Mutually dependent input sliders in R shiny

Question

I try to make two mutually dependent input sliders in a shiny app but don't seem to get it to work. I plan to use this kind of situation in a shiny app and for ease illustrate it here with a simplified 'glass half empty / full' example in which one slider should show glass fullness and one emptyness:

library(shiny)

ui =(pageWithSidebar(
  headerPanel("Glass fullness"),
  sidebarPanel(
    sliderInput("Full", "% water", min = 0, max = 1, value = 0.2),
    #display dynamic UI
    uiOutput("Empty")),
  mainPanel()
))

server = function(input, output, session){

  # make dynamic slider 
  output$Empty <- renderUI({
    sliderInput("Empty", "% air", min=0, max=1, value=1-input$Full)
  })

}

runApp(list(ui = ui, server = server))

I understand the value = 0.2 and value=1-input$Full cause above code only to make the second slider dependent on the first but not the other way around. My problem is that I want to make both dependent on each other. I tried adding an uiOutput("Full") & dynamic slider for Full but did not manage to work around circular coding.

The result of above code looks as follows:

Glass half empty/full screenshot

Any help greatly appreciated!

Answer 1

Hi define your sliders in the UI and updated them in the server when it's needed :

library(shiny)

ui =pageWithSidebar(
  headerPanel("Glass fullness"),
  sidebarPanel(
    sliderInput(inputId = "Full", label = "% water", min = 0, max = 1, value = 0.2),
    sliderInput(inputId = "Empty", label = "% air", min = 0, max = 1, value = 1 - 0.2),
    uiOutput("Empty")),
  mainPanel()
)

server = function(input, output, session){

  # when water change, update air
  observeEvent(input$Full,  {
    updateSliderInput(session = session, inputId = "Empty", value = 1 - input$Full)
  })

  # when air change, update water
  observeEvent(input$Empty,  {
    updateSliderInput(session = session, inputId = "Full", value = 1 - input$Empty)
  })

}

shinyApp(ui = ui, server = server)