title: Extract array from arrays of arrays
body: <p>I have this arrays:</p>
  
  <pre><code>arr = np.array([[[ -1.,  -1.,  -1.,  0.,   0.,   0.],
                  [ 0.1,  0.1,  0.1,  2.,   3.,   4.]], # &lt;-- this one
  
                 [[ -1.,  -1.,  -1.,  0.,   0.,  -1.],
                  [ 0.1,  0.1,  0.1, 16.,  17.,  0.1]], # &lt;-- and this one
  
                 [[ -1.,  -1.,  -1.,  0.,   0.,   0.],
                  [ 0.1,  0.1,  0.1,  4.,   5.,   6.]], # &lt;-- and this one
  
                 [[  0.,   0.,   0., -1.,   0.,   0.],
                  [  1.,   2.,   3., 0.1,   1.,   2.]], # &lt;-- and this one
  
                 [[ -1.,  -1.,   0.,  0.,   0.,   0.],
                  [ 0.1,  0.1,   1.,  9.,  10.,  11.]]]) # &lt;-- and the last one
  </code></pre>
  
  <p>I want to extract the 2nd array in each array, and the result should be as follows:</p>
  
  <pre><code>res = [[ 0.1,  0.1,  0.1,  2.,   3.,   4.],
         [ 0.1,  0.1,  0.1, 16.,  17.,  0.1],
         [ 0.1,  0.1,  0.1,  4.,   5.,   6.],
         [  1.,   2.,   3., 0.1,   1.,   2.],
         [ 0.1,  0.1,   1.,  9.,  10.,  11.]]
  </code></pre>
  
  <p>I want to get the <code>res</code> in one line of code, I tried this but it didn't work</p>
  
  <pre><code>arr[:][1] # select the element 1 in each array
  # I got
  array([[ -1. ,  -1. ,  -1. ,   0. ,   0. ,  -1. ],
         [  0.1,   0.1,   0.1,  16. ,  17. ,   0.1]])
  </code></pre>
  
  <p>Can anyone explain why?</p>
  
  <p>The only solution that I found is to explicitly indicate each index (<code>arr[0][1]...</code>), which I didn't like.</p>