dplyr: Use a custom function in summarize() after group_by()

Question

How can we use a custom function after group_by()? I checked similar posts (1, 2, and 3), but my current code returns the same values for all groups.

Answer 1

It's easier to understand if you start by writing it without an extra function. In that case it would be:

df %>%
  group_by(village) %>%
  summarize(Y_village = mean(Y[Z == z]))

## A tibble: 2 x 2
#  village Y_village
#  <fct>       <dbl>
#1 a            450.
#2 b            700.

Hence, your function should be something like

Y_hat_village <- function(Ycol, Zcol, z){
  mean(Ycol[Zcol == z])
}

And then using it:

df %>%
  group_by(village) %>%
  summarize(Y_village = Y_hat_village(Y, Z, z))

Note that the function I wrote only deals with atomic vectors which you can supply directly from within summarise. You don't need to supply the whole data.frame into it.