title: PySpark replace null in column with value in other column
body: <p>I want to replace null values in one column with the values in an adjacent column ,for example if i have</p>
  
  <pre><code>A|B
  0,1
  2,null
  3,null
  4,2
  </code></pre>
  
  <p>I want it to be:</p>
  
  <pre><code>A|B
  0,1
  2,2
  3,3
  4,2
  </code></pre>
  
  <p>Tried with</p>
  
  <pre><code>df.na.fill(df.A,"B")
  </code></pre>
  
  <p>But didnt work, it says  value should be a float, int, long, string, or dict</p>
  
  <p>Any ideas?</p>