title: numpy logical operation on 2D array
body: <p>Suppose we have a set of values: <code>n</code>-rows and <code>6</code> columns (n,6).
  How to replace 4th element in a row to be equal to the 3rd element if the 3rd element is more than the 4th?</p>
  
  <p>I tried to do it in such a way:. </p>
  
  <pre><code>griddata[:,3][griddata[:,2] &gt; griddata[:,3]] = griddata[:,2]
  </code></pre>
  
  <blockquote>
    <p>TypeError: 'numpy.float64' object does not support item assignment</p>
  </blockquote>