title: Replace value with the name of its respective column
body: <p>I have a data frame:</p>
  
  <pre><code>Code    401k    CVS
  101A            true
  231N    true
  FD54    true
  99JB    
  85F4    true
  </code></pre>
  
  <p>I'm trying to replace the "true" character values with the respective column name (e.g. "401k"). This is my desired output:</p>
  
  <pre><code>Code    401k    CVS
  101A            CVS
  231N    401k
  FD54    401k
  99JB    
  85F4    401k
  </code></pre>