title: Scala - Fill "null" column with another column
body: <p>I want to replicate the <a href="https://stackoverflow.com/questions/30357276/pandas-fillna-with-another-column">problem mentioned here</a> in Scala DataFrames. I have tried using the following approaches, to no success so far.</p>
  
  <p>Input</p>
  
  <pre><code>Col1  Col2
  A       M
  B       K
  null    S
  </code></pre>
  
  <p>Expected Output</p>
  
  <pre><code>Col1  Col2
  A       M
  B       K
  S &lt;---- S
  </code></pre>
  
  <p>Approach 1</p>
  
  <pre><code>val output = df.na.fill("A", Seq("col1"))
  </code></pre>
  
  <p>The <code>fill</code> method does not take a column as the (first) input.</p>
  
  <p>Approach 2</p>
  
  <pre><code>val output = df.where(df.col("col1").isNull)
  </code></pre>
  
  <p>I cannot find a suitable method to call after I have identified the null values.</p>
  
  <p>Approach 3</p>
  
  <pre><code>val output = df.dtypes.map(column =&gt;
    column._2 match {
      case "null" =&gt; (column._2 -&gt; 0)
    }).toMap
  </code></pre>
  
  <p>I get a <code>StringType</code> error.</p>