R Shiny Error in [.data.frame: undefined columns selected

Question

I am trying to build simple csv upload example in Shiny. Upload csv file, store header in Select Input control and display each column in table output. As soon as I upload any csv file it shows me below error

Error in [.data.frame: undefined columns selected

However if the same csv file is uploaded again it doesn't show the error.

    library(shiny)
    library(shinyjs)

    ui <- pageWithSidebar(
           headerPanel("CSV Upload test"),
             sidebarPanel(
               fileInput("datafile", h5("Choose CSV file:"),
                            accept=c('text/csv', 'text/comma-separated-values,text/plain')),
                            htmlOutput("varselect", inline=TRUE),
               selectInput("vars", label = h5("Select a variable:"), choice=NULL, selected = NULL, 
                                                multiple=FALSE, selectize=TRUE),
               actionButton("unselect", label="Clear All"),
            br()  
            ),
          mainPanel(dataTableOutput("table")))

    server <- function(session,input, output) {  
         
         Dataset <- reactive({    
         infile <- input$datafile
         if (is.null(infile)) {
         return(NULL)
         }
         read.csv(infile$datapath)
         })

       observe({
       if (identical(Dataset(), '') || identical(Dataset(),data.frame())) 
       return(NULL)
       updateSelectInput(session, inputId="vars",
                  choices=names(Dataset()), selected=NULL)
       })

      observe({
      if (input$unselect > 0) {
      if (identical(Dataset(), '') || identical(Dataset(),data.frame())) 
      return(NULL)
  
      updateSelectInput(session, inputId="vars",
                    choices=names(Dataset()), selected=NULL)
     }
    })

   output$table <- renderDataTable({
   if (is.null(input$vars) || length(input$vars)==0) return(NULL)
       validate(need(Dataset(), "Awaiting data"))
   return(Dataset()[,input$vars,drop=FALSE])
   })
   }

 shinyApp(ui, server)

Also in the selectInput function if I channge option for multiple = TRUE this error at the beginning of csv upload doesn't show.

 selectInput("vars", label = h5("Select a variable:"), choice=NULL, selected = NULL, 
            multiple=FALSE, selectize=TRUE)    

Appreciate if someone can help to make me understand what is going on here. Any work around?

Answer 1

In addition to the anwser of @YBS, it is a bit more cleaner to use req

library(shiny)
library(shinyjs)

ui <- pageWithSidebar(
  headerPanel("CSV Upload test"),
  sidebarPanel(
    fileInput("datafile", h5("Choose CSV file:"),
              accept=c('text/csv', 'text/comma-separated-values,text/plain')),
    htmlOutput("varselect", inline=TRUE),
    selectInput("vars", label = h5("Select a variable:"), choice=NULL, selected = NULL, 
                multiple=FALSE, selectize=TRUE),
    actionButton("unselect", label="Clear All"),
    br()  
  ),
  mainPanel(dataTableOutput("table")))

server <- function(session,input, output) {  
  
  Dataset <- reactive({
    req(input$datafile)
    infile <- input$datafile
    try( {
      read.csv(infile$datapath)
    })
  })
  
  observeEvent(Dataset(), {
    updateSelectInput(session, inputId="vars",
                      choices=names(Dataset()), selected=NULL)
    
  })
  
  observeEvent(input$unselect, {
    req(Dataset())
    updateSelectInput(session, inputId="vars",
                     choices=names(Dataset()), selected=NULL)
  })
  
  output$table <- renderDataTable({
    validate( need(Dataset(), "Awaiting data")
            , need(input$vars, "Select variable")
            )
    return(Dataset()[,input$vars,drop=FALSE])
  })
}

shinyApp(ui, server)

Answer 2

The following eliminates your error:

  output$table <- renderDataTable({
    validate(need(Dataset(), "Awaiting data"))
    if (is.null(Dataset()) || is.null(input$vars) || length(input$vars)==0) {
      return(NULL)
    } else{
      selected_vars <- names(Dataset()) %in% input$vars
      Dataset()[,selected_vars,drop=FALSE]
    }
  })