title: incompatible pointer types assigning to 'nsmutablearray ' from 'nsarray '
body: <pre><code>NSSortDescriptor *sortDescriptor = [[NSSortDescriptor alloc] initWithKey:@"self" ascending:YES];
  
  sortedFloats = [Arr_distance sortedArrayUsingDescriptors:
                      [NSMutableArray arrayWithObject:sortDescriptor]];
  </code></pre>
  
  <p>where <code>sortedFloats</code> is NSMutableArray and <code>Arr_distance</code> is also NSMutableArray   </p>
  
  <p>I got the warning <code>Incompatible pointer types assigning to 'NSMutableArray ' from 'NSArray '</code>  still result is correct, but whats the warning about?</p>