title: convert np.nan into value in numpy array
body: <pre><code>import numpy as np
  
  a = np.array([1,2,3,4,5,6,7,8,9])
  b = np.where (a==3,np.nan,a)
  print (b) #OK, can convert certain values (e.g. 3) into np.nan
  
  c = np.where (b==np.nan,3,b)
  
  print (c)
  </code></pre>
  
  <p>But does not work! Could not convert <code>np.nan</code> into 3.
  How can  I convert <code>np.nan</code> in the array <code>c</code> into the value 3?
  The result (array <code>c</code>) should be same as the original array <code>a</code>.</p>