Calling variable in df within function

Question

I'm getting the error message that "cyl" not found. Pretty sure it has to do with tidy eval, but I can't figure out what else I need to do.

I'm looking to evaluate all values in the cyl column and raise a flag if none are greater than zero.

check_percent_fun = function(df, var){
  
apply(df, MARGIN=2, stopifnot("not greater than 0" =
            {{var}} > 0 )
)
}

check_percent_fun(mtcars, cyl)

Answer 1

Tidy eval is a framework for "non-standard evaluation" used in tidyverse packages like dplyr. While there are instances in base R of "non-standard evaluation" like subset() or lm, they don't specifically use the "tidy eval" framework of NSE, so they would have no idea what to do with {{ }}, for instance.

Here's a dplyr way to do this (as far as I can discern), which uses rlang's tidy eval:

library(dplyr)
check_gt0 <- function(df, var) {
    df |>
      summarize(across({{var}}, ~stopifnot("not greater than 0" = .x > 0)))
}

With cyl we get no error, but with vs we trigger the stopifnot, since it has zeroes:

check_gt0(mtcars, cyl)
# data frame with 0 columns and 1 row

check_gt0(mtcars, vs)
# Error in `summarize()`:
#ℹ In argument: `across(vs, ~stopifnot(`not greater than 0` = .x > 0))`.
#Caused by error in `across()`:
#! Can't compute column `vs`.
#Caused by error in `stopifnot()`:
#! not greater than 0
#Run `rlang::last_trace()` to see where the error occurred.

Another possible variation using dplyr::pull(), another function that uses tidy eval.

check_gt0_b <- function(df, var) {
  stopifnot("not greater than 0" = dplyr::pull(df, {{var}}) > 0)
}