title: Spark, add new Column with the same value in Scala
body: <p>I have some problem with the <code>withColumn</code> function in Spark-Scala environment.
  I would like to add a new Column in my DataFrame like that:</p>
  
  <pre><code>+---+----+---+
  |  A|   B|  C|
  +---+----+---+
  |  4|blah|  2|
  |  2|    |  3|
  | 56| foo|  3|
  |100|null|  5|
  +---+----+---+
  </code></pre>
  
  <p>became:</p>
  
  <pre><code>+---+----+---+-----+
  |  A|   B|  C|  D  |
  +---+----+---+-----+
  |  4|blah|  2|  750|
  |  2|    |  3|  750|
  | 56| foo|  3|  750|
  |100|null|  5|  750|
  +---+----+---+-----+
  </code></pre>
  
  <p>the column D in one value repeated N-time for each row in my DataFrame.</p>
  
  <p>The code are this:</p>
  
  <pre><code>var totVehicles : Double = df_totVehicles(0).getDouble(0); //return 750
  </code></pre>
  
  <p>The variable totVehicles returns the correct value, it's works!</p>
  
  <p>The second DataFrame has to calculate 2 fields (id_zipcode, n_vehicles), and add the third column (with the same value -750):</p>
  
  <pre><code>var df_nVehicles =
  df_carPark.filter(
        substring($"id_time",1,4) &lt; 2013
      ).groupBy(
        $"id_zipcode"
      ).agg(
        sum($"n_vehicles") as 'n_vehicles
      ).select(
        $"id_zipcode" as 'id_zipcode,
        'n_vehicles
      ).orderBy(
        'id_zipcode,
        'n_vehicles
      );
  </code></pre>
  
  <p>Finally, I add the new column with <code>withColumn</code> function:</p>
  
  <pre><code>var df_nVehicles2 = df_nVehicles.withColumn(totVehicles, df_nVehicles("n_vehicles") + df_nVehicles("id_zipcode"))
  </code></pre>
  
  <p>But Spark returns me this error:</p>
  
  <pre><code> error: value withColumn is not a member of Unit
           var df_nVehicles2 = df_nVehicles.withColumn(totVehicles, df_nVehicles("n_vehicles") + df_nVehicles("id_zipcode"))
  </code></pre>
  
  <p>Can you help me?
  Thank you very much!</p>