title: Changing categorical variable to numeric
body: <p>my data set is</p>
  
  <pre><code>Type
  IL 
  DT
  DT
  FC
  MB
  IL
  </code></pre>
  
  <p>I want to change <code>IL=1</code>, <code>DT=2</code>, <code>FC=3</code>, <code>MB=4</code>
  So the result data will be</p>
  
  <pre><code>Type
  1
  2
  2
  3
  4
  1
  </code></pre>