title: Take average of columns in a numpy array
body: <p>I have taken data from a csv file using numpy. numpy array has dimensions : 100*20. How do i take average of columns (say col 3,5,8) and replace them with a new column containing average of these 3 cols</p>
  
  <p>If </p>
  
  <pre><code>   col3 = 1,2,3,4
     col5 = 2,3,4,8
     col8 = 3,4,5,6
  </code></pre>
  
  <p>then I want to remove these 3 columns and insert a new column in which each entry contains an average of values in these 3 columns </p>
  
  <p>I want to insert a new column: 2,3,4,6, delete previous 3 cols and dimension of final array to be 100*28</p>
  
  <p>Is there any numpy function to do this ?</p>