Shiny: How to make reactive value initialize with default value

Question

Consider the following actionButton demo: http://shiny.rstudio.com/gallery/actionbutton-demo.html

server.R:

shinyServer(function(input, output) {    # builds a reactive expression that only invalidates    # when the value of input$goButton becomes out of date    # (i.e., when the button is pressed)   ntext <- eventReactive(input$goButton, {     input$n   })    output$nText <- renderText({     ntext()   }) }) 

ui.R:

shinyUI(pageWithSidebar(   headerPanel("actionButton test"),   sidebarPanel(     numericInput("n", "N:", min = 0, max = 100, value = 50),     br(),     actionButton("goButton", "Go!"),     p("Click the button to update the value displayed in the main panel.")   ),   mainPanel(     verbatimTextOutput("nText")   ) )) 

In this example, prior to the action button being pressed, the right-hand side panel is empty. I would instead like the text with default value "50" to be rendered by default.

How to I get the output to display with default inputs if the action button has not yet been pressed?

Answer 1

eventReactive also takes ignoreNULL as documented here, which lets you initialise the object without an if statement.

By adding the ,ignoreNULL = FALSE to the original post (give or take some formatting), verbatimTextOutput shows 50 on startup.

This makes for a bit of economy on the server side I guess.

ui <- fluidPage(titlePanel("actionButton test"),                 sidebarLayout(                   sidebarPanel(                     numericInput(                       "n",                       "N:",                       min = 0,                       max = 100,                       value = 50                     ),                     br(),                     actionButton("goButton", "Go!"),                     p("Click the button to update the value displayed in the main panel.")                   ),                   mainPanel(verbatimTextOutput("nText"))                 ))  server <- function(input, output) {    ntext <- eventReactive(input$goButton, {     input$n   }   # Adding this parameter to the original example makes it work as intended   # with 50 in the output field to begin with   , ignoreNULL = FALSE   )    output$nText <- renderText({     ntext()   }) }  shinyApp(ui = ui, server = server) 

Answer 2

    shinyServer(function(input, output) {       values <- reactiveValues(default = 0)        observeEvent(input$goButton,{            values$default <- input$goButton       })       # builds a reactive expression that only invalidates        # when the value of input$goButton becomes out of date        # (i.e., when the button is pressed)       ntext <- eventReactive(input$goButton, {            input$n       })        output$nText <- renderText({          if(values$default == 0){               50          }          else{             ntext()          }       })     })