title: Randomly select index positions in numpy
body: <pre><code>import numpy as np
  data  = np.array([[0,1,2,3,4,7,6,7,8,9,10], 
      [3,3,3,4,7,7,7,8,11,12,11],  
      [3,3,3,5,7,7,7,9,11,11,11],
      [3,4,3,6,7,7,7,10,11,17,11],
      [4,5,6,7,7,9,10,11,11,11,11]])
  required = np.where(data==11)
  print required
  
  (array([1, 1, 2, 2, 2, 3, 3, 4, 4, 4, 4], dtype=int64), array([ 8, 10,  8,  9, 10,  8, 10,  7,  8,  9, 10], dtype=int64))
  </code></pre>
  
  <p>From the <strong>required</strong> how can I randomly select (with out re-occurring) only 3 index positions?</p>
  
  <p><strong>The answers should be index positions as a subset of required.</strong></p>
  
  <p>my trial:</p>
  
  <pre><code>result = np.random.choice(required, 3, replace=False)
  print result
  
  ValueError: a must be 1-dimensional
  </code></pre>
  
  <p>any idea to solve this problem???</p>