Passing strings as variables from a Shiny app into dplyr

I’ve been sort of waiting until I understood this thoroughly, and I was going to write a very detailed blog post about it, and although I do understand it a lot better now than I did, I’m still not at the point where I would write an authoritative blog post about it confidently because there are too many things that I don’t understand.

However, I’m aware that there are people out there right now who are trying to write dplyr code that takes strings as variable inputs, passed in from a Shiny interface, and I know how difficult it is to Google, because I Googled it myself. So I’m going to at least show you what to do, and talk a bit about it, and then maybe I’ll come back to it when I’ve understood it better myself.

So you quite often have a combo box in Shiny applications that returns a variable name, and you want to ask dplyr to filter/ select/ whatever on that name. So you have the variable name in the form of a string, and you want to pass it into dplyr. The old way, which still works, but is now deprecated, uses the filter_ function which evaluates a whole string as if it were dplyr code. So, for example, you might do this:

<pre class="brush: r; title: ; notranslate" title="">
library(tidyverse)

input = data.frame("variable" = "cty")

mpg %>%
  filter_(paste(input$variable, " > 18"))

input = data.frame("variable" = "hwy")

mpg %>%
  filter_(paste(input$variable, " > 18"))

I’ve given the dataframe the name “input” to make it look like a Shiny application. So imagine your user clicks on “cty” in your app, which makes input$variable equal to “cty”. Now you just paste that together with a filter condition (“> 18”) and pass the whole thing to filter_(). Now your user perhaps sets the variable to “hwy”, and the calculation can be done with the new value.

So what’s the new way? Well, as I mentioned I don’t understand it deeply, so read the following at your own risk, but essentially what you’re doing is described in this blog post about dplyr programming. You need to quote the variable names which sort of makes them variable-y (told you I didn’t really understand) and then you need to unquote them. Normally you would be doing this:

<pre class="brush: r; title: ; notranslate" title="">

input = quo(cty)

mpg %>%
  filter(!!(input) > 18)

input = quo(hwy)

mpg %>%
  filter(!!(input) > 18)

So (I think!) you’re saying to R- hey, listen, R, input is cty as a variable name. Wherever you see input, pretend it’s cty but as a variable name. This is called quoting. Then you have to unquote, using !!(). So you’re saying- that thing I mentioned about input being cty quoted? Well, I want it now, unquote input (using !!()) and then use that variable name in the following.

So you’re basically doing that except! Except you’re quoting a string, not a bare variable name. To quote strings, use sym. Other than that, it’s the same. So do this:

<pre class="brush: r; title: ; notranslate" title="">

input = sym("cty")

mpg %>%
  filter(!!(input) > 18)

input = sym("hwy")

mpg %>%
  filter(!!(input) > 18)

So now you can pass variable names from comboboxes in Shiny applications using dplyr.

I’m sorry I don’t have the detail as nailed as I would like, I wouldn’t advice sitting any exams after reading this, but it should get you coding, at least, which is the first step. If the fog ever truly clears for me I promise I’ll come back and write more.