This document discusses compiler construction and error handling. It describes the basic tasks of an error handler as detection, reporting, and recovery of errors encountered during compilation. Errors are classified and the document focuses on lexical analysis. The lexical analyzer takes source code as character streams and breaks it into tokens based on predefined patterns and grammar rules. It generates errors for invalid tokens and works closely with the syntax analyzer by passing tokenized data.

1. Compiler Construction
By Noor wali khan
UOCH
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2. Error Handler
• An error is an abnormal condition in source program, which
either stope the compilation or generate undesired results.
• A compiler must not stop at each error.
• It must somehow handle the error, so that compilation
proceeds up to the end of the program.
• At the end a list of errors messages is provided to the user.
• This is the job of the error handler, and also marks the place
where the errors occur.

3. Error Handler
• Error handler contains a set of routines for handling error
encountered in any phase of the compiler.
• The basic tasks of the compiler are:
• Detection
• Reporting
• Recovery
• During compilation process each phase may encounters errors and
has to deal with them.
• For this purpose the Error handler contains error handling routines.

4. Classification of Errors

5. Lexical analyzer
• It is the first phase of compiler.
• It is also known as linear analyzer or Scanner.
• It scans or reads the source program from left to right as a stream of
characters, and it groups these characters into a unit called “Tokens”.
OR
• It takes the source code program as a stream of characters and
broken them into tokens.

6. Lexical Analyzer
• Token:
•A sequence of characters having a collective meaning.
•Lexemes are said to be a sequence of characters
(alphanumeric) in a token.
•If the lexical analyzer finds a token invalid, it generates
an error.
•There are some predefined rules for every lexeme to
be identified as a valid token.
•These rules are defined by grammar rules, by means
of a pattern.

7. Lexical Analyzer (Token Cont.)
• A pattern explains what can be a token, and these
patterns are defined by means of regular expressions.
• In programming language, keywords, constants,
identifiers, strings, numbers, operators and
punctuations symbols can be considered as tokens
For example, in C language, the variable declaration line
int value = 100;
contains the tokens:
int (keyword), value (identifier), = (operator), 100 (constant) and ;
(symbol).

8. Lexical Analyzer (Token Cont.)
• The lexical analyzer works closely with the syntax
analyzer.
• It reads character streams from the source code,
checks for legal tokens, and passes the data to the
syntax analyzer when it demands.
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