Infix-to-Postfix.c

#include<stdio.h>
#include<stdlib.h>
#include<ctype.h>
#define MAX 100
char infix[200], postfix[200], STACK[MAX];
int top=-1;
int isEmpty()
{
  if(top==-1)
    return 1;
  else
    return 0;
}
int isFull()
{
  if(top==MAX-1)
    return 1;
  else
    return 0;
}
void PUSH(char ch)
{
  if(isFull())
  {
    printf("\nStack Overflow");
    exit(0);
  }
  else
    STACK[++top]=ch;
}
char POP()
{
  if(isEmpty())
  {
    printf("\nStack Underflow");
    exit(0);
  }
  else
  {
    char ch=STACK[top--];
    return (ch);
  }
}
int precedence(char ch)
{
  if(ch=='(' || ch==')')
    return 0;
  if(ch=='^')
    return 3;
  else if (ch=='*' || ch=='/' || ch=='%')
    return 2;
  else if(ch=='+' || ch=='-')
    return 1;
  else
  {
    printf("\nUnknown operator entered\n");
    exit(0);
  }
}
void infix_to_postfix()
{
  int i=0, j=0; char ch, tmp;
  for(;infix[i]!='\0';i++)
  {
    ch=infix[i];
    if(ch==' ')
      continue;
    if(isalnum(ch))
      postfix[j++]=ch;
    else
    {
      if(isEmpty())
      {
        precedence(ch);
        PUSH(ch);
        continue;
      }
      if(!isFull())
      {
        if(ch=='(')
        {
          PUSH(ch);
          continue;
        }
        else if(ch==')')
        {
          tmp=POP();
          while(tmp!='(')
          {
            postfix[j++]=tmp;
            tmp=POP();
          }
          continue;
        }
        int pch=precedence(ch), pst=precedence(STACK[top]);
        while(pch<=pst)
        {
          if(isEmpty())
            break;
          tmp=POP();
          if(tmp=='(')
          {}
          else
            postfix[j++]=tmp;
          if(top==-1)
          {
            pst=-1;
            continue;
          }
          pst=precedence(STACK[top]);
        }
        PUSH(ch);
      }
      else
      {
        printf("\nStack Overflow. Please reduce the equation length");
        exit(0);
      }
    }
  }
  while(!isEmpty())
    postfix[j++]=POP();
}
int main()
{
  printf("\nEnter the Infix Expression: ");
  gets(infix);
  infix_to_postfix();
  printf("\nThe expression after converting to Postfix is: %s",postfix);
}