Use of `case_when()`

[grahamison]

I'd be surprised if someone was able to complete exercise 2.2 in the time without prior experience of case_when(), and the complexity might be off-putting to some. Could we convert to an if_else() example?

[pbbgss]

I don't think case_when() is a relevant thing to include in "writing functions" training. Flow control and vectorised/non vectorised if/else are very different things

[grahamison]

We could just take out exercise 2.2.