/*
 * evaluate single digit operant postfix expression
 * modified from textbook example
 * HBF
*/
#include <stdio.h>
#include <conio.h>

#define MAX 100

int evaluatePostfix(char exp[]);
void push(int stk[], int *top, int val);
void pop(int stk[], int *top);
int  peek(int stk[], int top);

int main() {
  char postfixstr[100] = "934*8+4/-";
  printf("%s = %d", postfixstr, evaluatePostfix(postfixstr));
  return 0;
}

int evaluatePostfix(char exp[]) {
  int stk[MAX];
  int top = -1;

  char *p = exp;
  int opr1, opr2, value;
  while ( *p ) {
    
    if (*p >= '0' && *p <= '9') {
      push(stk, &top, (int)(*p - '0'));
    }  
    else {
      opr2 = peek(stk, top);
      pop(stk, &top);
      
      opr1 = peek(stk, top);
      pop(stk, &top);
      
      switch (*p) {
      case '+':
        value = opr1 + opr2;
        break;
      case '-':
        value = opr1 - opr2;
        break;
      case '/':
        value = opr1 / opr2;
        break;
      case '*':
        value = opr1 * opr2;
        break;
      case '%':
        value = opr1 % opr2;
        break;
      }
      push(stk,  &top, value);
    }
    p++;
  }
  
  return (peek(stk, top));
}

void push(int stk[], int *top, int val) {
  if (*top == MAX-1) {
    printf("\n STACK OVERFLOW");
  } else {
    *top = *top + 1;
    stk[*top] = val;
  }
}

void pop(int stk[], int *top) {
  if (*top == -1) {
    printf("\n STACK UNDERFLOW");
  } else {
    *top = *top - 1;
  }
}

int peek(int stk[], int top) {
  if (top == -1) {
    printf("\n STACK IS EMPTY");
    return -1;
  } else
    return (stk[top]);
}

/*
934*8+4/- = 4
*/