Lex program to implement a simple Calculator

Lex is a computer program that generates lexical analyzers. Lex reads an input stream specifying the lexical analyzer and outputs source code implementing the lexer in the C programming language.

The commands for executing the LEX program are:

lex abc.l (abc is the file name)
cc lex.yy.c -efl
./a.out 

Let’s see LEX program to implement a simple calculator.

Examples:

Input : 
3+3
Output :
6.0

Input : 
5*4
Output : 
20.0 

Below is the implementation:

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

% {
  int op = 0,i;
  float a, b;
% }

dig [0-9]+|([0-9]*)"."([0-9]+)
add "+"
sub "-"
mul "*"
div "/"
pow "^"
ln \n
%%

/* digi() is a user defined function */
{dig} {digi();}
{add} {op=1;}
{sub} {op=2;}
{mul} {op=3;}
{div} {op=4;}
{pow} {op=5;}
{ln} {printf("\n The Answer :%f\n\n",a);}

%%
digi()
{
 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(int argv,char *argc[])
{
 yylex();
}

yywrap()
 {
  return 1;
 }

Output: