shiny - expression in a variable for plot functions

Question

I'm new to shiny, I found in document like renderPlot, the first parameter expr can be saved in a variable. I'm interested if it's possible to parse text string to an expression and then pass it to render functions.

I did some test, with renderPrint:

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

the output is displaying x<-faithful[, 2], and if I wrap parse with eval

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

then it's printing the data frame.

And the renderPlot function is not working for me if I pass an expression variable, below code will plot a histagram

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')
})

However, if I parse the same code from string and save as expression, it doesn't work

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")
    )
  )
))

server.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)
})

the output is a empty image, I also tried wrapping expression with eval, it's the same output

app link: https://jerryjin.shinyapps.io/expr/

Question

What is the right way to render expression from variable?

thank you!

You only parsed the string, you didn't eval() it in your Shiny code. But really [eval/parse](https://stackoverflow.com/questions/13649979/what-specifically-are-the-dangers-of-evalparse) should be avoided. There are almost always more "R-like" ways to do things. It would be better to describe what problem you are "really" trying to solve here — MrFlick, Jun 02 '17 at 13:53
@MrFlick, i know this is weird question, i'm trying to load code from file and render them dynamically — jerry, Jun 02 '17 at 14:02

score 1 · Accepted Answer · answered Jun 02 '17 at 13:51

1

You should wrap the expression with eval and it should be in curly braces as it is a multi-line expression.

Try this:

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

answered Jun 02 '17 at 13:51

SBista

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thank you @SBista, you saved my day, it works great! – jerry Jun 02 '17 at 14:00

shiny - expression in a variable for plot functions

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