Shiny and Tidyeval Part 1

Note: sometimes I write these posts to teach myself a better way to do things in R.

I have been avoiding tidyeval somewhat, because I seem to have a bit of a learning block about it. I’m going to try to write some posts that help me understand what’s going on with Tidy Evaluation.

Using sym() in a Shiny App

One fairly simple Shiny Application might be selecting a column of the dataset and then doing something with it, such as using it in a select() or filter() statement. Say we had a simple app to produce histograms, and we wanted to change the column that is being displayed on the histogram.

Try this app out by running the following command. The code is here.

runGist("https://gist.github.com/laderast/a5205554324306e642b2df9f80ed6409", display.mode="showcase")

Our input is a select input called numeric_var, which returns a single column name as a character In our server logic, we’ve built a reactive called selected_data, which returns the selected column as a vector using pull().

  selected_data <- reactive({
    ## input$numeric_var is a character, so we cast it to symbol
    var_name <- sym(input$numeric_var)

    ## Now we evaluate it with !!
    out_col <- iris %>% pull(!!var_name)
  })

The question is: how do we pass the input value into pull()? We first have to use rlang::sym() to pass our character in as a symbol that we’re calling var_name. But the issue is that our reactive doesn’t know which environment to look in.

We want our reactive to look for the column name within the environment of the iris tibble. This is where the !! (bang-bang) comes in. It says, ‘look for the value’ within the tibble.

Using syms() in a Shiny App

What if wanted to pass in multiple variables from a select box? We’ll need to wrap our input with syms(), which takes a list.

Let’s do a slightly different version where we’re visualizing a box plot and we want to select multiple columns to display in our dataset from a selectInput where we’ve specified the multiple=TRUE argument.

Our setup is similar, but different. Because we have multiple values, we have to use syms() to wrap the input from input$numeric_vars. Then we can evaluate it with !!! (the triple bang).

  selected_data <- reactive({
    ## input$numeric_var is a character vector, so we cast it to symbol
    var_list <- syms(input$numeric_vars)

    ## Now we evaluate it with !!!
    out_col <- iris %>% select(!!!var_list)
  })

Try this app out. The Code is here.

runGist("https://gist.github.com/laderast/952120ac46d1f27c2d2dba5bd1ab5d10", display.mode="showcase")