C Program for Infix to Prefix Conversion

#define SIZE 50            /* Size of Stack */
#include<string.h>
#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],prfx[50],ch,elem;
int i=0,k=0;
printf("\n\nRead the Infix Expression ? ");
scanf("%s",infx);
push('#');
strrev(infx);
while( (ch=infx[i++]) != '\0')
{
if( ch == ')') push(ch);
else
if(isalnum(ch)) prfx[k++]=ch;
else
if( ch == '(')
{
while( s[top] != ')')
prfx[k++]=pop();
elem=pop(); /* Remove ) */
}
else
{       /* Operator */
while( pr(s[top]) >= pr(ch) )
prfx[k++]=pop();
push(ch);
}
}
while( s[top] != '#')     /* Pop from stack till empty */
prfx[k++]=pop();
prfx[k]='\0';          /* Make prfx as valid string */
strrev(prfx);
strrev(infx);
printf("\n\nGiven Infix Expn: %s  Prefix Expn: %s\n",infx,prfx);
}