/* Program to evaluate a postfixed expression entered by the user */
/* 71gh73n3d by 5kull_m4c1n705h */

#include <stdio.h>
#include <conio.h>
#include <math.h>
#include <ctype.h>
#define MAX 40

struct Stack
{
  int top;
  int ele[MAX];
};

typedef struct Stack lifo;

void Push(lifo *t, int ch)
{
  if (t -> top == MAX)
    printf("\nERR: Stack overflow.");
  else
    t -> ele[++t -> top] = ch;
}

int Pop(lifo *t)
{
  if (t -> top == -1)
    printf("\nERR: Stack underflow.");
  else
    return (t -> ele[t -> top--]);
}

int Peep(lifo *t)  /* returns top element from stack without removing it */
{
  return (t -> ele[t -> top]);
}

int Rank(char ch)
{
if ((ch == '+') || (ch == '-') || (ch == '*') || (ch == '/') || (ch == '$'))
  return -1;
else
  return 1;
}

float EvalPostfix(lifo *t, char *exp)
{
float op1, op2, value;

while (*exp)
{
	if ((*exp == ' ') || (*exp == '\t'))
		exp++;
	if (isdigit(*exp))
		Push(t, *exp - '0');	/* typecasts symbol to numeral */
	else
	{
		op2 = Pop(t);
		op1 = Pop(t);

		switch (*exp)
		{
			case '+':
				value = op1 + op2;
				break;
			case '-':
				value = op1 - op2;
				break;
			case '*':
				value = op1 * op2;
				break;
			case '/':
				value = op1 / op2;
				break;
			case '$':
				value = pow(op1, op2);
				break;
		}
		Push(t, value);
	}
	exp++;
}
return (Pop(t));
}

main()
{
char postfix[MAX];
int rank;
lifo *stack;
int i;
float result;

clrscr();

stack -> top = -1;  /* stack initialized */
printf("Enter the postfix expression : ");
scanf("%s", postfix);

rank = 0;
/* a prefix or postfix expression is valid iff rank is 1 */
for (i = 0; i < strlen(postfix); i++)
	rank = rank + Rank(*(postfix + i));

if (rank != 1)
{
	printf("\nInvalid postfix expression.");
	printf("\n\nProgram over. Press any key to exit...");
	getch();
	exit(-1);
}

result = EvalPostfix(stack, postfix);
printf("The result of the expression is : %f", result);

printf("\n\nProgram over. Press any key to exit...");
getch();
}