selectinput - RShiny中用户选择输入的动态平均值

Question

我正在尝试计算用户使用选择的变量的平均值selectInput并将其输出到表格中。显然，问题应该来自as.numeric(input$var_dropdown). 任何想法将不胜感激！

library(rshiny)
library(dplyr)

ui <- shinyUI(fluidPage(
  sidebarLayout(
    sidebarPanel(
      uiOutput("select")
    ),
    mainPanel(
      DT::dataTableOutput("table_with_mean")
    )
  )
)
)

server <- function(input, output, session){
  output$select <- renderUI({
    selectInput("var_dropdown", "Variable:", names(iris))
  })
  
  output$table_with_mean <- DT::renderDataTable({
    iris %>% 
      summarise("Mean" = mean(as.numeric(input$var_dropdown)))
  })
  
}

shinyApp(ui, server)

错误：

Warning in mean(as.numeric(input$var_dropdown)) :
  NAs introduced by coercion