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!
For me, the "revelation" came from reading r4ds, which addresses all these (and especially will introduce you to pivot_*() instead of gather() and join_*() instead of merge(), which are easier to remember). There is an interactive course on RStudio cloud with the same content (will probably require free registration).
Otherwise, this Coursera course looks like it should be really good, though I haven't tried it myself. That edX course also seems to include the same themes, but with more on web scraping, text mining, ... which may or may not be relevant to you.