In Shiny I want to updateSelectInput() only once during UI load

Question

I have a drop-down (SelectInput) that I want to update only once and load it up with a list of items programmatically during the upload of UI. I was putting it in a Render function but the problem is it resets again and again.

Answer 1

selectInput has parameters that allows you to set the initial state. Among these parameters, you can use choices for providing options and selected for having defaults. Please run ?shiny::selectInput for additional details.

Rendering it in server side or preferably using updateSelectInput would be of help if you want to update it upon user interaction in a reactive context.

Here is a minimal example:

library(shiny)

ui <- fluidPage(
  selectInput(
    inputId = "digits_input", 
    label = "Digits:", 
    choices = 0:9
    ## other arguments with default values:
    # selected = NULL,
    # multiple = FALSE,
    # selectize = TRUE, 
    # width = NULL, 
    # size = NULL
  ),

  selectInput(
    inputId = "letters_input", 
    label = "Lower case letters:", 
    choices = letters,
    selected = c("a", "b", "c"), # initially selected items 
    multiple = T # to be able to select multiple items
  ),

  actionButton(
    inputId = "update",
    label = "Capitalize"
  )

)

server <- function(session, input, output) {
  observeEvent(input$update, {
    updateSelectInput(
      session,
      inputId = "letters_input",
      label = "Upper case letters:",
      choices = LETTERS,
      selected = c("A", "B", "C")
    )
  })
}

shinyApp(ui = ui, server = server)