How to create IF statement with reactive values in R ( Shiny )

Question

Beginner to R and shiny here! Tried to make a minimal working example... I want to check a condition on a reactive input value. What am I doing wrong?

library(shiny)

ui<-fluidPage(

  numericInput(inputId="a", label=NULL, value=0),
  textOutput(outputId="out")
)

server <- function(input, output) {
  x <- reactive(input$a)
  if (x() < 4) 
    {y<-1}
  else
  {y<-0}

  output$out <- renderText({y})
}

shinyApp(ui = ui, server = server)

The error message:

Operation not allowed without an active reactive context. (You tried to do something that can only be done from inside a reactive expression or observer.)

As the message says, you can only use x() within a reactive context (for example, inside observe, local, render.... functions). Also you y value must be defined outside the function and in the same level or upper level of where it is used. — Xiongbing Jin
– Xiongbing Jin, Commented Jun 8, 2016 at 18:47
@warmoverflow : So there is no way to check a condition on a reactive value? Sorry super new to R — Lee88
– Lee88, Commented Jun 8, 2016 at 18:50
You can but must be inside one of those context. For example you can check it right inside renderText — Xiongbing Jin
– Xiongbing Jin, Commented Jun 8, 2016 at 18:51
BTW input$a is already reactive by itself so you don't need to enclose it with reactive and assign to a new variable. — Xiongbing Jin
– Xiongbing Jin, Commented Jun 8, 2016 at 18:52
@warmoverflow : I see. That's unfortunate. This was a minimal working example for trying to do a large amount of computation outside of renderOutputs, because my real code is (1) much longer, and (2) has multiple outputs based on y, and recomputing it inside each renderOutput is really slow. Thank you — Lee88
– Lee88, Commented Jun 8, 2016 at 18:55

timelyportfolio · Accepted Answer · 2016-06-08 19:16:55Z

12

You just need to use reactive with your if so that shiny knows that y changes when x does.

library(shiny)

ui<-fluidPage(

  numericInput(inputId="a", label=NULL, value=0),
  textOutput(outputId="out")
)

server <- function(input, output) {
  x <- reactive(input$a)
  y <- reactive( if (x()<4) 1 else 0 )

  output$out <- renderText({ y() })
}

shinyApp(ui = ui, server = server)

edited Jun 8, 2016 at 19:16

timelyportfolio

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answered Jun 8, 2016 at 18:53

John Paul

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1 Comment

Lee88 Over a year ago

Ah wow great! Assigning the entire IF statement to a variable is not intuitive to me, but you were right.

timelyportfolio · Accepted Answer · 2016-06-08 19:29:51Z

8

The answer above from John Paul is certainly acceptable, but I thought you might like to see another way as a part of your learning process. I will let StackOverflow sort out which is more advisable.

library(shiny)

ui<-fluidPage(

  numericInput(inputId="a", label=NULL, value=0),
  textOutput(outputId="out")
)

server <- function(input, output) {
  state <- reactiveValues()

  observe({
    state$x <- input$a
    state$y <- ifelse(state$x < 4, 1, 0)
  })

  output$out <- renderText({ state$y })
}

shinyApp(ui = ui, server = server)

answered Jun 8, 2016 at 19:29

timelyportfolio

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3 Comments

John Paul Over a year ago

Do you know if there is a reason why using the reactiveValues would be preferable here? I get the impression that the RStudio folks like this better but I don't fully understand what the advantage might be.

timelyportfolio Over a year ago

I watched the videos from the conference, and I adopted the approach, but unfortunately don't know why. I'm about to really dig into the shiny internals, so maybe then I will figure it out. In terms of a big complicated project, I think the reactiveValues approach simplifies some of the code. I am hoping others on StackOverflow will enlighten us.

Lee88 Over a year ago

@timelyportfolio Oh I like this! I'll have to play around with reactiveValues() and Observe() but it seems much easier, not having to keep track of which values are reactive, and making sure all downstream variables are assigned with reactive()

travismark · Accepted Answer · 2016-06-09 22:25:20Z

4

here's my attempt. 1) as stated, you don't need to wrap input$a in reactive context and save as x. just use input$a 2) you don't need reactiveValues in this simple example. just save y as a reactive variable. then, in the renderText, access by calling the function ("y()")

library(shiny)

ui<-fluidPage(

  numericInput(inputId="a", label=NULL, value=0),
  textOutput(outputId="out")
)

server <- function(input, output) {

  y <- reactive({
    if (input$a < 4) {
      return(1)
    } else {
      return(0)
    }
  }
  )

  output$out <- renderText({y()})
}

shinyApp(ui = ui, server = server)

answered Jun 9, 2016 at 22:25

travismark

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