Convert infix expression to postfix

#include<stdio.h>
#include<conio.h>
#include<ctype.h>
struct stack
{
	int top;
   char operator[40];
};
int main (void)
{
	struct stack s;
   char input[40],optop,output[40];
   int i,j=0,result,op1,op2,value;
   s.top =-1;
   clrscr();
   printf("Enter in INFIX form: ");
   i=0;
   while((input[i]=getchar())!='\n')
   {
		if(isalpha(input[i]))
      {
			output[j]=input[i];
	 		j++;
      }
      else
      {
			while(!empty(&s) && prcd(s.operator[s.top],input[i]))
	 		{
				optop=s.operator[s.top];
	    		pop(&s);
	    		output[j]=optop;
	    		j++;
	 		}
      	if(empty(&s)||input[i]!= ')')
      	push(&s,input[i]);
			else
			pop(&s);
      }
      i++;
   }
   while(!empty(&s))
   {
      optop=s.operator[s.top];
      pop(&s);
      output[j]=optop;
      j++;
   }
   printf("Postfix form is:\n");
   output[j]='\0';
   j=0;
   while(output[j]!='\0')
   {
		printf("%c",output[j]);
      j++;
   }
   getch();
}

push(ps,n)
struct stack *ps;
int n;
{
	ps->top++;
   ps->operator[ps->top]=n;
}

pop(ps)
struct stack *ps;
{
	ps->top--;
}

empty(ps)
struct stack *ps;
{
	if(ps->top==-1)
	return 1;
   else
	return 0;
}

prcd(stktop,op)
char stktop,op;
{
	if((stktop=='+'||stktop=='-')&&(stktop=='*'||stktop=='/'))
   return 0;
   if(stktop=='(')
   	return 0;
   if(stktop!=')'&& op=='(')
   	return 0;
   if(stktop!='('&& op==')')
   	return 1;
}