Infix to postfix code in data structures and algorithms?

#define SIZE 50 /* Size of Stack */
#include <ctype.h>
#include <stdio.h>
char s[SIZE];
int top = -1; /* Global declarations */
int 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 ‘/’:
case ‘%’:
return 3;
case ‘^’:
return 4;
}
}
void main() /* Main Program */
{
char infx[50], pofx[50], ch, elem;
int i = 0, k = 0;
printf(“\n\nEnter 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);
}