title: Use a generated string in the select expr of dataframe
body: <p>I am very new to scala programming, so this might be a basic question</p>
  
  <p>I am planning to create a dataframe dynamically.</p>
  
  <p>This is my end goal :</p>
  
  <pre><code>val df2 = df1.select("col1","col2","col3")
  </code></pre>
  
  <p>I have a function which generates these column names as below and saved to a variable like this :</p>
  
  <pre><code>scala&gt; val colVar = generateColSelectionString(4)
  colVar: String = col1,col2,col3
  </code></pre>
  
  <p>Now,</p>
  
  <p>How do I do something like this:</p>
  
  <pre><code>val df2 = df1.select(colVar)
  </code></pre>