Experiment No 2

 Exp no 2.  Parsing a Command Line:

The command line parameters are the parameters that are appearing on the shell prompt.

The command line interface is the interface which allows the user to interact with the computer by typing the commands.

In C we can pass these parameters to the main function in the form of character array.

For example:

                   $ cp abc.txt pqr.txt

Here

                   argv[0]=cp

                   argv[1]=abc.txt

                   argv[2]=pqr.txt

Now in this experiment we want to handle the command line parameters with using LEX specification file.

Example: Lex program to check well formedness of the parenthesis by using command line arguments.

LEX Program:

%{

#include<stdio.h>

#include<string.h>

char temp[10];

int i=0,openbracket=0,closebracket=0;

extern FILE *yyin;

%}

%%

"("[()]*")"";" {

strcpy(temp,yytext);

printf("%s\n",temp);

i=0;

while(temp[i]!=';')

{

if(temp[i]=='(')

openbracket++;

if(temp[i]==')')

closebracket++;

else;

i++;

}

if(openbracket==closebracket)

printf("Well formed input!\n");

else

printf("not well formed!\n");

}

%%

main(int argc,char *argv[])

{

FILE *fp=fopen(argv[1],"r");

yyin=fp;

yylex();

fclose(fp);

}

yywrap()

{

          return 1;

}

Input file named “b.c” contains ( ( )( ) ) ;

Program Explanation:

          In the above program, we have used a file b.c which is our input file. While running the LEX program we have given the program via the input file “b.c”using command line parameter.

Note that,

 argv[0]=”./a.out”

 argv[1]=”b.c”

In the main() function we have opened the file “b.c” in read mode by

Fp=fopen(argv[1],”r”);

The file pointer fp of the opened file “b.c” will then be assigned to the standard input file pointer yyin as,

                   Extern FILE *yyin;

Then the yylex routine is invoked to call LEX.

The logic of the above program is very simple. We have maintained one temp array in which the input expression is copied.

For example:

(

(

)

(

)

)

;

                                                temp[]

Then we have maintained two counters ‘openbracket’ and ‘closebracket’.

As soon as we come across the openbracket the ‘openbracket’ counter will be incremented and as soon as we come across the closing bracket, the ‘closebracket’ counter will be incremented.

The regular expression specifies that opening bracket should occur prior to closing brace. The same logic works until we get the semicolon.

If both the ‘openbracket’ & ‘closebracket’ counters are same then declare “Well formed input!”

Otherwise declare “not well formed!”

Steps to Execute:

1.     vi well.l

2.     lex well.c

3.     cc lex.yy.c

4.     ./a.out b.c

Output:

Content of input file	Ouput
e.g ( ( ) ( ) ) ;	Well formed input!