Evaluating postfix expression with a pointer?

Question

I have to evaluate a postfix expression in C. This wouldn't be too must of a challenge but instead of using subscript notation I have to interact only using a pointer. I'm extremely new to pointers and need some help converting the code I have, using subscript notation into one that only interacts with a pointer. Any help would be greatly appreciated.

//libraries
#include <stdio.h>    //for input and output          
#include <ctype.h>    //for isdigit function
#include <math.h>     //for pow function
#define size 50       //size of the array

//global variables
int stack[size];
int top=-1;       

//Prototypes
void push(float);
int pop();


//Enter main function
int main(void)
 {                         
char exp[50], c;
int i=0;
float x, y;

//Prompt user to enter equation
 printf("Please enter your expression in postfix notation:\n");
 //store equation in character array exp
 scanf("%s", exp);

//Begin while loop to read through the array
while( (c = exp[i++]) != '\0')
 {
     //If it is a operand, push
     if(isdigit(c))
         push(c-'0');

     //If it is an operator push two operands on top and evaluate
    else
     {        
      x = pop();
      y = pop();

      //Determining operation done
     switch(c)
     {
     case '+':
         push(x + y);
         break;
     case '-':
         push(x - y);
         break;
      case '*':
          push(x * y);
          break;
     case '/':
         push(x / y);
         break;
     case '^':
         push(pow(x,y));
     }
     }
 }

 printf("\n\n Evaluated expression will equal: %d\n",stack[top]);
}



void push(float z)
{                       
 stack[++top] = z;
}



int pop()
{                      
 return(stack[top--]);
}

Answer 1

Usually, it's enough to remember that, with a definition like:

int xyzzy[10];

these two are equivalent:

xyzzy[4]
*(xyzzy + 4)

So that's how you can get access to array elements without an actual subscript.

In other words, rather than:

stack[++top] = z;
return(stack[top--]);

you can instead use:

*(++top + stack) = z;
return *(top-- + stack);