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YACC program for Conversion of Infix to Postfix expression

  • Difficulty Level : Expert
  • Last Updated : 02 May, 2019
Geek Week

Problem: Write a YACC program for conversion of Infix to Postfix expression.

Explanation:
YACC (Yet another Compiler-Compiler) is the standard parser generator for the Unix operating system. An open source program, yacc generates code for the parser in the C programming language. The acronym is usually rendered in lowercase but is occasionally seen as YACC or Yacc.

Examples:

Input: a*b+c 
Output: ab*c+

Input: a+b*d
Output: abd*+ 

Lexical Analyzer Source Code:




%{
  /* Definition section */
%}
ALPHA [A-Z a-z]
DIGIT [0-9]
  
/* Rule Section */
%%
{ALPHA}({ALPHA}|{DIGIT})*  return ID;
{DIGIT}+                   {yylval=atoi(yytext); return ID;}
[\n \t]                    yyterminate();
.                          return yytext[0];
%%

Parser Source Code:






%{
   /* Definition section */
   #include <stdio.h>
   #include <stdlib.h>
%}
  
%token    ID
%left    '+' '-'
%left    '*' '/'
%left    UMINUS
  
/* Rule Section */
%%
  
S  :  E
E  :  E'+'{A1();}T{A2();}
   |  E'-'{A1();}T{A2();}
   |  T
   ;
T  :  T'*'{A1();}F{A2();}
   |  T'/'{A1();}F{A2();}
   |  F
   ;
F  :  '('E{A2();}')'
   '-'{A1();}F{A2();}
   |  ID{A3();}
   ;
  
%%
  
#include"lex.yy.c"
char st[100];
int top=0;
  
//driver code
int main()
{
    printf("Enter infix expression:  "); 
    yyparse();
    printf("\n");
    return 0;
}
A1()
{
    st[top++]=yytext[0];
}
  
A2()
{
    printf("%c", st[--top]);
}
  
A3()
{
    printf("%c", yytext[0]);
}

Output:

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