Using dplyr::filter when the condition is a string
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This took me a while to figure out and so I thought I would post this as future reference. Let’s say I have the mtcars
data and I want to filter for just the rows with cyl == 6
. I would do something like this:
library(tidyverse) data(mtcars) mtcars %>% filter(cyl == 6) # mpg cyl disp hp drat wt qsec vs am gear carb # Mazda RX4 21.0 6 160.0 110 3.90 2.620 16.46 0 1 4 4 # Mazda RX4 Wag 21.0 6 160.0 110 3.90 2.875 17.02 0 1 4 4 # Hornet 4 Drive 21.4 6 258.0 110 3.08 3.215 19.44 1 0 3 1 # Valiant 18.1 6 225.0 105 2.76 3.460 20.22 1 0 3 1 # Merc 280 19.2 6 167.6 123 3.92 3.440 18.30 1 0 4 4 # Merc 280C 17.8 6 167.6 123 3.92 3.440 18.90 1 0 4 4 # Ferrari Dino 19.7 6 145.0 175 3.62 2.770 15.50 0 1 5 6
What if I had the filter condition as a string instead? The code below doesn’t work:
filter_string <- "cyl == 6" mtcars %>% filter(filter_string) # Error: Problem with `filter()` input `..1`. # x Input `..1` must be a logical vector, not a character. # Input `..1` is `filter_string`. # Run `rlang::last_error()` to see where the error occurred.
This is one possible solution:
mtcars %>% filter(!! rlang::parse_expr(filter_string)) # mpg cyl disp hp drat wt qsec vs am gear carb # Mazda RX4 21.0 6 160.0 110 3.90 2.620 16.46 0 1 4 4 # Mazda RX4 Wag 21.0 6 160.0 110 3.90 2.875 17.02 0 1 4 4 # Hornet 4 Drive 21.4 6 258.0 110 3.08 3.215 19.44 1 0 3 1 # Valiant 18.1 6 225.0 105 2.76 3.460 20.22 1 0 3 1 # Merc 280 19.2 6 167.6 123 3.92 3.440 18.30 1 0 4 4 # Merc 280C 17.8 6 167.6 123 3.92 3.440 18.90 1 0 4 4 # Ferrari Dino 19.7 6 145.0 175 3.62 2.770 15.50 0 1 5 6
This can be useful if you are trying to running several different filters automatically. In the following contrived example, I want to compute the mean MPG for two different slices of the data:
filters <- c("carb == 4", "am == 1") for (filter in filters) { print(paste0("Mean mpg for ", filter, ": ", mtcars %>% filter(!! rlang::parse_expr(filter)) %>% summarize(mean_mpg = mean(mpg)) %>% pull())) # [1] "Mean mpg for carb == 4: 15.79" # [1] "Mean mpg for am == 1: 24.3923076923077" }
You can read about parse_expr
here and about !!
here. I don’t fully understand tidy evaluation at this point, but the code above should work in a wide variety of situations.
(Disclaimer: There was a reference I came across for the !!
+ rlang::parse_expr
trick that I can’t find now. If anyone knows where it is please let me know and I’ll acknowledge it here in the references.)
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