/*PROGRAM TO COMPUTE THE RESULT FOR A GIVEN POSTFIX EXPRESSION*/
#include<stdio.h>
#include<conio.h>
#include<ctype.h>
#include<string.h>
#include<stdlib.h>

struct st
{
 int data;
 struct st *next;
};
typedef struct st st;
st *base,*newnode,*old;
char operators[]={'+','-','*','/','%','\0'};

void push(int num)
{
 if(base==NULL)
 {
  base=(st *)malloc(sizeof(st));
  base->next=NULL;
  base->data=num;
  old=base;
 }
 else
 {
  newnode=(st *)malloc(sizeof(st));
  newnode->next=NULL;
  newnode->data=num;
  old->next=newnode;
  old=newnode;
 }
}

int pop()
{
 int num;
 st *temp;
 if(base==NULL)
  return(NULL);
 if(old==base)
 {
  num=base->data;
  free(base);
  base=NULL;
 }
 else
 {
  temp=old;
  old=base;
  while(old->next!=temp)
   old=old->next;
  num=temp->data;
  free(temp);
 }
 return(num);
}

int isoperator(char op)
{
 int i;
 for(i=0;operators[i]!='\0';i++)
 {
  if(op==operators[i])
   return(1);
 }
 return(0);
}

void accept(char ch,char *ipstr)
{
 char *p;
 p=ipstr;
 while(1)
 {
  if(isdigit(ch))
  {
   *p=ch;
   ch=getche();
   p++;
   *p='\0';
  }
  if(ch==' ')
   break;
 }
}

void main()
{
 int num,op1,op2;
 char ch,ipstr[100];
 clrscr();
 printf("Program to evaluate a postfix expression\nSYMANTICS:\n\tSeperate data items\(strictly integers) with blank spaces\n\tHit return\(Enter\) to view result of the computation\n");
 printf("\nEnter the postfix expression: ");
 base=NULL;
 while(1)
 {
  ch=getche();
  if(ch==13)
   break;
  if(isdigit(ch))
  {
   accept(ch,ipstr);
   num=atoi(ipstr);
   push(num);
  }
  if(isoperator(ch))
  {
   op1=pop();
   op2=pop();
   if(op1==NULL||op2==NULL)
   {
    textcolor(WHITE+BLINK);
    printf("\n");
    cprintf("\nERROR");
    textcolor(7);
    printf("\nProbable causes:\n\tBad formatting/formulating or typing postfix expression\n\tStack overflow\n\tDivide error\nAborting. Press a key...");
    getch();
    exit(EXIT_FAILURE);
   }
   switch(ch)
   {
    case '+': num=op1+op2;
	      break;
    case '-': num=op1-op2;
	      break;
    case '*': num=op1*op2;
	      break;
    case '/': num=op1/op2;
	      break;
    case '%': num=op1%op2;
	      break;
    default:  printf("\nCondition not incorporated for operator.\nCheck operators array. \nAborting. Press a key...");
	      getch();
	      exit(EXIT_FAILURE);
	      break;
   }
   push(num);
  }
 }
 num=pop();
 printf("\nFinal answer of computation=%d",num);
 getch();
 exit(EXIT_SUCCESS);
}