title: Replace missing values in one table to values in another table by joining column name to row
body: <p>I am trying to replace missing values in one table to values in another table by joining column name to row. Below is an example:</p>
  
  <p>df1</p>
  
  <pre><code>A  B  C  D
  1  3  4  *
  4  *  5  9
  0  *  2  *
  1  2  *  4
  </code></pre>
  
  <p>df2</p>
  
  <pre><code>Variable  Value
  A  2
  B  1
  C  9
  D  0
  </code></pre>
  
  <p>Result data set:</p>
  
  <pre><code>A  B  C  D
  1  3  4  0
  4  1  5  9
  0  1  2  0
  1  2  9  4
  </code></pre>