title: Explain this 4D numpy array indexing intuitively
body: <pre><code>x = np.random.randn(4, 3, 3, 2)
  print(x[1,1])
  
  output:
  [[ 1.68158825 -0.03701415]
  [ 1.0907524  -1.94530359]
  [ 0.25659178  0.00475093]]
  </code></pre>
  <p>I am python newbie. I can't really understand 4-D array index like above. What does x[1,1] mean?</p>
  <p>For example, for vector</p>
  <pre><code>a = [[2][3][8][9]], a[0] = 2, a[3] = 9. 
  </code></pre>
  <p>I get this but I don't know what x[1,1] refers to.</p>
  <p>Please explain in detail. Thank you.</p>