Can't use !!arg in dplyr for mutate call

Question

I can use !! to filter by a user-given variable but not to modify that same variable. The following function throws an error when created, but it works just fine if I delete the mutate call.

avg_dims <- function(x, y) {
  y <- enquo(y)
  x %>%
    filter(!!y != "TOTAL") %>%
    mutate(!!y = "MEAN") %>%
    group_by(var1, var2)
}

Can you try with `!! y :=` in `mutate` i.e. `mutate(!!y := "MEAN")` — akrun, Jan 26 '19 at 17:16
Now I can create the function but this happens when trying to execute `Error: The LHS of := must be a string or a symbol` — overdisperse, Jan 26 '19 at 17:18
Oh, okay, it should be `mutate(!! rlang::quo_name(y) := "MEAN")` — akrun, Jan 26 '19 at 17:19
That worked. Thanks! If you post as answer I'll award credit. — overdisperse, Jan 26 '19 at 17:20

score 8 · Accepted Answer · answered Jan 26 '19 at 17:23

The naming of the column on the lhs of assignment goes along with the assignment operator (:=) instead of the = operator. Also, the names should be either string or symbol. So, we can convert the quosure ('y' from enquo) to string (quo_name) and then do the evaluation (!!)

avg_dims <- function(x, y) {
 y <- enquo(y)
 y1 <- rlang::quo_name(y)
 x %>%
    filter(!!y != "TOTAL") %>%
    mutate(!!y1 := "MEAN") %>%
    group_by(var1, var2)
  }

avg_dims(df1, varN)

data

set.seed(24)
df1 <- data.frame(var1 = rep(LETTERS[1:3], each = 4), 
      var2 = rep(letters[1:2], each = 6), 
      varN = sample(c("TOTAL", "hello", 'bc'), 12, replace = TRUE), 
     stringsAsFactors = FALSE)

This helped me, but honestly, I do not understand it. Any more clarity anyone can add? Thanks in advance. — Sam, Jan 11 '21 at 03:58

Can't use !!arg in dplyr for mutate call

1 Answers1

data