How to do a fct_drop within a function (using tidy eval)? - tidyeval

Question

Using the diamonds dataset...
Trying to create a function that will allow me to plot either cut or color on the x-axis...
...but first I want to filter the selected column to show only a certain number of levels.
I've got the filter working but the levels are still present... and they will show up in the chart. I need to do a fct_drop() on the selected column
Please see the code below for a reproducible example:
library(tidyverse)
diamonds <- diamonds %>%
mutate(cut = factor(cut),
color = factor(color))
reduce_for_plot <- function(data, column, how_many_levels) {
column2 <- enquo(column)
of_interest <- unique(data[[deparse(substitute(column))]])[1:how_many_levels]
data %>%
filter(!! column2 %in% of_interest)
# here is where I then do some kind of mutate... to fct_drop the selected column
# this line seems to work
# value_to_put_in <- fct_drop(data[[deparse(substitute(column))]])
# but this line doesn't
# data <- data %>%
# mutate(!! column = value_to_put_in)
}
diamonds %>%
reduce_for_plot(color, 1)

Answer 1

You were almost there! The problem in your code is that R doesn't allow ! on the LHS of =. So you need to use the fake operator := instead.
reduce_for_plot <- function(data, column, how_many_levels) {
col_expr <- enquo(column)
col_name <- rlang::as_name(col_expr)
of_interest <- unique(data[[col_name]])[1:how_many_levels]
data <- data %>%
filter(!!col_expr %in% of_interest)
value_to_put_in <- fct_drop(data[[col_name]][of_interest])
data %>%
mutate(!!col_name := value_to_put_in)
}
As you can see I have replaced all deparse(substitute(column)) by as_name(enquo(column)). However you can avoid these entirely by doing the computations in data context, which I think yields nicer code:
reduce_for_plot <- function(data, column, how_many_levels) {
column <- enquo(column)
data %>%
filter(!!column %in% unique(!!column)[1:how_many_levels]) %>%
mutate(!!column := fct_drop(!!column))
}