Postfix implementation in C using Stack

postfix in C

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

#define SIZE 50
char s[SIZE];
int top =-1;

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

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


int pr(char elem)
{
	switch(elem)
	{
		case '#' = retun 0;
		case '(' = return 1;
		case '+' :
		case '-' :
					return 2;
		case '*' :
		case '/' : return 3;
	}
}

void main()
{
	char infix[50],postfix[50]; 
	char ch,elem;
	int i,k=0;
	printf("Read infix expression\n");
	scanf("%s",&infix);
	push('#');
	while(ch = infix[i++] != '/0')
	{
		if (ch == '(') 
			push(ch);
		else
		if (isalnum(ch)) 
			postfix[k++] = ch;
		else
		if (ch == ')')
		{
			while(s[top] != '(')
			postfix[k++] = pop();
			elem = pop();
		}
		else
		{
			while(pr(s[top]) >= pr(ch))
				postfix[k++] = pop();
				push(ch);
		}
	}	
	while(s[top] != '#')
		postfix[k++] = pop();
		postfix[k] = '/0';
		printf("\n \n given infix expression:%s postfix expression:%s\n", infix,postfix);
	
}