title: python numpy: Change the column type of a numpy matrix
body: <p>I have a numpy matrix X, and I tried to change the datatype of column 1 using the code below:</p>
  
  <pre><code>X[:, 1].astype('str')
  print(type(X[0, 1]))
  </code></pre>
  
  <p>but I got the following result:</p>
  
  <pre><code>&lt;type 'numpy.float64'&gt;
  </code></pre>
  
  <p>Anyone know why the type was not changed to str ? 
  And what is a correct way to change the column type of X?Thanks!</p>