title: Python numpy array strange return
body: <p>I am relatively new to python. I have a numpy array that has 3 dimensions. I know we can display only few elements using <code>:</code>. </p>
  
  <p>It seems to work just fine while I'm doing it starting from a small value, but at one point, it returns something different than a matrix.</p>
  
  <p>I want to get the mean value for the array. So, for instance, given an array <code>c</code>, I do <code>numpy.mean(c[0:200][0:200][0:200])</code>. This works just fine. But increasing the starting point (i.e. <code>c[200:][200:][200:]</code>) doesn't work and returns <code>nan</code>. So, printing the result explains the nan value. But I don't get why <code>c[200:][200:][200:]</code> returns this kind of answer. </p>
  
  <p>Here's two examples:</p>
  
  <pre><code>In [68]: c.shape
  Out[68]: (448, 433, 446)
  
  In [63]: c[100:][100:][100:]
  Out[63]: 
  array([[[ 0.,  0.,  0., ...,  0.,  0.,  0.],
          [ 0.,  0.,  0., ...,  0.,  0.,  0.],
          [ 0.,  0.,  0., ...,  0.,  0.,  0.],
          ..., 
          [ 0.,  0.,  0., ...,  0.,  0.,  0.],
          [ 0.,  0.,  0., ...,  0.,  0.,  0.],
          [ 0.,  0.,  0., ...,  0.,  0.,  0.]],
  
         [[ 0.,  0.,  0., ...,  0.,  0.,  0.],
          [ 0.,  0.,  0., ...,  0.,  0.,  0.],
          [ 0.,  0.,  0., ...,  0.,  0.,  0.],
          ..., 
          [ 0.,  0.,  0., ...,  0.,  0.,  0.],
          [ 0.,  0.,  0., ...,  0.,  0.,  0.],
          [ 0.,  0.,  0., ...,  0.,  0.,  0.]],
  
         [[ 0.,  0.,  0., ...,  0.,  0.,  0.],
          [ 0.,  0.,  0., ...,  0.,  0.,  0.],
          [ 0.,  0.,  0., ...,  0.,  0.,  0.],
          ..., 
          [ 0.,  0.,  0., ...,  0.,  0.,  0.],
          [ 0.,  0.,  0., ...,  0.,  0.,  0.],
          [ 0.,  0.,  0., ...,  0.,  0.,  0.]],
  
         ..., 
         [[ 0.,  0.,  0., ...,  0.,  0.,  0.],
          [ 0.,  0.,  0., ...,  0.,  0.,  0.],
          [ 0.,  0.,  0., ...,  0.,  0.,  0.],
          ..., 
          [ 0.,  0.,  0., ...,  0.,  0.,  0.],
          [ 0.,  0.,  0., ...,  0.,  0.,  0.],
          [ 0.,  0.,  0., ...,  0.,  0.,  0.]],
  
         [[ 0.,  0.,  0., ...,  0.,  0.,  0.],
          [ 0.,  0.,  0., ...,  0.,  0.,  0.],
          [ 0.,  0.,  0., ...,  0.,  0.,  0.],
          ..., 
          [ 0.,  0.,  0., ...,  0.,  0.,  0.],
          [ 0.,  0.,  0., ...,  0.,  0.,  0.],
          [ 0.,  0.,  0., ...,  0.,  0.,  0.]],
  
         [[ 0.,  0.,  0., ...,  0.,  0.,  0.],
          [ 0.,  0.,  0., ...,  0.,  0.,  0.],
          [ 0.,  0.,  0., ...,  0.,  0.,  0.],
          ..., 
          [ 0.,  0.,  0., ...,  0.,  0.,  0.],
          [ 0.,  0.,  0., ...,  0.,  0.,  0.],
          [ 0.,  0.,  0., ...,  0.,  0.,  0.]]])
  
  In [67]: c[200:][200:][200:]
  Out[67]: array([], shape=(0, 433, 446), dtype=float64)
  </code></pre>