Though I'm using R for quite a while and (with Google) I'm almost always able to realize all my projects, I still lack a sound skill set about how to manipulate data efficiently. For example, I often have to think about whether I have to use gather(), %>%, mutate(), merge() and so on.
I believe someone with my experience (2-3 years) should be much more fluent with this.
Do you know a good resource, preferably an online course, which aims at this and is worth it, especially in terms of teaching?
Thanks a lot in advance!