Demo updateSelectInput in R Shiny

server.r

library(shiny)
shinyServer(function(input, output, session){

  observe(
    {input$Year
    # Update based on the year change event
    updateSelectInput(session, "Month", "Month", choices = data$Month[data$Year==input$Year])
 }
    )
  
  observe(
    {
      input$Month
      # Update based on the month change event
      updateSelectInput(session, "Person", "Person", 
                        choices = data$Name[data$Month==input$Month & data$Year==input$Year])})
  
})

ui.r

library(shiny)
# Creating a dummy dataset just for demo sake
data = data.frame(Year=c("2002","2003","2004","2003","2001","2002", "2001"), 
                  Month = c("Jan", "Feb", "Mar", "Jan", "Dec", "Jan", "Nov"),
                  Name =c("Sam", "Paul", "James", "Ana","Rose", "Juan", "Tim"),
                  row.names=NULL, stringsAsFactors = FALSE)

data$Year = factor(data$Year, levels = c("2001", "2002", "2003", "2004"))
shinyUI(fluidPage(
  
  selectInput("Year", "Year", choices = levels(data$Year) , 
              selected = levels(data$Year)[1]),
  # Choice of this select input based on the first year that appears on level & which was actaully selected in the year select input
  selectInput("Month", "Month", choices = data$Month[data$Year==levels(data$Year)[1]]),
  # Choice of this select input based on the first year that appears on level & which was actaully selected in the year select input
  selectInput("Person", "Person", choices = data$Name[data$Year==levels(data$Year)[1]])

  )

  
)

I got the following error msg when trying to run your code. Any suggestion on how to fix?

Warning: Error in $: object of type 'closure' is not subsettable
Stack trace (innermost first):
    57: updateSelectInput
    56: observerFunc [C:\Users\s\R\testInput/server.R#7]
     1: shiny::runApp
Warning: Error in $: object of type 'closure' is not subsettable
Stack trace (innermost first):
    57: updateSelectInput
    56: observerFunc [C:\Users\s\R\testInput/server.R#15]
1: shiny::runApp