Lexical Analysis.pdf

COMPILER DESIGN

1. LEXICALANALYSER(SCANNER)

2. •Lexicalanalysisisthefirstphaseofacompiler.Ittakesmodifiedsourcecodefrom
languagepreprocessorsthatarewrittenintheformofsentences.Thelexicalanalyzer
breaksthesesyntaxesintoaseriesoftokens,byremovinganywhitespaceorcomments
inthesourcecode.
•LexicalAnalysisisthefirstphaseofthecompileralsoknownasascanner.Itconverts
theHighlevelinputprogramintoasequenceofTokens.
•Ifthelexicalanalyzerfindsatokeninvalid,itgeneratesanerror.Thelexical
analyzerworkscloselywiththesyntaxanalyzer.Itreadscharacterstreamsfromthe
sourcecode,checksforlegaltokens,andpassesthedatatothesyntaxanalyzer
whenitdemands.
•Tokens
Lexemesaresaidtobeasequenceofcharacters(alphanumeric)inatoken.There
aresomepredefinedrulesforeverylexemetobeidentifiedasavalidtoken.These
rulesaredefinedbygrammarrules,bymeansofapattern.Apatternexplainswhat
canbeatoken,andthesepatternsaredefinedbymeansofregularexpressions.

3. •Lexical Analysis can be implemented with theDeterministic finite Automata.
•The output is a sequence of tokens that is sent to the parser for syntax analysis

4. •What is a token?
A lexical token is a sequence of characters that can be treated as a unit in the grammar
of the programming languages.
Example of tokens:
•Type token (id, number, real, . . . )
•Punctuation tokens (IF, void, return, . . . )
•Alphabetic tokens (keywords)
Keywords; Examples-for, while, if etc.
Identifier; Examples-Variable name, function name, etc.
Operators; Examples '+', '++', '-' etc.
Separators; Examples ',' ';' etc
Example of Non-Tokens:
•Comments, preprocessordirective, macros, blanks, tabs, newline, etc.

5. Lexeme:Thesequenceofcharactersmatchedbyapatterntoform
thecorrespondingtokenorasequenceofinputcharactersthatcomprisesasingletokenis
calledalexeme.
eg-“float”,“abs_zero_Kelvin”,“=”,“-”,“273”,“;”
How Lexical Analyzer functions
1. Tokenization i.e. Dividing the program into valid tokens.
2. Remove white space characters.
3. Remove comments.
4. It also provides help in generating error messages by providing row numbers and column
numbers.

6. Suppose we pass a statement through lexical analyzer –
a= b + c; It will generate token sequence like this:
id=id+id; Where each id refers to it’s variable in the symbol table referencing all details
For example, consider the program
int main()
{ // 2 variables
int a, b; a = 10;
return 0;
}
All the valid tokens are:
'int’ 'main’ '(‘ ')’ '{‘ 'int’ 'a' ‘, ' 'b’ ';' ‘ a' '=‘ ‘ 10’ ';’
'return’ '0' ';’ '}'
Above are the valid tokens.
You can observe that we have omitted comments.

7. Exercise 1:
Count number of tokens :
int main()
{
int a = 10, b = 20;
printf("sum is :%d", a + b );
return 0;
}

8. Answer: Total number of token: 27.
Exercise 2:
Count number of tokens :
int max(int i);
•Lexical analyzer first readintand finds it to be valid and accepts as token
•maxis read by it and found to be a valid function name after reading(
•intis also a token , then againias another tokenand finally;
Answer: Total number of tokens 7

9. Basic Terminologies
What’s a lexeme?
A lexeme is a sequence of characters that are included in the source program according to the
matching pattern of a token. It is nothing but an instance of a token.
What’s a token?
Tokens in compiler design are the sequence of characters which represents a unit of
information in the source program.
What is Pattern?
A pattern is a description which is used by the token. In the case of a keyword which uses
as a token, the pattern is a sequence of characters.

10. RolesoftheLexicalanalyzer:
Lexicalanalyzerperformsbelowgiventasks:
•Helpstoidentifytokenintothesymboltable
•Removeswhitespacesandcommentsfromthesourceprogram
•Correlateserrormessageswiththesourceprogram
•Helpsyoutoexpandsthemacrosifitisfoundinthesourceprogram
•Readinputcharactersfromthesourceprogram
Lexical Errors
A character sequence which is not possible to scan into any valid token is
a lexical error.

11. •Lexicalerrorsarenotverycommon,butitshouldbemanagedbyascanner
•Misspellingofidentifiers,operators,keywordareconsideredaslexicalerrors
•Generally,alexicalerroriscausedbytheappearanceofsomeillegalcharacter,mostly
atthebeginningofatoken.