Shiny: How to make reactive value initialize with default value - r

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# when the value of input$goButton becomes out of date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# when the value of input$goButton becomes out of date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()
}
})
})