title: How do I return the first non-NULL value in a series of columns for every row? And the second non-NULL value?
body: <p>I have the following organizational data:</p>
  
  <pre><code>EmployeeID &lt;- c(10:15)
  Job.Title &lt;- c("Program Manager", "Development Manager", "Developer" , "Developer", "Developer", "Summer Intern")
  Level.1 &lt;- c(1,1,1,1,1,1)
  Level.2 &lt;- c(2,2,2,2,2,2)
  Level.3 &lt;- c("",10,10,10,10,10)
  Level.4 &lt;- c("","",11,11,11,11)
  Level.5 &lt;- c("","","","","",12)
  Level.6 &lt;- c("","","","","","")
  Pay.Type &lt;- c("Salary", "Salary", "Salary", "Salary", "Salary", "Hourly")
  acme = data.frame(EmployeeID, Job.Title, Level.1, Level.2, Level.3, Level.4, Level.5, Level.6, Pay.Type)
  
  acme
  
    EmployeeID           Job.Title Level.1 Level.2 Level.3 Level.4 Level.5 Level.6 Pay.Type
  1         10     Program Manager       1       2                                   Salary
  2         11 Development Manager       1       2      10                           Salary
  3         12           Developer       1       2      10      11                   Salary
  4         13           Developer       1       2      10      11                   Salary
  5         14           Developer       1       2      10      11                   Salary
  6         15       Summer Intern       1       2      10      11      12           Hourly
  </code></pre>
  
  <p>For each row, I need to identify the first non-NULL value for Level.1 through Level.6, starting from the right with Level.6, then, Level.5, then Level.4 and so on.  I also need to identify the second non-Null value in this same pattern.  The identified values for each row need to be put in new columns, so the final tables looks like this:</p>
  
  <pre><code>  EmployeeID           Job.Title Level.1 Level.2 Level.3 Level.4 Level.5 Level.6 Pay.Type Supervisor Manager
  1         10     Program Manager       1       2                                   Salary          2       1
  2         11 Development Manager       1       2      10                           Salary         10       2
  3         12           Developer       1       2      10      11                   Salary         11      10
  4         13           Developer       1       2      10      11                   Salary         11      10
  5         14           Developer       1       2      10      11                   Salary         11      10
  6         15       Summer Intern       1       2      10      11      12           Hourly         12      11
  </code></pre>