title: How to apply a function to a column of a Spark DataFrame?
body: <p>Let's assume that we have a Spark DataFrame</p>
  
  <pre><code>df.getClass
  Class[_ &lt;: org.apache.spark.sql.DataFrame] = class org.apache.spark.sql.DataFrame
  </code></pre>
  
  <p>with the following schema</p>
  
  <pre><code>df.printSchema
  root
  |-- rawFV: string (nullable = true)
  |-- tk: array (nullable = true)
  |    |-- element: string (containsNull = true)
  </code></pre>
  
  <p>Given that each row of the <code>tk</code> column is an array of strings, how to write a Scala function that will return the number of elements in each row?</p>