C Program for Infix to Postfix Conversion

#define SIZE 50            /* Size of Stack */
#include <ctype.h>
char s[SIZE];
int top=-1;       /* Global declarations */
 
push(char elem)
{                       /* Function for PUSH operation */
s[++top]=elem;
}
 
char pop()
{                      /* Function for POP operation */
return(s[top--]);
}
 
int pr(char elem)
{                 /* Function for precedence */
switch(elem)
{
case '#': return 0;
case '(': return 1;
case '+':
case '-': return 2;
case '*':
case '/': return 3;
}
}
 
main()
{                         /* Main Program */
char infx[50],pofx[50],ch,elem;
int i=0,k=0;
printf("\n\nRead the 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(); /* Remove ( */
}
else
{       /* Operator */
while( pr(s[top]) >= pr(ch) )
pofx[k++]=pop();
push(ch);
}
}
while( s[top] != '#')     /* Pop from stack till empty */
pofx[k++]=pop();
pofx[k]='\0';          /* Make pofx as valid string */
printf("\n\nGiven Infix Expn: %s  Postfix Expn: %s\n",infx,pofx);
}