dynamic data transforms + input options in shiny for Gokhan

app.R

library(shinydashboard)
library(dplyr)

ui <- navbarPage("test",
                 tabPanel("dataBuilder", icon = icon("database"),
                          fluidRow(
                            box(width = 3, title = "Data Upload", solidHeader = TRUE, status = "primary",
                                helpText("Choose CSV File (Max size 10 MB)"),
                                fileInput("file1", "",
                                          multiple = FALSE,
                                          accept = c("text/csv",
                                                     "text/comma-separated-values,text/plain",
                                                     ".csv"))
                            ),
                            box(width = 2, title = "Outcome", solidHeader = T, 
                                status = "primary",
                                helpText("Select a variable to predict"),
                                selectizeInput("yvar", label = "", choices = character(0))
                            ),
                            box(width = 2, title = "Duration", solidHeader = T,
                                status = "primary",
                                helpText("Select the duration variable"),
                                selectizeInput("tvar", label = "", choices = character(0)),
                                checkboxInput("splines", "Include cubic splines?", FALSE)),
                            box(width = 5, title = "Predictors", solidHeader = T, status = "primary",
                                helpText("Select the independent variables"),
                                selectizeInput("xvar", label = "", choices = character(0), multiple = T,
                                               width = "500px")
                            )),
                          
                          column(width = 12, DT::dataTableOutput("table"))
))


server <- shinyServer(function(input, output, session) {
  
  rawdata <- reactive({
    req(input$file1)
    
    inFile <- input$file1
    
    df <- read.csv(inFile$datapath)
  })
  
  observe({
    updateSelectizeInput(session, "yvar", choices = names(rawdata()), selected = names(rawdata())[1])
  })
  
  observe({
    nums <- colnames(select_if(rawdata(), "is.numeric"))
    ints <- colnames(select_if(rawdata(), "is.integer"))
    digits <- c(nums, ints)
    updateSelectizeInput(session, "tvar", choices = digits)
  })
  
  data_transform <- reactive({
    req(input$tvar)
    
    if(input$splines) {
      rawdata() %>% 
        mutate(square = (!! rlang::sym(input$tvar))^2,
               cube = (!! rlang::sym(input$tvar))^3)
    } else {
      rawdata()
    }
  })
  
  observe({
    nms <- names(data_transform())[names(data_transform()) != input$yvar]
    updateSelectizeInput(session, "xvar", choices = nms)
  })
  
  output$table <- DT::renderDataTable({
    DT::datatable(data_transform())
  })
  
})

shinyApp(ui = ui, server = server)

What does the code do after we select the variables, I do not see any results displayed! Please suggest.