title: #define or #if statement error
body: <p>I have read many definitions of # if and # define statement.<br>
  I tried to use the way I read, but only with the error "<strong>Invalid token at start of a preprocessor expression</strong>" on the line that defines it as a comment below:</p>
  
  <pre><code>#import &lt;Foundation/Foundation.h&gt;
  #import &lt;UIKit/UIKit.h&gt;
  
  #define is_ipad         (UI_USER_INTERFACE_IDIOM() == UIUserInterfaceIdiomPad)
  #define is_ipad_hd      ( (is_ipad == 1 &amp;&amp; [UIScreen mainScreen].bounds.size.width &gt; 1024 ) ? YES : NO)
  #define is_iphone_hd    ([UIScreen mainScreen].bounds.size.width &gt; 480 ? YES : NO)
  #define device_width    480.0f
  #define device_height   320.0f
  
  #if (is_ipad_hd == YES) // Error here
    #define device_width       = 2048.0f
    #define device_height      = 1496.0f
  #endif
  </code></pre>
  
  <p>Why it works in simple tutorials and when we try something more complex these things happen!</p>