title: How to loop rows and columns in pandas while replacing values with a constant increment
body: <p>I am trying to replace values in a dataframe by <code>0</code>. the first column I need to replace the 1st <code>3</code> values, the next column the 1st <code>6</code> values so on so forth increasing by <code>3</code> every time</p>
  
  <pre><code>a=np.array([133,124,156,189,132,176,189,192,100,120,130,140,150,50,70,133,124,156,189,132])
  b = pd.DataFrame(a.reshape(10,2), columns= ['s','t'])
  for columns in b:
      yy = 3
      for i in xrange(yy):
          b[columns][i] = 0 
      yy += 3
  print b
  </code></pre>
  
  <p>the outcome is the following </p>
  
  <pre><code>      s    t
  0     0    0
  1     0    0
  2     0    0
  3   189  189
  4   132  132
  5   176  176
  6   189  189
  7   192  192
  8   100  100
  9   120  120
  </code></pre>
  
  <p>I am clearly missing something really simple, to make the loop replace <code>6</code> values instead of only <code>3</code> in column <code>t</code>, any ideas?</p>