Postfix expression C++

I made a program and I have a problem here.

Problems ::
1. This code accepts 1 digit only if you enter 5 6 + this program shows 11 but it you enter 11 22 + it shows 4. please help me out to change it upto 3 digits.

2. If a user enters 2 operators like 30 40 + * it should display an error "Two operators are not allowed"

3. if user enters 6 2 3 + - 3 8 2 / + * 2 ^ 3 + result 52 should appear.

here is the code.....

Code:

 


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


#include <iostream>
#include <stdlib.h>
#include <string>
#include <sstream>
#include <fstream>
#include <cstdio>
#include <iomanip>
#include <cstdlib>


#define MAX 50
#define EMPTY -1




struct stack
{
    int data[MAX];
    int top;
};
 
void emptystack(struct stack* s)
{
    s->top = EMPTY;
}
void push(struct stack* s,int item)
{
    if(s->top == (MAX-1))
    {
        printf("\nSTACK FULL");
    }
    else
    {
        ++s->top;
        s->data[s->top]=item;
    }
}
 
int pop(struct stack* s)
{
    int ret=EMPTY;
    if(s->top == EMPTY)
        printf("\nSTACK EMPTY");
    else
    {
        ret= s->data[s->top];
        --s->top;
    }
    return ret;
}
void display(struct stack s)
{


    while(s.top != EMPTY)


    {


        printf("\n%d",s.data[s.top]);


        s.top--;


    }


}


 


int evaluate(char *postfix)


{


    char *p;


    struct stack stk;


    int op1,op2,result;


 


    emptystack(&stk);


    p = &postfix[0];


 


    while(*p != '\0')


    {


       /* removes tabs and spaces */


        while(*p == ' ' || *p == '\t')


        {


            p++;


        }


      /* if is digit */


        if(isdigit(*p))


        {


            push(&stk,*p - 48);


        }


        else


        {


           /* it is an operator */


            op1 = pop(&stk);


            op2 = pop(&stk);


 


            switch(*p)


            {


                case '+':
                    result = op2 + op1;
                    break;
                case '-':


                    result = op2 - op1;
                    break;


                case '/':


                    result = op2 / op1;
                    break;
                case '*':


                    result = op2 * op1;
                    break;


                case '%':


                    result = op2 % op1;
                    break;
                    
                case '^':


                    result = op2 ^ op1;
                    break;


                default:


                    printf("\nInvalid Operator");


                    return 0;


            }


            push(&stk,result);


        }


        p++;


    }


    result = pop(&stk);


    return result;


}


 


int main()


{


    char exp[MAX];


    printf("Enter Postfix Exp<b></b>ression : ");


    gets(exp);
    


    printf("%s EQUALS %d\n",exp,evaluate(&exp[0]));
    
    gets(exp); 
    return 0;


}

Can you give some general idea what this program is for?
Some examples? (there are some but the line is messed up so i didn't understand what you meant).

1. Headers: you don't need <stdlib.h> or <stdio.h> since you've already got the more suitable equivalents <cstdlib> and <cstdio>. As for <ctype.h>, the equivalent is <cctype>.



2. Avoid gets as it is an unsafe function. Prefer instead fgets as it allows you to specify the maximum size of the string you wish to accept from the user.



3. A bit of a nit but... you've posted this in the C++ forum but your I/O is decidedly straight vanilla C. There is nothing in the program, other than the headers you are including, that says "this is a C++ program".



4.

1. This code accepts 1 digit only if you enter 5 6 + this program shows 11 but it you enter 11 22 + it shows 4. please help me out to change it upto 3 digits.

Well, this can be done several ways. I'd probably suggest parsing the input using strtok. Each token you get you can attempt to convert to a number (strtol for example) to see if it is a value or not. If it is a value that you can convert, then push it onto your stack. If not, then you check if is an operator and proceed accordingly.



5.

2. If a user enters 2 operators like 30 40 + * it should display an error "Two operators are not allowed"

I'd suggest making sure there are at least two values on your stack before you attempt to resolve an operator. That would detect these conditions and you could then display an error message to the user. In this case the 30 40 + would evaluate to 70 which would be on the stack... once you parse the asterisk from the user you'd look at the stack and see that there was only one value present which would be a big clue that something was wrong.


6.

3. if user enters 6 2 3 + - 3 8 2 / + * 2 ^ 3 + result 52 should appear.

I haven't looked too deeply at your code however if you at least take care of the other issues from your post (#1 and #2) then you'd be a lot closer to having this issue resolved. Fixing those problems might even take care of this one for you all on its own.