title: How to replace values in a data frame with another value
body: <p>I have huge data set. The columns contain values like <code>A,B,C,D,E,F,G,H</code> and I need to replace them with <code>1,2,3,4...</code> </p>
  
  <pre><code>[1] "C" "C" "C" "C" "C" "A" "H" "G" "G" "G" "G" "G" "G" "G" "C" "C" "C" "C" "C"
  [20] "C" "B" "B" "B" "H" "H" "H" "H" "H" "H" "G" "C" "A" "A" "A" "A" "A" "A" "A"
  [30]----
  </code></pre>
  
  <p>Another similar problem is values in one column are more than <code>1000</code> and I need to replace them by unique numbers.</p>