Postfix Calculations In C

#include<stdio.h>
#include<conio.h>
#define STACKSIZE 20
typedef struct{
int top;
int items[STACKSIZE]; /*stack can contain up to 20 integers*/
}STACK;
void push(STACK *, int);
int pop(STACK *);
int calculate(char []);
int main()
{
int result;
char E[50];
printf("Enter your Postfix expression(don't use space character):\n");
scanf("%s",E);
result = calculate(E);
printf("The result of the Postfix expression %s=%d\n",E,result);
return 0;
}
int calculate(char exp[])
{
STACK s;
s.top =-1;/*indicates that the stack is empty at the beginning*/
int i,num1,num2,value;
for(i=0; exp[i]!='\0';i++){
if(exp[i] >='0' && exp[i] <='9') /*checks if exp[i] has a digit*/
push(&s,(int)(exp[i] -'0')); /*converts digit into integer*/
else{
num1=pop(&s);
num2=pop(&s);
switch(exp[i]){
case '+': value=num2+num1;break;
case '-': value=num2-num1;break;
case '*': value=num2*num1;break;
case '/': value=num2/num1;break;
default : printf("Illegal Operator\n");
exit(1);
}
push(&s,value);
}
}
return pop(&s);
}
void push(STACK *Sptr, int ps) /*pushes ps into stack*/
{
if(Sptr->top == STACKSIZE-1){
printf("Stack is full\n");
exit(1); /*exit from the function*/
}
else {
Sptr->top++;
Sptr->items[Sptr->top]= ps;
}
}
int pop(STACK *Sptr)
{
int pp;
if(Sptr->top == -1){
printf("Stack is empty\n");
exit(1); /*exit from the function*/
}
else {
pp = Sptr->items[Sptr->top];
Sptr->top--;
}
return pp;
}