R闪亮的渲染：找不到对象“输出”

Question

这是我第一次接触RShiny，遇到了一些问题。这个想法是添加两个选择按钮：第一个是纽约区域，然后选择社区。结果-在Airbnb上可用。但是，即使在图形部分之前，此代码也不起作用。

我有一个数据框，其中包括每个公寓的此类列：neighbourhood_groups（曼哈顿，布朗克斯等），邻里（纽约市5个行政区的221个），room_type，价格和空房情况。稍后，我想通过ggplot2软件包添加更多的数量图。

library(shiny)

# Define UI for application that draws a histogram
ui <- fluidPage(

    # Application title
    titlePanel("New-York City apartment offer on Airbnb "),

    # Sidebar with a slider input for number of bins 
    sidebarLayout(
        sidebarPanel(
            selectInput("area",
                        "Neighbourhood:",
                        choices = c('Manhattan', 'Brooklyn', 'Bronx', 'Queen', 'Staten Island')),
            uiOutput("special"),
            selectInput("room",
                        "Room Type:",
                        choices = unique(nyc_df$room_type)),
            radioButtons("graph",
                         "Graph type:",
                         choices = c("Area plot", "Line plot"),
                         selected = "Area plot")
        ),

        # Show a plot of the generated distribution
        mainPanel(
        )
    )
)

a <- unique(nyc_df[nyc_df$neighbourhood_group == 'Brooklyn', 6])
b <- unique(nyc_df[nyc_df$neighbourhood_group == 'Manhattan', 6])
c <- unique(nyc_df[nyc_df$neighbourhood_group == 'Queen', 6])
d <- unique(nyc_df[nyc_df$neighbourhood_group == 'Staten Island', 6])
e <- unique(nyc_df[nyc_df$neighbourhood_group == 'Bronx', 6])

output$special <- renderUI({
    switch(input$area,
            "Brooklyn" = selectInput("nhd", "Neighbourhood:",
                    choices = a),
            "Manhattan" = selectInput("nhd", "Neighbourhood:",
                    choices = b),
            "Queens" = selectInput("nhd", "Neighbourhood:",
                    choices = c), 
            "Staten Island" = selectInput("nhd", "Neighbourhood:",
                    choices = d),
            "Bronx" = selectInput("nhd", "Neighbourhood:",
                    choices = e))

})




# Define server logic required to draw a histogram
server <- function(input, output) {

    output$distPlot <- renderPlot({

        area_data <- nyc_df[nyc_df$neighbourhood_group == req(input$area), ]
        data <- area_data %>% filter(NHD == req(input$nhd),
                                     Room == req(input$room))
        barplot(data)
    })
}

# Run the application 
shinyApp(ui = ui, server = server)