3. Implementation of Lexical Analyzer using Lex Tool

Ex. No: 3

Date:

3.  Implementation of Lexical Analyzer using  Lex Tool

AIM:

                   To write a lex program to implement the lexical analyzer

INTRODUCTION

Lex is officially known as a "Lexical Analyzer". It’s main job is to break up an input stream into more into meaningful units, or tokens. For example, consider breaking a text file up into individual words. More pragmatically, Lex is a tool for automatically generating a lexer ( also known as scanner) starting from a lex specification.

Lex specifications:

A Lex program (the .l file ) consists of three parts:

declarations

%%

translation rules

%%

auxiliary procedures

1. The declarations section includes declarations of variables, manifest constants (A manifest constant is an identifier that is declared to represent a constant. e.g. # define PIE 3.14), and regular definitions.

2. The translation rules of a Lex program are statements of the form :

p1 {action 1}

p2 {action 2}

p3 {action 3}

… …

… …

where each p is a regular expression and each action is a program fragment describing what

action the lexical analyzer should take when a pattern p matches a lexeme. In Lex the actions are written in C.

3. The third section holds whatever auxiliary procedures are needed by the actions.

Alternatively, these procedures can be compiled separately and loaded with the lexical

analyzer.

A Lex program has the following form:

declarations

%%

translation rules

%%

auxiliary functions

The declarations section includes declarations of variables, manifest constants and regular

definitions.

The translation rules each have the form Pattern { Action }

Each pattern is a regular expression, which may use the regular definitions of the declaration section. The actions are fragments of code, typically written in C, although many variants of Lex using other languages have been created.

STEP BY STEP PROCEDURE FOR RUNNING A LEX AND YACC PROGRAM

For compiling a lex program

1. write the lex program in a file and save it as file.l (where file is the name of the file).

2. open the terminal and navigate to the directory where you have saved the file.l

3. type - lex file.l

4. then type - cc lex.yy.c

5. then type - ./a.out or a.exe

For compiling lex and yacc together

1. write lex program in a file file.l and yacc in a file file.y

2. open the terminal and navigate to the directory where you have saved the files.

3. type lex file.l

4. type yacc file.y

5. type cc lex.yy.c y.tab.h

6. type ./a.out or a.exe

ALGORITHM:

1.Start the program

2. Pass  file.c via command line arguments as Input file and include the yylex() tool for input scanning.

3.Define the alphabets and numbers

4.Print the preprocessor, function, keyword using yytext.lex tool

5.Print the relational, assignment and all the operator using yytext() tool

6.Also scan and print where the loop ends and begins.

7.Use yywrap() to enter an error.

8.Stop the program.

PROGRAM

CDP3.l  (Save the following code snippets in CDP3.l)

%{

int COMMENT=0;

%}

identifier [a-zA-Z][a-zA-Z0-9]*

%%

#.* { printf("\n%s is a PREPROCESSOR DIRECTIVE",yytext);}

int |

float |

char |

double |

while |

for |

do |

if |

break |

continue |

void |

switch |

case |

long |

struct |

const |

typedef |

return |

else |

goto {printf("\n\t%s is a KEYWORD",yytext);}

"/*" {COMMENT = 1;}

"*/" {COMMENT = 0;}

{identifier}\( {if(!COMMENT)printf("\n\nFUNCTION\n\t%s",yytext);}

\{ {if(!COMMENT) printf("\n BLOCK BEGINS");}

\} {if(!COMMENT) printf("\n BLOCK ENDS");}

{identifier}(\[[0-9]*\])? {if(!COMMENT) printf("\n %s IDENTIFIER",yytext);}

\".*\" {if(!COMMENT) printf("\n\t%s is a STRING",yytext);}

[0-9]+ {if(!COMMENT) printf("\n\t%s is a NUMBER",yytext);}

\)(\;)? {if(!COMMENT) printf("\n\t");ECHO;printf("\n");}

\( ECHO;

= {if(!COMMENT)printf("\n\t%s is an ASSIGNMENT OPERATOR",yytext);}

\<= |

\>= |

\< |

== |

\> {if(!COMMENT) printf("\n\t%s is a RELATIONAL OPERATOR",yytext);}

%%

int main(int argc,char **argv)

{

if (argc > 1)

{

FILE *file;

file = fopen(argv[1],"r");

if(!file)

{

printf("could not open %s \n",argv[1]);

exit(0);

}

yyin = file;

}

yylex();

printf("\n\n");

return 0;

} int yywrap()

{

return 0;

}

ODDREVEN.C

#include <stdio.h>

int main()

{

    int number;

    printf("Enter an integer: ");

    scanf("%d", &number);

    if(number % 2 == 0)

        printf("%d is even.", number);

    else

        printf("%d is odd.", number);

    return 0;

}

  ═══════════=============Output ═════════════════════════

D:\Mohanraj\CS6612 COMPILER LAB\lex_yacc>flex CDP3.l

D: \Mohanraj \CS6612 COMPILER LAB\lex_yacc>gcc lex.yy.c

D: \Mohanraj \CS6612 COMPILER LAB\lex_yacc>a.exe ODDREVEN.c

#include <stdio.h> is a PREPROCESSOR DIRECTIVE

        int is a KEYWORD

FUNCTION

        main(

        )

 BLOCK BEGINS

        int is a KEYWORD

 number IDENTIFIER;

FUNCTION

        printf(

        "Enter an integer: " is a STRING

        );

FUNCTION

        scanf(

        "%d" is a STRING, &

 number IDENTIFIER

        );

FUNCTION

        if(

 number IDENTIFIER %

        2 is a NUMBER

        == is a RELATIONAL OPERATOR

        0 is a NUMBER

        )

FUNCTION

        printf(

        "%d is even." is a STRING,

 number IDENTIFIER

        );

        else is a KEYWORD

FUNCTION

        printf(

        "%d is odd." is a STRING,

 number IDENTIFIER

        );

        return is a KEYWORD

        0 is a NUMBER;

 BLOCK ENDS

        ( Ctl+C to Stop Program Execution)

                                                                            

                                    

RESULT:

Thus the C program to implement the lexical analyzer using LEX tool was executed and the output is verified.