title: Objective c add string object to an array
body: <p>I've looked everywhere for this, online, on stack overflow and cannot still work out what I'm doing wrong.</p>
  
  <p>I'm trying to add an element to an existing NSMutableArray. But it crashes on line 4:</p>
  
  <pre><code>-[__NSArrayI addObject:]: unrecognized selector sent to instance 0x897b320
  </code></pre>
  
  <p>The code:</p>
  
  <pre><code>NSMutableArray *mystr = [[NSMutableArray alloc] init];
  
  mystr = [NSArray arrayWithObjects:@"hello",@"world",@"etc",nil];
  
  NSString *obj = @"hiagain";
  
  [mystr addObject:obj];
  </code></pre>
  
  <p>What am I doing wrong? This is driving me crazy!!!</p>