R shiny input in one input to form input of the next input

Question

I am subsetting a dataframe in R shiny using the following example

if (interactive()) {

  ui <- fluidPage(
  selectInput("variable", "Variable:",
            c("Cylinders" = "cyl",
              "Transmission" = "am",
              "Gears" = "gear")),
   tableOutput("data")
  )

   server <- function(input, output) {
   output$data <- renderTable({
   mtcars[, c("mpg", input$variable), drop = FALSE]
   }, rownames = TRUE)
 }

shinyApp(ui, server)
}

This works with a predetermined set of columns

I have written the following code with a slider to create inputs that will act as column numbers for the MTcars dataset used in the above example. Is there a way to use the number indicated on th slider a an input for the column number and dynamically subset the dataframe in shiny

I have written the folowing code but it doesn't seem to work

if (interactive()) {

 ui <- fluidPage(
 sliderInput(inputId = "slider1", label = "numberinput", min = 1, max = 10, 
 value = 1, step = 1),

 selectInput(inputId = "Columnno", label = "Column Number",choices = 
 as.numeric(input$slider1)),
tableOutput("data")
  )

 server <- function(input, output) {
 output$data <- renderTable({
  mtcars[, c("mpg", input$Columnno), drop = FALSE]
  }, rownames = TRUE)
  }

  shinyApp(ui, server)
  }

I request some guidance in this regard.

Answer 1

IF you want just one extra column you can do this:

library(shiny)

ui <- fluidPage(
  sliderInput(inputId = "slider1", label = "numberinput", min = 1, max = 10, value = 1, step = 1),
  selectInput(inputId = "Columnno", label = "Column Number",choices = NULL,selected = NULL),
  tableOutput("data")
)

server <- function(input, output,session) {

  observeEvent(input$slider1,{
    updateSelectInput(session,"Columnno",choices = as.numeric(1:input$slider1), selected = as.numeric(input$slider1))
  })
  output$data <- renderTable({
    req(input$Columnno)
    mtcars[, c("mpg", colnames(mtcars)[as.numeric(input$Columnno)]), drop = FALSE]
  }, rownames = TRUE)
}

shinyApp(ui, server)