title: How to change multiple columns' types in pyspark?
body: <p>I am just studying pyspark. I want to change the column types like this:</p>
  
  <pre><code>df1=df.select(df.Date.cast('double'),df.Time.cast('double'),
            df.NetValue.cast('double'),df.Units.cast('double'))
  </code></pre>
  
  <p>You can see that df is a data frame and I select 4 columns and change all of them to double. Because of using select, all other columns are ignored. </p>
  
  <p>But, if df has hundreds of columns and I just need to change those 4 columns. I need to keep all the columns. So, how to do it?</p>