LEX AND YACC

Implementation of Calculator using LEX and YACC.

Description:

Lex is used for breaking up an input stream into more usable elements. Lex is used for tokenizing  input

i.e separating  input into the lowest-level objects that your grammar defines.

YACC is used for analyzing the structure of the input stream.YACC stands for “Yet Another Compiler

Compiler”. YACC  is a parser that tokenizes input provided by Lex. YACC performs parsing of grammer.

//LEX FILE

%{

#include <math.h>

#include "y.tab.h"

extern double vblt[26];

%}

%%([0-9]+|([0-9]*\.[0-9]+)([eE][+-]?[0-9]+)?) {yylval.dval=atof(yytext);

return NUMBER;}

[\t ] ;

sqrt {return SQRT;}

log {return LOG;}

[a-z] { yylval.vblno=yytext[0]-'a';

return NAME; }

"$" {return 0;}

\n|. return yytext[0];

%%

//yacc file

%{

#include<stdio.h>

#include <math.h>

double vblt[26];

%}

/*%union is used to collect all the attributes which are going to used in our program */

%union {

float dval;

int vblno;

}

/*this is used to know yacc file that which attribute is attached to which terminal */

%token <vblno> NAME%token <dval> NUMBER%token SQRT LOG

/* Priority & associativity */

%left '-' '+'%left '*' '/'

%nonassoc UMINUS

/*this is used to know yacc file that which attribute is attached to which non-terminal */

%type <dval> expression

%%

statement_list:statement '\n'| statement_list statement '\n'

;

statement: NAME '=' expression {vblt[$1]=$3;}| expression {printf("=%f\n",$1);}

;

expression: expression '+' expression {$$=$1+$3;}

| expression '-' expression {$$=$1-$3;}

| expression '*' expression {$$=$1*$3;}

| expression '/' expressionif($3==0.0)

yyerror("divide by zero");

else

$$=$1/$3;}

| '-' expression %prec UMINUS {$$=-$2;}

| '(' expression ')' {$$=$2;} | SQRT '(' expression ')' {$$=sqrt($3);} | LOG '(' expression ')' {$$=log($3);}

| NUMBER

| NAME {$$=vblt[$1];};

%%

main()

{ //extern double sqrt(),exp(),log();

yyparse(); return 0;

}

int yyerror(char *s)

{

printf("%s",s);

return 0;

}