驗證使用者是否已做出選擇

回想影片中的內容：雖然為選擇器輸入設定預設值通常是好做法，但如果不想設定預設值，你可以拋出自訂錯誤訊息，提示使用者需要做什麼，才能讓應用程式順利執行。

在前一個練習中我們看到，若 pickerInput() 沒有預設值，圖形就會是空白。這一題要改成不要顯示空白圖，而是顯示自訂錯誤訊息，告訴使用者需要做出正確的選擇，才能讓應用程式運作。

本練習屬於課程

使用 R 的 Shiny 建立網頁應用程式

檢視課程

練習說明

新增一則自訂錯誤訊息，預設就會顯示，提醒使用者需要選擇「心理健康」或「身體健康」作為輸入。

動手互動練習

試著完成這個範例程式碼，體驗一下這個練習。

ui <- fluidPage(
  titlePanel("2014 Mental Health in Tech Survey"),
  sidebarPanel(
    sliderTextInput(
      inputId = "work_interfere",
      label = "If you have a mental health condition, do you feel that it interferes with your work?", 
      grid = TRUE,
      force_edges = TRUE,
      choices = c("Never", "Rarely", "Sometimes", "Often")
    ),
    checkboxGroupInput(
      inputId = "mental_health_consequence",
      label = "Do you think that discussing a mental health issue with your employer would have negative consequences?", 
      choices = c("Maybe", "Yes", "No"),
      selected = "Maybe"
    ),
    pickerInput(
      inputId = "mental_vs_physical",
      label = "Do you feel that your employer takes mental health as seriously as physical health?", 
      choices = c("Don't Know", "No", "Yes"),
      multiple = TRUE
    )    
  ),
  mainPanel(
    plotOutput("age")  
  )
)

server <- function(input, output, session) {
  output$age <- renderPlot({
    # MODIFY CODE BELOW: Add validation that user selected a 3rd input
    
    
    
    
    
    

    mental_health_survey %>%
      filter(
        work_interfere == input$work_interfere,
        mental_health_consequence %in% input$mental_health_consequence,
        mental_vs_physical %in% input$mental_vs_physical
      ) %>%
      ggplot(aes(Age)) +
      geom_histogram()
  })
}

shinyApp(ui, server)

編輯並執行程式碼