title: How to replace the value of a column base on the condition of another 2 columns?
body: <p>if the value in the column 'active' is equal to 0, or if the value in 'category' is equal to NaN, then the value in that specific row in 'position' should = 0. </p>
  
  <p>Before;</p>
  
  <pre><code>active position category
  1       1      NaN
  1       2      NaN
  1       1        a
  0       1        a
  0       1        a
  1       2        a
  1       1        b
  0       1        b
  1       2        b
  </code></pre>
  
  <p>after:</p>
  
  <pre><code>active position category
  1       0      NaN
  1       0      NaN
  1       1        a
  0       0        a
  0       0        a
  1       2        a
  1       1        b
  0       0        b
  1       2        b
  </code></pre>