title: ValueError: Unknown format code 'f' for object of type 'str'
body: <p>I get the following error when trying to write 2 NumPy arrays on a DAT file.</p>
  
  <pre><code>fo.write('{0:f} {1:f} \n'.format(np.array(p_initial),np.array(pv_za)))
  ValueError: Unknown format code 'f' for object of type 'str'
  </code></pre>
  
  <p>The full code is available here : <a href="https://stackoverflow.com/questions/15804936/typeerror-cant-multiply-sequence-by-non-int-of-type-float-prblem-with-nump/15804979?noredirect=1#15804979">TypeError: can&#39;t multiply sequence by non-int of type &#39;float&#39; : prblem with NumPy arrays</a></p>
  
  <p>I am quite new to Python. How does one write NumPy floating point arrays to DAT files so that they can be plotted using matplotlib or gnuplot ?</p>