lex program to implement a simple calculator.

 /*lex program to implement  a simple calculator.*/

%{

int op = 0,i;

float a, b;

%}

%option noyywrap

dig [0-9]+|([0-9]*)"."([0-9]+)

add "+"

sub "-"

mul "*"

div "/"

pow "^"

ln \n

%%

{dig} {compute();}

{add} {op=1;}

{sub} {op=2;}

{mul} {op=3;}

{div} {op=4;}

{pow} {op=5;}

{ln} {printf("\n The Answer :%f\n\n",a);}

%%

compute()

{

if(op==0)

/* atof() is used to convert  the ASCII input to float */

a=atof(yytext);

else

{

b=atof(yytext);

switch(op)

{

case 1:a=a+b;

        break;

case 2:a=a-b;

        break;

case 3:a=a*b;

        break;

case 4:a=a/b;

        break;

case 5:for(i=a;b>1;b--)

        a=a*i;

        break;

}

op=0;

}

}

main()

{

yylex();

}

Execution

$flex cal.lex

$ gcc lex.yy.c

$ ./a.out

2+3

The Answer :5.000000

2.2/1.2

 The Answer :1.833333

2.54-1.23

 The Answer :1.310000

2/3

 The Answer :0.666667