title: python pandas - change values in column to Boolean based on content of another column
body: <p>I have a dafatframe such as this: </p>
  
  <pre><code>    A      B   
  0  NaN   string1
  1  NaN   string2
  2  Nan   string1
  3  Nan   string1
  </code></pre>
  
  <p>How can I change all the values in column A such that they are boolean based on whether the entry at same index in column B contains a certain string "stringX"?   </p>