How do you filter by passing a string as a column name in a user-defined function?

Question

I'm writing a function where the user specifies the column they want to filter and what cutoff value they want. In this example, I want to filter out any pretest scores under 2. Here's a sample dataset:

library(dplyr)

test <- tibble(name = c("Corey", "Justin", "Sibley", "Kate"),
               pretest_score = c(1:4),
               posttest_score = c(5:8),
               final_score = c(9:12))


filter_function <- function(data, test_type = c(pretest, posttest, final), value) {
  
  test_character <- deparse(substitute(test_type))
  test_score <- paste0(test_character, "_score")
  
  data %>%
    filter({{test_score}} > value)
  
}

filter_function(test, test_type = pretest, value = 2)

I've also tried !!test_score, test_score (with nothing around it), and ensym(test_score) from rlang, all to no avail.

Note: I know that in this example, I could just specify pretest_score, posttest_score, etc as the test type, but in my real dataset, I have many dimensions for these tests that users can determine cutoffs for (pretest_score, pretest_date, pretest_location, etc.), so it's important that I merge the column prefix with the suffix (here, _score) within the function itself.

Thank you for any help!

Answer 1

Convert the character to symbol and evaluate with !!

filter_function <- function(data, test_type = c(pretest, posttest, 
      final), value) {
  
  test_character <- deparse(substitute(test_type))
  test_score <- paste0(test_character, "_score")
  
  data %>%
    filter(!! rlang::sym(test_score) > value)
  
}

-testing

> filter_function(test, test_type = pretest, value = 2)
# A tibble: 2 × 4
  name   pretest_score posttest_score final_score
  <chr>          <int>          <int>       <int>
1 Sibley             3              7          11
2 Kate               4              8          12