title: Fastest way to read every n-th row with numpy's genfromtxt
body: <p>I read my data with numpy's genfromtxt:</p>
  
  <pre><code>import numpy as np
  measurement = np.genfromtxt('measurementProfile2.txt', delimiter=None, dtype=None, skip_header=4, skip_footer=2, usecols=(3,0,2))
  rows, columns = np.shape(measurement)
  x=np.zeros((rows, 1), dtype=measurement.dtype)
  x[:]=394
  measurement = np.hstack((measurement, x))
  np.savetxt('measurementProfileFormatted.txt',measurement)
  </code></pre>
  
  <p>this works fine. But i want only ever <code>5-th</code>, <code>6-th</code> (so <code>n-th</code>)  row in the final Output file.
  According to <a href="http://docs.scipy.org/doc/numpy/reference/generated/numpy.genfromtxt.html" rel="nofollow">numpy.genfromtxt.html</a> there is no Parameter which would do that. I dont want to iterate the array. Is there a recommended way to deal with this problem?</p>