Add columns to a reactive data frame in Shiny and update them

Question

I would like to be able to calculate a new column of data based on dividing one column by another, with both original columns selected by a user input. I would like to have

Answer 1

You don't return anything in your reactive for selectedData2 you just do an incrementation <-, I think you should do this :

   function(input, output, session) {

  # Combine the selected input variables into a new data frame
  selectedData <- reactive({
    return(iris[, c(input$xcol, input$ycol),])
  })


  # divide one variable selection by the other
  selectedData2 <- reactive({
            new<-iris[, c(input$xcol)]/iris[, c(input$ycol)]
            return(new)

    })

  # create data output for selected variables
  output$view <- renderTable({selectedData()
  })

  # create data output for calculated variable
  output$view2 <- renderTable({selectedData2()
  })

}

Answer 2

You forget that iris is NOT a reactive element, so your code can't work. You have two options here:

• creating a reactive value to store that data frame, using reactive().
• creating a list of "global" reactive values in which you can store the updated data frame, using reactiveValues().

Using global reactive expressions with reactiveValues

Using reactiveValues you can make a list of reactive expressions much like input and output are. In the example below I use it to store the data frame iris as globals$mydf. Then you can use eg observe to change the value reactively, as in the following server function:

#Server
server <- function(input, output, session) {

  globals <- reactiveValues(
    mydf = iris
  )

  observe({
    thedf <- globals$mydf
    newvar <- thedf[[input$xcol]] / thedf[[input$ycol]]
    globals$mydf$ratio <- newvar
  })


  # create data output for selected variables
  output$view <- renderTable({
    globals$mydf
  })
}

using reactive()

You can make two reactive expressions that depend on eachother:

• one that selects the variables and calculates that ratio
• one that combines this result with the selected variables

Your server would look like this :

server <- function(input, output, session) {

  newdf <- reactive({
    cbind(
      iris[c(input$xcol, input$ycol)],
      Ratio = newvar()
    )
  })

  newvar <- reactive({
    iris[[input$xcol]] / iris[[input$ycol]]
  })

  # create data output for selected variables
  output$view <- renderTable({
    newdf()
  })

}

Although you beliefe this is not what you're looking for, you can use newdf() in other code just like you would use globals$mydf in the previous example. reactiveValues() especially pays off if different parts of your code have to be able to change the data frame.