闪亮 - 绘图函数变量中的表达式

Question

我是shiny的新手，我在renderPlot之类的文档中发现，第一个参数expr可以保存在变量中。 我很感兴趣是否可以将文本字符串解析为表达式，然后将其传递给渲染函数。

我做了一些测试，renderPrint:

code <- "x<-faithful[, 2]"
expr <- parse(text=code)
output$out1 <- renderPrint(expr)

输出显示x<-faithful[, 2]，如果我用eval 包装parse

code <- "x<-faithful[, 2]"
expr <- eval(parse(text=code))
output$out1 <- renderPrint(expr)

然后它正在打印数据框。

如果我传递一个表达式变量，renderPlot 函数对我不起作用，下面的代码将绘制一个直方图

output$distPlot <- renderPlot({
  x    <- faithful[, 2]  # Old Faithful Geyser data
  bins <- seq(min(x), max(x), length.out = 51)
  hist(x, breaks = bins, col = 'darkgray', border = 'white')
})

但是，如果我从字符串中解析相同的代码并保存为表达式，它就不起作用

ui.R

#
# This is the user-interface definition of a Shiny web application. You can
# run the application by clicking 'Run App' above.
#
# Find out more about building applications with Shiny here:
# 
#    http://shiny.rstudio.com/
#

library(shiny)

# Define UI for application that draws a histogram
shinyUI(fluidPage(

  # Application title
  titlePanel("Old Faithful Geyser Data"),

  # Sidebar with a slider input for number of bins 
  sidebarLayout(
    sidebarPanel(
       sliderInput("bins",
                   "Number of bins:",
                   min = 1,
                   max = 50,
                   value = 30)
    ),

    # Show a plot of the generated distribution
    mainPanel(
       plotOutput("distPlot")
    )
  )
))

服务器.R

#
# This is the server logic of a Shiny web application. You can run the
# application by clicking 'Run App' above.
#
# Find out more about building applications with Shiny here:
#
#    http://shiny.rstudio.com/
#

library(shiny)

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

  str<- c("x    <- faithful[, 2]  # Old Faithful Geyser data\n", "bins <- seq(min(x), max(x), length.out = 51)\n", "hist(x, breaks = bins, col = 'darkgray', border = 'white')")
  code <- paste(str, collapse = "")
  expr <- parse(text=code)
  print(code)
  print(expr)
  output$distPlot <- renderPlot(expr)
})

输出是空图像，我也试过用eval包裹表达式，输出是一样的

应用链接：https://jerryjin.shinyapps.io/expr/

问题

从变量渲染表达式的正确方法是什么？

谢谢！

Answer 1

你应该用 eval 包裹表达式并且它应该用大括号括起来，因为它是一个多行表达式。

试试这个：

output$distPlot <- renderPlot({eval(expr)})