Finally, a use for rapply

In the apply family of functions, rapply is the unloved ginger stepchild. While lapply, sapply and vapply make regular appearances in my code, and apply and tapply have occasional cameo appearances, in ten years of R coding, I’ve never once found a good use for rapply.

Maybe once a year I take a look at the help page, decide it looks to complicated, and ignore the function again. So today I was very pleased to have found a genuine use for the function. It isn’t life-changing, but it’s quite cute.

Complex classes often have a print method that hides their internals. For example, regression models created by glm are lists with thirty elements, but their print method displays only the call, the coefficients and a few statistics.

# From example(glm)
utils::data(anorexia, package = "MASS")
anorex.1 <- glm(Postwt ~ Prewt + Treat + offset(Prewt),
                family = gaussian, data = anorexia)
str(anorex.1)

To see everything, you need to use unclass.

unclass(anorex.1) #many pages of output

unclass has a limitation: it only removes the top level class, so subelements keep their classes. For example, compare:

class(unclass(anorex.1)$qr) # qr
class(unclass(anorex.1$qr)) # list

Using rapply, we can remove classes throughout the whole of the object, turning it into a list of simple objects.

rapply(anorex.1, unclass, how = "replace")

As well as allowing us to thoroughly inspect the contents of the object, it also allows the object to be used with other code that doesn’t understand particular classes.

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