title: replacing null value with 0 with special condition every other row
body: <p>This is the following subset of a dataset:    </p>
  
  <pre><code>A  B    C         D         R        sentence              ADR1         ADR2     
  112 135 21  EffexorXR.21    1    lack of good feeling.     good        feeling
  113 135 21  EffexorXR.21    1                               1
  115 136 21  EffexorXR.21    2    Feel disconnected        disconnected   feel    
  116 136 21  EffexorXR.21    2                                             0
  118 142 22  EffexorXR.22    1    Weight gain               gain         
  119 142 22  EffexorXR.22    1                                1             
  </code></pre>
  
  <p>In column ADR1 and ADR2, for each word, there should be 1 or 0 in the row blow it. if the value is missing, I need to replace it with "0". This is the output:</p>
  
  <pre><code>A  B    C         D         R        sentence              ADR1         ADR2     
  112 135 21  EffexorXR.21    1    lack of good feeling.     good        feeling
  113 135 21  EffexorXR.21    1                               1             0
  115 136 21  EffexorXR.21    2    Feel disconnected        disconnected   feel    
  116 136 21  EffexorXR.21    2                                 0            0
  118 142 22  EffexorXR.22    1    Weight gain               gain         
  119 142 22  EffexorXR.22    1                                1    
  </code></pre>
  
  <p>I tried </p>
  
  <pre><code>df[ADR1].fillna(0, inplace=True) and df[ADR2].fillna(0, inplace=True)
  </code></pre>
  
  <p>but this code produce the following df, which is not wanted </p>
  
  <pre><code> A  B    C         D         R        sentence              ADR1         ADR2     
  112 135 21  EffexorXR.21    1    lack of good feeling.     good        feeling
      113 135 21  EffexorXR.21    1                               1        0
      115 136 21  EffexorXR.21    2    Feel disconnected        disconnected   feel                                                                 0
      116 136 21  EffexorXR.21    2                                             0
      118 142 22  EffexorXR.22    1    Weight gain               gain           0
      119 142 22  EffexorXR.22    1                                1            0 
  </code></pre>