title: Using dplyr summarize with different operations for multiple columns
body: <p>Well, I know that there are already tons of related questions, but none gave an answer to my particular need.</p>
  
  <p>I want to use dplyr "summarize" on a table with 50 columns, and I need to apply different summary functions to these.</p>
  
  <p>"Summarize_all" and "summarize_at" both seem to have the disadvantage that it's not possible to apply different functions to different subgroups of variables.</p>
  
  <p>As an example, let's assume the iris dataset would have 50 columns, so we do not want to address columns by names. I want the sum over the first two columns, the mean over the third and the first value for all remaining columns (after a group_by(Species)). How could I do this?</p>