How can I create a conditional selectInput widget in R Markdown?

Question

The purpose is to choose a county from a state. I first create a selectInput widget for choosing a state. Then I create a selectInput widget for choosing a county from the selected state. In an R Markdown, the code is as follows:

inputPanel(
   selectInput(inputId = "State", label = "Choose a state:", choices = state.name),
   selectInput(inputId = "County", label = "Choose a county:", choices = input.State)
)

I guess the use of input.State is problematic, but I don't have any other idea.

Thanks for your time!

Answer 1

There are a number of ways to create conditional/dynamic UI in Shiny (see here). The most straightforward is usually renderUI. See below for a possible solution for you. Note that this requires Shiny so if you’re using R Markdown make sure to specify runtime: shiny in the YAML header.

library(shiny)

# I don't have a list of all counties, so creating an example:
county.name = lapply(
  1:length(state.name),
  function(i) {
    sprintf("%s-County-%i",state.abb[i],1:5)
  }
)
names(county.name) = state.name

shinyApp(

  # --- User Interface --- #

  ui = fluidPage(

    sidebarPanel(
      selectInput(inputId = "state", label = "Choose a state:", choices = state.name),
      uiOutput("county")
    ),

    mainPanel(
      textOutput("choice")
    )

  ),

  # --- Server logic --- #

  server = function(input, output) {
    output$county = renderUI({
      req(input$state) # this makes sure Shiny waits until input$state has been supplied. Avoids nasty error messages
      selectInput(
        inputId = "county", label = "Choose a county:", choices = county.name[[input$state]] # condition on the state
      )
    })

    output$choice = renderText({
      req(input$state, input$county)
      sprintf("You've chosen %s in %s",
              input$county,
              input$state)
    })
  }

)

Hope this helps!