Passing argument into function for group_by in dplyr

Question

I am trying to use group_by within a function call in dplyr (R) and I am getting unexpected results. Here is an example of what I am trying to do:

df = data.frame(a = c(0,0,1,1), b = c(0,1,0,1), c = c(1,2,3,4))

result1 = df %>%
  group_by(a,b) %>%
  mutate(d = sum(c))
result1$d

myFunc <- function(df, var) {
  output = df %>%
    group_by(a,!!var) %>%
    mutate(d = sum(c))
  return(output)
}

result2 = myFunc(df,"b")
result2$d

result1$d yields [1,2,3,4] which is what I expected. result2$d yields [3,3,7,7] which I do not want, and I am not sure what is going on.

It works to have b (without quotes) as the function argument, and {{var}} in place of !!var. Unfortunately, in my case, my column names are in string format (but maybe there is a way to transform the string beforehand so that it will work with the {{}} notation?)

Answer 1

If you want to pass a character object that can refer to a certain column of a data frame, you should use !!sym(var) :

myFunc <- function(df, var) {
  output = df %>%
    group_by(a, !!sym(var)) %>%
    mutate(d = sum(c))
  return(output)
}

myFunc(df, "b")

If you want to pass a data-masked argument, you should use {{ var }} or equivalently !!enquo(var) :

myFunc <- function(df, var) {
  output = df %>%
    group_by(a, {{ var }}) %>%
    mutate(d = sum(c))
  return(output)
}

myFunc(df, b)

Note that I pass "b" and b respectively into the function in the two different cases.

Answer 2

If we want to use quoting and unquoting instead of curlycurly {{}} the we should consider this basic procedure: https://tidyeval.tidyverse.org/dplyr.html

Creating a function around dplyr pipelines involves three steps: abstraction, quoting, and unquoting.

1. Abstraction step:

• Here we identify the varying steps. In our case var in group_by :

2. Quoting step:

• Identify all the arguments where the user is allowed to refer to data frame columns directly.
• The function can't evaluate these arguments right away.
• Instead they should be automatically quoted. Apply enquo() to these arguments

3. Unquoting step:

• Identify where these variables are passed to other quoting functions and unquote with !!.
• In this case we pass var to group_by() :

myFunc <- function(df, var) {
  var <- enquo(var)
  output = df %>%
    group_by(a,!!var) %>%
    mutate(d = sum(c))
  return(output)
}

result2 = myFunc(df,b)

output:

[1] 1 2 3 4

Answer 3

Just as I post a question, I come across something that works...

myFunc <- function(df, var) {
  output = df %>%
    group_by_at(.vars = c("a",var)) %>%
    mutate(d = sum(c))
  return(output)
}

result2 = myFunc(df,"b")