title: replace a string to number in a factor column
body: <p>I have a df where the class of a column is <code>factor</code>. However, 80% of its values are numeric/integer/float and the 20% are string <code>"No contact"</code>. How can I replace <code>"No contact"</code> with the value <code>0</code>?.</p>
  
  <p>I understand if I change the class of the column to numeric, it will give me an error (<a href="https://stackoverflow.com/questions/2288485/how-to-convert-a-data-frame-column-to-numeric-type">How to convert a data frame column to numeric type?</a>). So, I need to replace the value <code>"No contact"</code> first in order to change the column class.</p>
  
  <p>Thanks !</p>