title: Using "replaceObjectAtIndex" for array within array
body: <p>I have an array within an (mutable) array.  I am trying to replace certain objects with "replaceObjectAtIndex."</p>
  
  <p>I have tried:</p>
  
  <pre><code>[[mutableArrayName objectAtIndex:0]replaceObjectAtIndex:0 withObject:@"TEST"]; 
  </code></pre>
  
  <p>but I get the following error:</p>
  
  <p>-[__NSArrayI replaceObjectAtIndex:withObject:]: unrecognized selector sent to instance 0x4e24d70
  2011-03-17 17:02:07.008 Contact details[5145:207] <em>*</em> Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: '-[__NSArrayI replaceObjectAtIndex:withObject:]: unrecognized selector sent to instance 0x4e24d70'</p>
  
  <p>I tried this as well:</p>
  
  <pre><code>    [mutableArrayName replaceObjectAtIndex:[[mutableArrayName objectAtIndex:0]objectAtIndex:0] withObject:@"TEST"];
  </code></pre>
  
  <p>but I get the following error:</p>
  
  <p><strong>* Terminating app due to uncaught exception 'NSRangeException', reason: '*</strong> -[NSMutableArray replaceObjectAtIndex:withObject:]: index 16660 beyond bounds [0 .. 0]'</p>