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

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!