Understanding the Error: Unused Arguments in Shiny Application

As a developer working with Shiny applications, we have encountered various errors and challenges. In this article, we will delve into one such error that occurred in a Shiny application, specifically related to unused arguments.

Error Description

The error message “unused arguments (alist(, drop = FALSE))” was appearing in the Shiny application’s server logic. This error occurs when an argument is passed to a function but not used within its scope.

Background and Context

Shiny applications are built using R and utilize several libraries, including shiny and ggplot2. The application in question allowed users to upload data, plot the data, and exclude points by clicking or brushing the plot. However, due to an incorrect indexing of functions, the Shiny application encountered issues.

Step 1: Identifying the Issue

To debug this issue, we first examined the server function within the Shiny application’s code. We noticed that a reactive value vals was being used but its usage seemed suspicious.

# Define server logic required to plot data with a brush#
server <- function(input, output) {
  # ...
}

Step 2: Understanding Reactive Values

Reactive values in Shiny are functions that produce values as the input changes. The reactiveValues() function creates an object that stores values within it.

# Create reactive value 'vals'
vals <- reactiveValues(
  keeprows = rep(TRUE, nrow(datamerge()))
)

In this code snippet, we define a reactive value named “vals” containing a vector called keeprows. The nrow() function is used to determine the number of rows in the data frame datamerge(), and then it stores this value within the keeprows variable.

Step 3: Analyzing Code Usage

Looking closer at the code, we see that vals$keeprows is being assigned a new value:

# Toggle points that are clicked
observeEvent(input$plot1_click, {
  res <- nearPoints(datamerge(), input$plot1_click, allRows = TRUE)
  vals$keeprows <- xor(isolate(vals$keeprows), res$selected_)
})

Here, we observe that vals$keeprows is being updated when a point is clicked. However, there is no clear connection between the line of code and the variable name.

Step 4: Exploring Alternative Solutions

We discovered an alternative approach to using this reactive value by directly accessing its elements without referencing it as a vector (vals$keeprows):

# Toggle points that are clicked
observeEvent(input$plot1_click, {
  res <- nearPoints(datamerge(), input$plot1_click, allRows = TRUE)
  keeprows <- xor(vals$keeprows, res$selected_)
})

By making this change, we could use the keeprows value from within our Shiny application more effectively.

Step 5: Applying Changes to Main Function

We modified our main server function code as follows:

# Run the application 
shinyApp(ui = ui, server = function(input, output) {
  # ...
})

By changing server <- into shinyApp(ui = ui, server = server) we are defining a new function and using that one within our shiny app.

Step 6: Final Check

After applying these changes to the Shiny application’s code, we noticed that the “unused arguments” error had vanished. Our reactive value was now being utilized correctly throughout the code.

# Create reactive value 'vals'
vals <- reactiveValues(
  keeprows = rep(TRUE, nrow(datamerge()))
)

By identifying and fixing this issue, our Shiny application could display data points more effectively than ever before.

Last modified on 2023-05-08