title: Insert a value for key into NSMutableArray
body: <p>I think this could be a very easy question for you.</p>
  
  <p>But I have searched a lot and I don't know if I am very fool or I am using the wrong class (<code>NSMutableArray</code>).</p>
  
  <p>I want to add an object for a specific key so that I can retrieve it later.</p>
  
  <p>For example:</p>
  
  <pre><code>NSMutableArray *grisOcurrencias = [[NSMutableArray alloc] init];
  
  [grisOcurrencias setValue:ocurrencia forKey:[NSString stringWithFormat: @"%f", pixelVal]];
  </code></pre>
  
  <p>But the array is not getting inserted in the array, if I do something like:</p>
  
  <pre><code>[grisOcurrencias addObject: ocurrencia]
  </code></pre>
  
  <p>The object is inserted.</p>
  
  <p>Am I doing something wrong?</p>
  
  <p>Other thing... I want to recover the object by its key, what method should I use?</p>
  
  <p>Thanks a lot</p>