R Shiny: How to make the initial value for numericInput dynamic, based on user input

Question

I'm trying to make the value argument for shiny::numericInput() dynamic, based on input from a user.

Both code chunks below will run, but they fail to set a dynamic initial value.

Chunk 1:

idOptions <- c("1","2","3")

ui <- shiny::fluidPage(
  shiny::selectInput(inputId = "idSelection", "Identification: ", idOptions),
  shiny::numericInput("num", "Number associated with id:", value=shiny::verbatimTextOutput("numberOut")),
)

server <- function(input, output) {
  df <- data.frame(id = c("1","2","3"), number = c(100,227,7))
  output$numberOut <- shiny::renderText({ input$idSelection })
}

shiny::shinyApp(ui, server)

Chunk 2:

idOptions <- c("1","2","3")

ui <- shiny::fluidPage(
  shiny::selectInput(inputId = "idSelection", "Identification: ", idOptions),
  shiny::numericInput("num", "Number associated with id:", value=shiny::verbatimTextOutput("numberOut")),
)

server <- function(input, output) {
  df <- data.frame(id = c("1","2","3"), number = c(100,227,7))
  dfReactive <- shiny::reactive({
    dataOut <- df %>%
      dplyr::filter(., id %in% input$idSelection)
    return(dataOut)
  })
  shiny::observe({
    output$numberOut <- shiny::renderText({ 
      return(dfReactive()$number)
    })
  })
}

shiny::shinyApp(ui, server)

I want the initial value for shiny::numericInput to change like so:

When the user selects "1" for Identification, the initial value is 100:

When the user selects "2" for Identification, the initial value is 227:

The idea behind this is to have an appropriate initial value suggested to the user based on the identification of the input.

I'm guessing the problem might be with shiny::verbatimTextOutput("numberOut") , but I don't know a way to render a simple numeric value to pass into the value argument of shiny::numericInput .

Any thoughts?

Thanks much.

Answer 1

One way would be to use renderUI and do the computation on the server side:

idOptions <- c("1","2","3")

shiny::shinyApp(

  ui = shiny::fluidPage(
    shiny::selectInput(inputId = "idSelection", "Identification: ", idOptions),
    shiny::uiOutput("num")
  ),
  
  server = function(input, output) {
    
    df <- data.frame(id = c("1","2","3"), number = c(100,227,7))
    
    output$num <- shiny::renderUI({
      shiny::numericInput("num",
                          "Number associated with id:",
                          value = df$number[as.numeric(input$idSelection)])
    })
    
})

The alternative is to leave the input on the UI side and use updateNumericInput inside an observeEvent :

idOptions <- c("1","2","3")

shiny::shinyApp(
  
  ui = shiny::fluidPage(
    
    shiny::selectInput(inputId = "idSelection",
                       "Identification: ",
                       idOptions),
    
    shiny::numericInput("num",
                        "Number associated with id:",
                        value = NULL)
  ),
  
  server = function(input, output, session) {
    
    df <- data.frame(id = c("1","2","3"),
                     number = c(100,227,7))
    
    observeEvent(input$idSelection, {
      updateNumericInput(session,
                         inputId = "num",
                         value = df$number[as.numeric(input$idSelection)])
    })
  })