Program to evaluate an epression entered in postfix form

#include <"stdio.h">
#include <"conio.h">
#include <"stdlib.h">
#include <"math.h">
#include <"ctype.h">

#define MAX 50

struct postfix
{
int stack[MAX] ;
int top, nn ;
char *s ;
} ;

void initpostfix ( struct postfix * ) ;
void setexpr ( struct postfix *, char * ) ;
void push ( struct postfix *, int ) ;
int pop ( struct postfix * ) ;
void calculate ( struct postfix * ) ;
void show ( struct postfix ) ;

void main( )
{
struct postfix q ;
char expr[MAX] ;

clrscr( ) ;

initpostfix ( &q ) ;


printf ( "\nEnter postfix expression to be evaluated: " ) ;
gets ( expr ) ;

setexpr ( &q, expr ) ;
calculate ( &q ) ;
show ( q ) ;

getch( ) ;
}

/* initializes data members */
void initpostfix ( struct postfix *p )
{
p -> top = -1 ;
}

/* sets s to point to the given expr. */
void setexpr ( struct postfix *p, char *str )
{
p -> s = str ;
}

/* adds digit to the stack */
void push ( struct postfix *p, int item )
{
if ( p -> top == MAX - 1 )
printf ( "\nStack is full." ) ;
else
{
p -> top++ ;
p -> stack[p -> top] = item ;
}
}

/* pops digit from the stack */
int pop ( struct postfix *p )
{
int data ;

if ( p -> top == -1 )
{
printf ( "\nStack is empty." ) ;
return NULL ;
}

data = p -> stack[p -> top] ;
p -> top-- ;

return data ;
}

/* evaluates the postfix expression */
void calculate( struct postfix *p )
{
int n1, n2, n3 ;
while ( *( p -> s ) )
{
/* skip whitespace, if any */
if ( *( p -> s ) == ' ' || *( p -> s ) == '\t' )
{
p -> s++ ;
continue ;
}

/* if digit is encountered */
if ( isdigit ( *( p -> s ) ) )
{
p -> nn = *( p -> s ) - '0' ;
push ( p, p -> nn ) ;
}
else
{
/* if operator is encountered */
n1 = pop ( p ) ;
n2 = pop ( p ) ;
switch ( *( p -> s ) )
{
case '+' :
n3 = n2 + n1 ;
break ;

case '-' :
n3 = n2 - n1 ;
break ;

case '/' :
n3 = n2 / n1 ;
break ;

case '*' :
n3 = n2 * n1 ;
break ;

case '%' :
n3 = n2 % n1 ;
break ;

case '$' :
n3 = pow ( n2 , n1 ) ;
break ;

default :
printf ( "Unknown operator" ) ;
exit ( 1 ) ;
}

push ( p, n3 ) ;
}
p -> s++ ;
}
}

/* displays the result */
void show ( struct postfix p )
{
p.nn = pop ( &p ) ;
printf ( "Result is: %d", p.nn ) ;
}