Infix to postfix.c

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


char s[50];
int top = -1; 

push(char elem) {
 s[++top] = elem;
}

char pop() {
 return (s[top--]);
}

int pr(char elem) { 
 switch (elem) {
 case '#':
  return 0;
 case '(':
   return 1;
 case '+':
 case '-':
  return 2;
 case '*':
 case '/':
  return 3;
 }
}
pushit(int ele){                       
 s[++top]=ele;
}

int popit(){                      
 return(s[top--]);
}

 main() { 
  char infx[50], pofx[50], ch, elem;
 int i = 0, k = 0;     
 printf("Enter Infix expression :");
 scanf("%s", infx);
 push('#');
 while ((ch = infx[i++]) != '\0') {
  if (ch == '(')
   push(ch);
  else if (isalnum(ch))
   pofx[k++] = ch;
  else if (ch == ')') {
   while (s[top] != '(')
    pofx[k++] = pop();
   elem = pop(); 
  } else { 
   while (pr(s[top]) >= pr(ch))
    pofx[k++] = pop();
   push(ch);
  }
 }
  while (s[top] != '#') 
  pofx[k++] = pop();
 pofx[k] = '\0'; 
 printf("PostFix expression : %s",pofx);

 getch();
}