Printing output based on dynamic user input in R Shiny

Question

I am quite new to R shiny and so have not been able to figure out the solution from similar questions posted on this site. I am trying to read and use the input that a user provides to R shiny to generate an output.

I am trying to create a simple GUI where a user selects the name of a person (from a drop-down menu) and then enters his/her weight. If the height is above a certain threshold the output recommendation is "Gain Weight", else it is "Loose Weight".

Everything seems to be working fine, except for the following error from the Server.R file:

 Error in `$.shinyoutput`(output, value_weight) : 
 Reading objects from shinyoutput object not allowed

How can I read and use the variable 'value_weight' in an if-then-else condition?

Main.R

library(shiny)
runApp()

Server.R

function(input, output) {

 # You can access the value of the widget with input$select, e.g.
 output$value_name   <- renderPrint({ input$select })
 output$value_weight <- renderPrint({ input$num })

 if(output$value_weight > 150)
 {
   output$value_recommendation <- "Loose Weight"
 }
 else{
   output$value_recommendation <- "Gain Weight"
 }

 }

UI.R

 names_list <- list("Adam", "Jenna","Peter")

 fluidPage(
 selectInput("select", label = h3("Select Name"), choices = names_list, selected = 1),

 hr(),
 fluidRow(column(3, verbatimTextOutput("value_name"))),
 numericInput("num", label = h3("Enter Weight"), value = 0),

 hr(),
 fluidRow(column(3, verbatimTextOutput("value_weight"))),

 hr(),
 fluidRow(column(3, verbatimTextOutput("value_recommendation")))

   )

Gregor de Cillia · Accepted Answer · 2017-10-11 15:03:08Z

The problem in your code is the line

if(output$value_weight > 150)

Generally speaking, outputs are write-only objects in the server, while inputs are readonly. If you replace output$value_weight with input$num, everything should work fine. You also need to use a render-function for outputs: in this case renderPrint or renderText (see the documentation for the difference between those two render functions).

## server.R
function(input, output) {
  # You can access the value of the widget with input$select, e.g.
  output$value_name   <- renderPrint({ input$select })
  output$value_weight <- renderPrint({ input$num })

  output$value_recommendation <- renderPrint({
    if(input$num > 150)
      "Loose Weight"
    else
      "Gain weight"
  })
}

Another way to do this is using a call to the function reactive

## server.R
function(input, output) {
  # You can access the value of the widget with input$select, e.g.
  output$value_name   <- renderPrint({ input$select })
  value_weight <- reactive({ input$num })
  output$value_weight <- renderPrint({ value_weight() })

  output$value_recommendation <- renderPrint({
    if(value_weight() > 150)
      "Loose Weight"
    else
      "Gain weight"
  })
}

Batool · Accepted Answer · 2017-10-09 17:40:43Z

0

Using 'renderText' solved the issue!

Server.R

function(input, output) 
{

  output$value_market <- renderPrint({ input$select })
  output$value_demand <- renderPrint({ input$num })


  output$value_recommendation <- renderText({
  if(input$num > 150)
  {
    print("Loose Weight")
  }
  else{
    print("Gain Weight")
  }
  })
}

answered Oct 9, 2017 at 17:40

Batool

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1 Comment

Gregor de Cillia Over a year ago

Oh! Just saw you solved it yourself. Hopefully, my answer gives more insight about why you encountered this specific error message.

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