Parser for sample language using YACC.

***Note : While executing the program remove the comments in blue. Comments mentioned are for understanding purpose

calc.l:

%{
//declarations for c code
#include<stdio.h>
#include"y.tab.h"
//extern because it is declared in yacc file and used here   
extern int yylval;
%}

//regular expressions and corresponding actions for input streams read
%%
[0-9]+ {
    yylval=atoi(yytext);
    return NUMBER;
    }   
    //If any number is encountered yylval stores its value and we are returning token number which shows yylval's type ie int

[\t];    //we are ignoring white spaces
[\n] return 0; 
. return yytext[0];  //return all the other characters as it is
%%

int yywrap()   //returns 1 when end of file is reached otherwise its value is 0
{
return 1;
}


calc.y :

//for c declarations
%{
#include<stdio.h>
 #include<time.h>
int flag=0;
extern FILE* yyin;    //file pointer by default points to terminal
%}

//for declarations in yacc
%token NUMBER        //declaring token NUMBER
%left '+''-'        //to maintain associativity
%left '*''/''%'
%left '('')'

//context free grammar rules and corresponding action
%%
ArithematicExpression:E{
    printf("\n Result=%d\n",$$);
    return 0;
    }

E:E'+'E{$$=$1+$3;}    //$$ refers to non-terminal on left side, $1 to first E on right and so on
|E'-'E{$$=$1-$3;}    //Pseudo varibles in {} describe action to be performed for given grammar rule
|E'*'E{$$=$1*$3;}
|E'/'E{$$=$1/$3;}
|E'%'E{$$=$1%$3;}
|'('E')'{$$=$2;}
|NUMBER{$$=$1;}
;
%%

void main()
{
clock_t starttime;
clock_t endtime;
double timeinterval;
int i;
FILE* fp=fopen("expressions.txt","r");        //file contains expression to be evaluated
yyin=fp;

starttime=clock();
yyparse();     //yyparse() in turn calls yylex()

endtime=clock();
timeinterval=(double)(endtime-starttime)/CLOCKS_PER_SEC;
printf("Time required for execution=%lf Seconds.\n",timeinterval);

if(flag==0)
{
printf("\n ENTERED ARETHEMATIC EXPRESSION IS VALID \n\n");
}

}

void yyerror()
{
printf("\n ENTERED ARETHEMATIC EXPRESSION IS INVALID \n\n");
flag=1;
}


expressions.txt:
5+2*6
3*2+1

OUTPUT: