How can I set variable x as default when I upload a .CSV file on my App?

Question

My Shiny Application's purpose is to upload a.csv file and plot a ggplot graph with its data. When I upload a.csv file on my Shiny Application, the default variable is "y" for both input variables. However, when I change "y" to "x" on the "X Variable" input, the App works fine. I just have to set "x" for "X Variable" input and "y" for "Y Variable" input as default when I upload the.csv file.

Here follows the app.R code with my comments and Error Message I get when I upload the file.

library(shiny)
library(datasets)
library(ggplot2)

ui <- shinyUI(fluidPage(
  titlePanel("Column Plot"),
  tabsetPanel(
    tabPanel("Upload File",
             titlePanel("Uploading Files"),
             sidebarLayout(
               sidebarPanel(
                 fileInput('file1', 'Choose CSV File',
                           accept=c('text/csv', 
                                    'text/comma-separated-values,text/plain', 
                                    '.csv')),

                 tags$br(),
                 checkboxInput('header', 'Header', TRUE),
                 radioButtons('sep', 'Separator',
                              c(Comma=',',
                                Semicolon=';',
                                Tab='\t'),
                              ','),

                 selectInput('xcol', 'X Variable', "", selected = NULL),
                 selectInput('ycol', 'Y Variable', "", selected = NULL)

               ),
               mainPanel(
                 tableOutput('contents'),
                 plotOutput('MyPlot')
               )
             )
    )
  )
)
)


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

  data <- reactive({ 
    req(input$file1)

    inFile <- input$file1 

    df <- read.csv(inFile$datapath, header = input$header, sep = input$sep)

    ## Update inputs
    ## I've already tried to change the selected = names(df) to selected = NULL on this part of the server code, but it didn't work.

    updateSelectInput(session, inputId = 'xcol', label = 'X Variable',
                      choices = names(df), selected = names(df))
    updateSelectInput(session, inputId = 'ycol', label = 'Y Variable',
                      choices = names(df), selected = names(df)[2])

    return(df)
  })

  output$contents <- renderTable({
    data()
  })

  output$MyPlot <- renderPlot({

    xy <- data()[, c(input$xcol, input$ycol)]
    ggplot(data = xy, aes(x, y)) +
      geom_line() +
      geom_point()

  })
})

shinyApp(ui, server)

The error message I get is:

"Error: object 'x' not found".

Answer 1

and welcome to SO

Your issue here is that you update the selectInput() at the wrong time: the update is done once the server has finished computing, so here once the data() is done running.

So to be clear, you update your input after trying to select from it.

Other thing is that you need to sym() & !! the input$ to make it work with ggplot() (see: https://github.com/ColinFay/tidytuesday201942/blob/master/R/mod_dataviz.R#L184 for example)

Here is a working version:

library(shiny)
library(datasets)
library(ggplot2)
library(rlang)

ui <- shinyUI(fluidPage(
  titlePanel("Column Plot"),
  tabsetPanel(
    tabPanel("Upload File",
             titlePanel("Uploading Files"),
             sidebarLayout(
               sidebarPanel(
                 fileInput('file1', 'Choose CSV File',
                           accept=c('text/csv', 
                                    'text/comma-separated-values,text/plain', 
                                    '.csv')),

                 tags$br(),
                 checkboxInput('header', 'Header', TRUE),
                 radioButtons('sep', 'Separator',
                              c(Comma=',',
                                Semicolon=';',
                                Tab='\t'),
                              ','),

                 selectInput('xcol', 'X Variable', "", selected = NULL),
                 selectInput('ycol', 'Y Variable', "", selected = NULL)

               ),
               mainPanel(
                 tableOutput('contents'),
                 plotOutput('MyPlot')
               )
             )
    )
  )
)
)


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

  r <- reactiveValues(
    df = NULL
  )

  observe({
    req(input$file1$datapath)
    # req(input$xcol)
    # req(input$ycol)
    r$df <- read.csv(
      input$file1$datapath, 
      header = input$header, 
      sep = input$sep
    )
  })


  observeEvent( r$df   , {
    updateSelectInput(
      session, 
      inputId = 'xcol', 
      label = 'X Variable',
      choices = names(r$df), 
      selected = names(r$df)[1]
    )
    updateSelectInput(
      session, 
      inputId = 'ycol', 
      label = 'Y Variable',
      choices = names(r$df),
      selected = names(r$df)[2]
    )
  }, ignoreInit = TRUE)


  output$contents <- renderTable({
    req(r$df)
    head(r$df)
  })

  output$MyPlot <- renderPlot({
    req(r$df)
    req(input$xcol)
    req(input$ycol)
    ggplot(
      data = r$df, 
      aes(!!sym(input$xcol), !!sym(input$ycol))
    ) +
      geom_line() +
      geom_point()

  })
})

shinyApp(ui, server)