title: Concat multiple columns of a dataframe using pyspark
body: <p>Suppose I have a list of columns, for example:</p>
  
  <pre class="lang-python prettyprint-override"><code>col_list = ['col1','col2']
  df = spark.read.json(path_to_file)
  print(df.columns)
  # ['col1','col2','col3']
  </code></pre>
  
  <p>I need to create a new column by concatenating <code>col1</code> and <code>col2</code>. I don't want to hard code the column names while concatenating but need to pick it from the list.</p>
  
  <p>How can I do this?</p>