Intermediate Code Generator
• After semantic analysis, some compilers generate an explicit
intermediate representation of the source program.
• This representation should be easy to produce and easy to translate
into the target program.
13. Code Optimization
• It is used to improve the intermediate code
• So the output can run fast and take less space
• It removes unnecessary lines of code and arrange the sequence in
order to speed up the execution.
14. Code Generation
• The final phase of the compiler is the generation of target code,
consisting of relocatable machine code or assembly code.
• The intermediate instructions are each translated into sequence of
machine instructions that perform the same task.
15. Symbol table Manager
• An essential function of a compiler is to record the identifiers used in
the source program and collect its information.
• A symbol table is a data structure containing a record for each
identifier with fields for attributes.(such as, its type, its scope, if
procedure names then the number and type of arguments etc.,)
• The data structure allows to find the record for each identifier and
store or retrieve data from that record quickly.
Example: sum=initial + value * 10 1 sum
2 initial
3 value
4 ….
16. Error Handling and Reporting
• Each phase can encounter errors. After detecting an error, a phase must deal that
error, so that compilation can proceed ,allowing further errors to be detected.
• Lexical phase can detect error where the characters remaining in the input do not
form any token.
• The syntax and semantic phases handle large fraction of errors. The stream of
tokens violates the syntax rules are determined by syntax phase.
• During semantic, the compiler tries to detect constructs that have the right
syntactic structure but no meaning to the operation involved.
18. Role of lexical Analyzer
• As the first phase of a compiler, the main task of the lexical analyzer is
to read the input characters of the source program, group them into
lexemes, and produce as output a sequence of tokens for each lexeme
in the source program.
• The stream of tokens is sent to the parser for syntax analysis.
• It is common for the lexical analyzer to interact with the symbol table
as well.
• When the lexical analyzer discovers a lexeme constituting an
identifier, it needs to enter that lexeme into the symbol table.
19. Contd..
• In some cases, information regarding the kind of identifier may be
read from the symbol table by the lexical analyzer to assist it in
determining the proper token it must pass to the parser.
20. Interaction between the lexical analyzer and the parser
• The interaction is implemented by having the parser call the lexical
analyzer.
• The call, suggested by the getNextToken command, causes the
lexical analyzer to read characters from its input until it can identify
the next lexeme and produce for it the next token, which it returns to
the parser.
• Since the lexical analyzer is the part of the compiler that reads the
source text, it may perform certain other tasks besides identification of
lexemes
21. Contd..
• One such task is stripping out comments and whitespace (blank,
newline, tab, and perhaps other characters that are used to separate
tokens in the input).
• Another task is correlating error messages generated by the compiler
with the source program.
• lexical analyzers are divided into a cascade of two processes:
• Scanning consists of the simple processes that do not require tokenization of the
input, such as deletion of comments and compaction of consecutive whitespace
characters into one.
• Lexical analysis proper is the more complex portion, where the scanner produces the
sequence of tokens as output.
22. Lexical Analysis and Parsing
• Simplicity of design – avoid complexity to work together .
• Improving Compiler Efficiency - A separate lexical analyzer allows us to apply
specialized techniques that serve only the lexical task, not the job of parsing. In
addition, specialized buffering techniques for reading input characters can speed
up the compiler significantly.
• Enhancing Compiler Portability -Input-device-specific peculiarities can be
restricted to the lexical analyzer.
23. Tokens, Patterns, Lexemes
• A token is a pair consisting of a token name and an optional
attribute value.
• A pattern is a description of the form that the lexemes of a token
may take.
• A lexeme is a sequence of characters in the source program that
matches the pattern for a token and is identified by the lexical analyzer
as an instance of that token.
26. Lexical Errors
• Some errors are out of power of lexical analyzer to recognize:
✓fi(a==f(x))..
• However it may be able to recognize errors like:
✓d=2r
• Such errors are recognized when no pattern for tokens matches a character
sequence
28. Error Recovery
• Panic Mode: Successive characters are ignored until we reach to a well
formed token
• Delete one character from the remaining input
• Insert a missing character into the remaining input
• Replace a character by another character
• Transpose two adjacent characters
30. Input Buffering
• Input buffer helps to find the correct lexeme.
• Sometimes lexical analyzer needs to look ahead some symbols to decide about
the token to return
- in C language: we need to look after -,= or < to decide what token to return
- in FORTRAN: DO 5 I=1.25
• We need to introduce a two buffer scheme to handle large look-aheads safely
31. Buffer Pairs
Two pointers to the input are maintained:
I. Pointer lexemeBegin, marks the beginning of the current lexeme,
whose extent we are attempting to determine.
II. Pointer forward scans ahead until a pattern match is found
32. Sentinels
• A sentinel is a special character or token used to mark the end of
a sequence.
• In the context of a lexical analyzer, a sentinel can be utilized to
indicate the end of the input source code or the end of a specific
token.
33.
34. QUIZ
1. ______________ is the heart of the compiler.
2.what will be the next phase of Intermediate code generator?
3. What is the output of Lexical analyzer?
4. Token consist of pairs, what are they?
5. What is the command ,the parser generate to lexical analyzer?
6. What is the drawback in 1-Buffer?
7. What the two pointers used in Buffering technique?
8. What are the Back-end in Phases of compiler?
36. SPECIFICATION OF TOKENS
• In theory of compilation regular expressions are used to formalize the
specification of tokens.
• The specification of tokens depends on the pattern of the lexeme.
Example:
letter(letter+digit)*
• Each regular expression is a pattern specifying the form of strings.
39. Strings and Languages
Symbol:
• A symbol is an abstract entity that we shall not define formally.
Examples:
• Letters: A|B|C…..|Z |a|b….|z
• Digit:0,1,2…9
• Special Characters
40. Contd..
Alphabet:
• Alphabet is a non-empty finite set of symbols. it is denoted by the
symbol Ɛ(Sigma).
Example:
• Ɛ={0,1} is the binary alphabet which consisting of digits
• Ɛ={a,b,c} is an alphabet which consisting of letters.
• ASCII is an alphabet ;it is used in many softwares.
• Unicode is an alphabet , which includes 1,00,000 characters.
41. Contd..
String:
• A String is defined as finite sequence of symbols over an alphabet Ɛ .
It is denoted as ‘w’.
• In language theory, the terms “sentence” and “word” are often used as
synonyms for “string”
Examples:
1.Alphabet Ɛ={ a, b }
Strings: w = {a, b, aa, ab, ba, bb, aaa, aab,……..}
2. Ɛ={ 0, 1 }
Strings: w = { 0, 1, 00, 01, 10, 11, 000, 001, 010, 011,100,101,110,111,
0000,....}
42. Different Notation used in a string
• Length
• Prefix:- any number of leading symbols
• Proper Prefix :- any number of leading symbols without € and
original string
• Suffix:- tailing symbols
• Proper Suffix:- any number of trailing symbols without € and
original string
• Substring
46. Definition
- A regular expression over Ɛ can be defined as
• ɸ is a regular expression for empty set
• Ɛ is a regular expression for null string {Ɛ}
• If ‘a’ is a symbol in Ɛ then ‘a’ is RE for the set {a}.
• If R and S are two RE then
• UNION of two R+E is a RE
• CONCATENATION of two RS is a RE
• KLEEN CLOSURE of any R* S* is a RE
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