title: pyspark : Convert DataFrame to RDD[string]
body: <p>I'd like to convert <code>pyspark.sql.dataframe.DataFrame</code> to <code>pyspark.rdd.RDD[String]</code></p>
  
  <p>I converted a DataFrame <code>df</code> to RDD <code>data</code>:</p>
  
  <pre><code>data = df.rdd
  type (data)
  ## pyspark.rdd.RDD 
  </code></pre>
  
  <p>the new RDD <code>data</code> contains <code>Row</code> </p>
  
  <pre><code>first = data.first()
  type(first)
  ## pyspark.sql.types.Row
  
  data.first()
  Row(_c0=u'aaa', _c1=u'bbb', _c2=u'ccc', _c3=u'ddd')
  </code></pre>
  
  <p>I'd like to convert <code>Row</code> to list of <code>String</code> , like example below:</p>
  
  <pre><code>u'aaa',u'bbb',u'ccc',u'ddd'
  </code></pre>
  
  <p>Thanks</p>