Set a default value in shiny inputs (in case the user deletes it in the UI)

Question

I am trying to set a default (or fallback) value for numericInput() in my shiny app to prevent NAs.

I am aware that the NA can be dealt with later in the server.r, but was wondering if there is a more elegant way of replacing the value within the input whenever a user deletes it in the ui.

Answer 1

The best way is to use the validate package with need() (see this SO thread), but here is something simpler and closer to what you are asking for:

library(shiny)

ui <- fluidPage(
  numericInput("obs", "Observations:", 10, min = 1, max = 100),
  verbatimTextOutput("value")
)

server <- function(input, session, output) {
  
  dafault_val <- 0
  
  observe({
    if (!is.numeric(input$obs)) {
      updateNumericInput(session, "obs", value = dafault_val)
    }
  })
  
  output$value <- renderText({ input$obs })
}

shinyApp(ui, server)

Answer 2

I'd recommend using library(shinyvalidate), which is RStudios "official" way to solve this:

library(shiny)
library(shinyvalidate)

ui <- fluidPage(
  numericInput(
    inputId = "myNumber",
    label = "My number",
    value = 0,
    min = 0,
    max = 10
  ),
  textOutput("myText")
)

server <- function(input, output, session) {
  iv <- InputValidator$new()
  iv$add_rule("myNumber", sv_required(message = "Number must be provided"))
  iv$add_rule("myNumber", sv_gte(0))
  iv$add_rule("myNumber", sv_lte(10))
  iv$enable()
  
  output$myText <- renderText({
    req(iv$is_valid())
    input$myNumber
  })
  
}

shinyApp(ui, server)