Yacc program to evaluate POSTFIX expression

%{#include<stdio.h>
#include<ctype.h>
%}
%token num
%left '+''-'
%left '*' '/'
%right '^'
%%
s:e'\n'{printf("\n%d",$1);}
e:e e'+'{$$=$1+$2;}
|e e'-'{$$=$1-$2;}
|e e'*'{$$=$1*$2;}
|e e'/'{$$=$1/$2;}
|num
;
%%
yylex()
{
int c;
c=getchar();
if(isdigit(c))
{ yylval=c-'0';
 return num;
}return c;
}
int main()
{
 yyparse();
return 1;
}
int yyerror()
{
return 1;
}
int yywrap()
{
 return 1;
}

Illustraion:

Evaluation of postfix expression can be done very easily.

34+
3--> store value in $1
4--> store value in $2
+--> add $1 and $2 and store result in $$

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Posted in: Language processing programs , YACC