R: Further subset a selection using the pipe %>% and placeholder

Question

I recently discovered the pipe operator %>%, which can make code more readable. Here is my MWE.

library(dplyr)                                          # for the pipe operator
library(lsr)                                            # for the cohensD function

set.seed(4)                                             # make it reproducible
dat <- data.frame(                                      # create data frame
    subj = c(1:6),
    pre  = sample(1:6, replace = TRUE),
    post = sample(1:6, replace = TRUE)
)

dat %>% select(pre, post) %>% sapply(., mean)           # works as expected

However, I struggle using the pipe operator in this particular case

dat %>% select(pre, post) %>% cohensD(.$pre, .$post)    # piping returns an error
cohensD(dat$pre, dat$post)                              # classical way works fine

Why is it not possible to subset columns using the placeholder .in combination with $? Is it worthwhile to write this line using a pipe operator %>%, or does it complicate syntax? The classical way of writing this seems more concise.

Answer 1

Since you're going from a bunch of data into one (row of) value(s), you're summarizing. in a dplyr pipeline you can then use the summarize function, within the summarize function you don't need to subset and can just call pre and post

Like so:

dat %>% select(pre, post) %>% summarize(CD = cohensD(pre, post)) 

(The select statement isn't actually necessary in this case, but I left it in to show how this works in a pipeline)

Answer 2

This would work:

dat %>% select(pre, post) %>% {cohensD(.$pre, .$post)}

Wrapping the last call into curly braces makes it be treated like an expression and not a function call. When you pipe something into an expression, the . gets replaced as expected. I often use this trick to call a function which does not interface well with piping.

What is inside the braces happens to be a function call but could really be any expression of . .

Answer 3

It doesn't work because the . operator has to be used directly as an argument, and not inside a nested function (like $...) in your call.

If you really want to use piping, you can do it with the formula interface, but with a little reshaping before (melt is from reshape2 package):

dat %>% select(pre, post) %>% melt %>% cohensD(value~variable, .)
#### [1] 0.8115027