title: Get vector of column names depending on logicals of same dataframe
body: <p>I have a named dataframe containig logicals with missings and I want to get a vector with the column names where values are <code>TRUE</code> (going down the rows and, if multiple <code>TRUE</code>s in one row, going from left to right). Here an example:</p>
  <pre><code>df &lt;- data.frame(a= c(FALSE, NA, TRUE, TRUE),
                   b= c(TRUE, FALSE, FALSE, NA),
                   c= c(TRUE, TRUE, NA, NA))
  df
  #       a     b    c
  # 1 FALSE  TRUE TRUE
  # 2    NA FALSE TRUE
  # 3  TRUE FALSE   NA
  # 4  TRUE    NA   NA
  
  expected &lt;- c(&quot;b&quot;, &quot;c&quot;, &quot;c&quot;, &quot;a&quot;, &quot;a&quot;)
  </code></pre>
  <p>Going from first to last row we see <code>TRUE</code> in the first row. Here are multiple <code>TRUE</code>s, thus we go from left to right and get <code>&quot;b&quot;</code> and <code>&quot;c&quot;</code>. In second tow we get <code>&quot;c&quot;</code>, and so on.</p>
  <p>How to do this (in an elegant way)?</p>