Want to share your content on R-bloggers? click here if you have a blog, or here if you don't.

As an experienced dplyr user since almost day one, I thought I knew every aspect of it. But when my new colleague, who is learning dplyr from scratch, asked me to explain the peeling of group layers with summarise, I was like, what? Turns out this actually is a thing. Let me show the example from the dplyr introduction:

Notice that the newly generated data.frame is grouped by year and month only, not by date anymore, which is the effect of the forementioned peeling. The idea is that if you want to aggregate your data further, in most cases you would aggregate it with one layer off since you already exploited the initial grouping.

So if you want to calculate the amount of flights per month and year and then per year, you don’t have to regroup anymore.

And finally:

How does dplyr determine which layer to peel off? Well it seems to depend on the order of the groups:

This produces the same results as in the first daily computation, but obviously now year got peeled off.

Although it might help you write code faster (by saving you a group_by-clause), in my humble opinion it deteriorates readability. Or would you instantly know what to expect from this code, which is basically the summary of the ones above:

To understand the result, you have to count the peeled off layers, and see which ones are affected. You can’t simply rely on the group_by anymore while reading the code. Now imagine you are reviewing code or looking at old code of yours. Everytime you see a wild summarise without a directly preceeding group_by you will have to go through the whole code looking for the previous group_by and then search all summarise statement and count the peeling.

Peeling onions isn’t fun either. Lilly Martin Spencer, Peeling Onions, ca. 1852.

While it might be a convenient way for quick and dirty analysis, I would not use it in production code and my best practice is to always explicitly use a group_by statement right before a summarise. Keeping in mind that the behaviour of group_by already changed from adding groups to overriding groups completely in dplyr 2.0, safest thing you can do is even to add an ungroup statement right before grouping.

Another problem is that some users might not be aware of this behaviour (I wasn’t, therefore the blog post) and might be surprised by the somewhat counterintuitive results.

So be aware with this behaviour and you can keep enjoying dplyr!

Peeling of group layers. was originally published by Kirill Pomogajko at Opiate for the masses on August 07, 2015.