Program to create Parser for IF-ELSE Statement using Flex(Lex) and Bison(Yacc)

'IF -ELSE' Statement is commonly used in all programming languages to implement various logical operations.
The control flows by checking a specific condition-if('cond) then expression if the condition is not satisfied the control flows 
to the else statement else-expression.


Parsing if-else statement is done by definitions of a grammar which uses Regular expressions.Compiler construction tools Flex(Lex) and Bison(Yacc) in Linux(Ubuntu) are used to define the grammar and basic operations .On compilation a c program is generated which is actually the parser for 'IF-ELSE' statement.A parser generates a Parse tree for Intermediate code generation.


Following are complete source codes for Flex and Bison programs to create a Parser for 'IF-ELSE' Statement which accepts valid 'IF-ELSE' Statements

 /*PARSER FOR IF ELSE STATEMENT*/  
 //c-madeeasy.blogspot.com  
 /*YACC PROGRAM*/  
 %{  
 #include<stdio.h>  
 #include<stdlib.h>  
 %}  
 %token num alpha LT GT EQ LE GE NE AND OR INC DEC END  
 %left '+''-'  
 %left '*''/'  
 %right '^'  
 %right '='  
 %nonassoc UMINUS  
 %nonassoc IF  
 %nonassoc ELSE  
 %left GE NE LT GT LE EQ  
 %left AND OR  
 %%  
 S:ST END{printf("\n Accepted\n");exit(0);}  
 ST:IF'('F')''{'ST'}'%prec IF  
  |IF'('F')''{'ST'}'ELSE'{'ST'}'  
  |E';'  
  |E';'ST  
  F:C LO C  
  |C  
 LO:AND  
  |OR  
  C:E RELOP E  
  |E  
  E:alpha '='E  
  |E'+'E   
  |E'-'E   
  |E'*'E   
  |E'/'E  
  |E'^'E  
  |'('E')'   
  |'-'E %prec UMINUS  
  |alpha  
  |num  
  |alpha INC  
  |alpha DEC  
 RELOP:LT  
    |GT  
    |EQ  
    |LE  
    |GE  
    |NE  
   ;  
 %%  
 #include"lex.yy.c"  
 int main()  
 {   
 yyparse();  
 yylex();  
 return END;  
 }  
 yyerror(char *s)  
 {  
  printf("\nError");  
 }  
 /*LEX PROGRAM*/  
 %{  
 #include"pgm4.tab.h"  
 %}  
 %%  
 "if" {return IF;}  
 "else" {return ELSE;}  
 "&&" {return AND;}  
 "||" {return OR;}  
 "<=" {return LE;}  
 ">=" {return GE;}  
 ">" {return GT;}  
 "<" {return LT;}  
 "!=" {return NE;}  
 "++" {return INC;}  
 "--" {return DEC;}  
 "==" {return EQ;}  
 [0-9]+ {return num;}  
 [a-zA-Z]+ {return alpha;}  
 [\t];  
 [\n];   
 "$" {return END;}  
 .  {return yytext[0];}  
 %%  
 /*OUTPUT  
 if(i>0)  
 {  
 a=0;  
 c++;  
 }  
 else  
 {  
 h;  
 }  
 $  
 Accepted  
 if(a>l)   
 {  
 w;  
 }  
 $  
 Accepted*/