library("shiny")

choicesONE <- c("a","b","c","d","e")

choicesTWO <- c("a","c","e")

ui <- shinyUI(fluidPage(

  sidebarLayout(

    sidebarPanel(

      selectizeInput(inputId="topic",
                     label = ("Topic"),
                     choices=NULL,
                     multiple = T,
                     options=list(maxItems = 1,
                                  placeholder="Please choose...")),

      checkboxInput("sub", "Show only subchoices", value = FALSE, width = NULL)

    ),

    mainPanel(

    )

  )

))

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

  #------- Initialize the Memory ----------

  choice <- reactiveValues(selection = NULL)

  #------ Whenever the inputs are changed, it only modifies the memory----

  observeEvent(input$topic,{

    choice$selection <- input$topic

  })

  #------ Update UI element using the values stored in memory ------

  observe({

    if(input$sub==T) {

      updateSelectizeInput(session,
                           server = T,
                           'topic',
                           choices = choicesTWO,
                           selected = choice$selection)

    } else {

      updateSelectizeInput(session,
                           server = T,
                           'topic',
                           choices = choicesONE,
                           selected = choice$selection)


    }


  })



}

shinyApp(ui = ui, server = server)