Understanding Reactivity

server.R

require(shiny)
require(rCharts)

inputChoices <- c("A", "B", "C", "D")
adf <- data.frame(val1=sample(inputChoices), val2=sample(inputChoices))


shinyServer(function(input, output, session){
    
  	input2Choices <- reactive({
      inputChoices[-grep(input$input1, inputChoices)]
  	})

  	output$input1 <- renderUI({
  		selectInput("input1", "Input 1:", choices=as.list(inputChoices))
  	})

  	output$input2 <- renderUI({
  		if(is.null(input$input1))
  			return()
  		selectInput("input2", "Input 2:", choices=input2Choices())

  	})
  	
   dataInput1 <- reactive({
    if(is.null(input$input1))
      return()
    switch(input$input1,
           "A" = 1,
           "B" = 2,
           "C" = 3,
           "D" = 4)
    })

    dataInput2 <- reactive({
    if(is.null(input$input2))
      return()
    switch(input$input2,
           "A" = 1,
           "B" = 2,
           "C" = 3,
           "D" = 4)
    })

    getData <- function(){
      data1 <- dataInput1()
      data2 <- dataInput2()
      result <- adf[which(adf$val1 == input$input1),2]
      #print(result)
      return(result)
    }

  	output$show <- renderPrint({
       if(input$input1 == input$input2)
        return()
       z<-getData()
       #print(z)
  		 print(input$input1)
       print(input$input2)
       Sys.sleep(2)
  	})

})

ui.R

require(shiny)
require(rCharts)
shinyUI(pageWithSidebar(
  headerPanel("Dynamic UI inputs with Shiny"),
  sidebarPanel(
    uiOutput('input1'),
    uiOutput('input2')
  ),
  mainPanel(
   # showOutput("mychart", "rickshaw")
   verbatimTextOutput("show")
  )
))