#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <ctype.h>

typedef struct node
{
    int numdata;
    struct node * nextnode;
} node;

node * pushItem(node * stackTop, int number)
{
    static node * newItem;
	
    newItem = calloc(sizeof(node),1);
    newItem->numdata = number;
    newItem->nextnode = stackTop;
	
    return newItem;
}

node * popItem(node * stackTop)
{
    static node * temp;
   
    temp = stackTop->nextnode;
    free(stackTop);
	
    return temp;
}

void calculate(char * inString)
{
    char * strPointer = inString;
    char numString[20];
    int currentNum;
    int numLength;
    int assigned = 0;
    node * stackTop = NULL;
	
    if (strPointer[0] == '\n') return;
    while (strPointer[0] != '\0')
    {
        if (isdigit(strPointer[0]))
        {
             numLength = strspn(strPointer,"0123456789");
              if (assigned)
              {
                   stackTop = pushItem(stackTop, currentNum);
              } else assigned = 1;
             strncpy(numString, strPointer, numLength);
             numString[numLength]='\0';
             currentNum = atoi(numString);
             strPointer+= numLength;
        } else
        {
             switch (strPointer[0])
	{
    	    case '+': 
     	    {
                           if (stackTop != NULL)
		{
  		    currentNum+=stackTop->numdata;
    		    stackTop = popItem(stackTop);
		} else
		{
		    printf("Error! Too many arithmetic operations.\n");
		    return;
		}
    	    } break;
    	    case '-': 
    	    {
                           if (stackTop != NULL)
		{
  		    currentNum=stackTop->numdata-currentNum;
    		    stackTop = popItem(stackTop);
		} else
		{
		    printf("Error! Too many arithmetic operations.\n");
		    return;
		}
    	    } break;
    	    case '*': 
    	    {
                           if (stackTop != NULL)
		{
  		    currentNum*=stackTop->numdata;
    		    stackTop = popItem(stackTop);
		} else
		{
		    printf("Error! Too many arithmetic operations.\n");
		    return;
		}
    	    } break;
    	    case '/': 
    	    {
                           if (stackTop != NULL)
		{
  		    currentNum=stackTop->numdata/currentNum;
    		    stackTop = popItem(stackTop);
		} else
		{
		    printf("Error! Too many arithmetic operations.\n");
		    return;
		}
    	    } break;
    	    case ' ': break;
    	    case '\n':
    	    {
                           if (stackTop == NULL)
		{
		    printf("The result of the computation is: %d.\n",currentNum);
 		    return;
		} else
		{
		    printf("Error! Too many numbers.\n");
		    while (stackTop != NULL) stackTop = popItem(stackTop);
		    return;
		}
    	    }
    	    default:
    	    {
                           printf("Error! Illegal input.\n");
                           while (stackTop != NULL) stackTop = popItem(stackTop);
        	    	return;	
    	    }
	}
	strPointer++;
        }
    }
	
    return;
}

int main(void)
{
     char inString[50];

     printf("Please input a polish postfix expression.\n\n> ");
     while (strcmp(fgets(inString,50,stdin),"quit\n")!=0)
     {
          calculate(inString);
          printf("> ");
     }
		
     return 0;
}