INFIX to POSTFIX convertion

INFIX to POST FIX

#include<stdio.h> 

#include<conio.h>          

#include <ctype.h>

char s[20];

int top = -1;

void push(char elem)

{

 top++;

 s[top] = elem;

}

char pop()

{ 

return (s[top--]);

}

int pr(char elem)

{

 switch (elem)

 {

 case '(':

  return 1;

 case '+':

 case '-':

  return 2;

 case '*':

 case '/':

  return 3;

 }

 return 0;

}

void main()

 {

char infx[50], pofx[50], ch, elem;

 int i = 0, k = 0;

 printf("\n\nRead the Infix Expression ? ");

 scanf("%s", infx);

 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 (top !=-1 )

  pofx[k++] = pop();

 pofx[k] = '\0';

printf("\n\nGiven Infix Expn: %s  Postfix Expn: %s\n", infx, pofx);

}