title: How to create a factor variable from values from two other columns
body: <p>I want to add a new categorical variable based on values from two other columns. </p>
  
  <p>In the example below, I want to create the new variable "simple" using the semester and gender info. </p>
  
  <pre><code> semester gender score simple 
   01       F       152    F_01
   02       M       190    M_02
  </code></pre>
  
  <p>Can I do this with dplyr?
  cheers</p>