Using dplyr group_by in a function

Question

I am trying to use dplyr's group_by in a local function, example:

testFunction <- function(df, x) {
  df %>%
group_by(x) %>%
summarize(mean.Petal.Width = mean(Petal.Width))
}

testFunction(iris, Species)

and I get an error "... unknown variable to group by: x" I've tried group_by_ and it gives me a summary of the entire dataset. Anybody have a clue how I can fix this?

Thanks in advance!

Answer 1

Here is one way to work with the new enquo from dplyr , where enquo takes the string and converts to quosure which gets evaluated by unquoting ( UQ or !! ) in group_by , mutate , summarise etc.

library(dplyr)
testFunction <- function(df, x) {
 x <- enquo(x)
  df %>%
    group_by(!! x) %>%
     summarize(mean.Petal.Width = mean(Petal.Width))
 }

testFunction(iris, Species)
# A tibble: 3 x 2
#     Species mean.Petal.Width
#      <fctr>            <dbl>
#1     setosa            0.246
#2 versicolor            1.326
#3  virginica            2.026

Answer 2

I got it to work like this:

testFunction <- function(df, x) {
                      df %>%
                         group_by(get(x)) %>%
                         summarize(mean.Petal.Width = mean(Petal.Width))
                 }

testFunction(iris,"Species")

I changed x to get(x) , and Species to "Species" in testFunction(iris,...) .