title: How to sum two columns from two different dataframes?
body: <p>I have two dataframes <code>tmp</code> and <code>df</code>.</p>
  
  <p>I would like to do the following sum</p>
  
  <pre><code>df.iloc[:,2] = tmp.iloc[:,2] + df.iloc[:,2]
  
  tmp.iloc[:,2]
  
  0         NaN
  1         NaN
  2         NaN
  3      1693.0
  4      3511.0
  5         NaN
  6         NaN
  7      2285.0
  8     63269.0
  9         NaN
  10    16637.0
  11     4319.0
  12     5361.0
  13    32338.0
  14    61578.0
  15     9022.0
  16     5648.0
  17      190.0
  18        NaN
  19     3840.0
  20    13217.0
  21     8525.0
  22     3725.0
  23      552.0
  24        NaN
  25        NaN
  26     3914.0
  27     2328.0
  28     1980.0
  29    15436.0
  30     5284.0
  31      258.0
  Name: (HOMEWORK, GAID), dtype: float64
  
  
  df.iloc[:,2]
  
  0          NaN
  1          NaN
  2          NaN
  3       3403.0
  4       6893.0
  5          NaN
  6          NaN
  7       4605.0
  8     126664.0
  9          NaN
  10     33022.0
  11      8700.0
  12     10796.0
  13     64832.0
  14    123594.0
  15     18109.0
  16     11377.0
  17       394.0
  18         NaN
  19      7656.0
  20     26441.0
  21     16935.0
  22      7572.0
  23      1032.0
  24         NaN
  25         NaN
  26      7824.0
  27      4659.0
  28      3884.0
  29     30860.0
  30     10562.0
  31       510.0
  Name: (HOMEWORK, GAID), dtype: float64
  </code></pre>
  
  <p>but if try </p>
  
  <pre><code>df.iloc[:,2] = tmp.iloc[:,2] + df.iloc[:,2]
  </code></pre>
  
  <p>I get the following:</p>
  
  <pre><code>TypeError: Cannot convert bool to numpy.ndarray
  </code></pre>