program for Postfix evaluation

import java.io.*;
import java.io.DataInputStream;
class Stack
{
double items[]=new double[10];
int top;
Stack()
{
top=-1;
}
void push (double item)
{
if (top==9)
System.out.println("Stack overflow");
else
items[++top]=item;
}/*end push*/


boolean isempty()
{
if (top<0)
return true;
else
return false;
}/*end isempty*/


double pop()
{
if (isempty())
{
System.out.println("Stack underflow");
return 0;
}
else
return (items[top--]);
}/*end pop*/
}/*end class stack*/
//--------------------------------------------------------------------------------------------------------------------
class Functions
{
int isdigit(char symb)
{
if(symb>='0'&& symb<='9')
return 1; /*return 1 if symbol is digit*/
else
return 0;
}/*end isadigit*/
//--------------------------------------------------------------------------------------------------------------------
void eval(String expr)
{
int position;
char c;
double opnd1,opnd2,value;
Stack s=new Stack();
s.top=-1;
for(position=0;position!=expr.length(); position++)
{
c=expr.charAt(position);
if(isdigit(c)==1)
s.push((double)(c -'0'));
else
{
opnd2=s.pop();
opnd1=s.pop();
value=oper(c,opnd1,opnd2);
s.push(value);
}/*end else*/
}/*end for*/
value=s.pop();
if(!s.isempty())
System.out.println("\nExpression is wrong!!!!!!!!!!!");
else
System.out.println("\nResult of postfix expression is "+value);
}/*end eval*/
//--------------------------------------------------------------------------------------------------------------------
double oper(char symb,double opnd1,double opnd2)
{
switch(symb)
{/*return the result of operation*/
  case '+':return(opnd1+opnd2);
  case '-':return(opnd1-opnd2);                    
  case '*':return(opnd1*opnd2);
  case '/':return(opnd1/opnd2);
  case '$':return(Math.pow(opnd1,opnd2));
  default:System.out.println("\nillegal operator found");
}
return(0);
}/*end oper*/
}/*end class functions*/
//--------------------------------------------------------------------------------------------------------------------
class PostfixEval
{
public static void main(String args[])
{
Functions f=new Functions();
String expr=new String();
DataInputStream in=new DataInputStream(System.in);
System.out.println("\nUse digits(0-9)and operators(+,-,*,/,$)");
System.out.println("\nEnter an expression in postfix form :");
/*read posfix string and call eval*/
try
{
expr=in.readLine();
}
catch(Exception e)
{
System.out.println("I/O Error");
}
System.out.println("\nOriginal postfix expression is "+expr);
f.eval(expr);
}
}/*end class PostfixEval*/


/*
Sample Input and Output :
Enter an expression in postfix form :13*2+1+3/


Original postfix expression is 13*2+1+3/
Result of postfix expression is 2.000000


Enter an expression in postfix form :623+-382/+*2$3+


Original postfix expression is 623+-382/+*2$3+
Result of postfix expression is 52.000000


Enter an expression in postfix form :45&


Original postfix expression is 45&
illegal operator found


Enter an expression in postfix form :24-46*


Original postfix expression is 24-46*
Expression is wrong!!!!!!!!!!!


Enter an expression in postfix form :4*7


Original postfix expression is 4*7
stack underflow - expression is wrong
*/