Use dynamically generated column names in Shiny - r

Question

I have a Shiny app which contains two dataframes.
The user presses a button which creates new columns in the dataframe
I have a further selectInput box which I want to populate with the names of the dataframe columns as they appear in the Shiny app (ie if the user have pressed the button then the SlectInput box would contain the new column names).
My attempt so far:
---
title: "GUI"
output: html_document
runtime: shiny
---
```{r, echo=FALSE}
library(EndoMineR)
RV <- reactiveValues(data = mtcars)
shinyApp(
ui = fluidPage(
selectInput("variable", "Variable:",
colnames(RV$data)),
tableOutput("data")
),
server = function(input, output) {
observeEvent(input$doExtractor, {
mtcars$cyl2<-mtcars$cyl*10
})
}
)
```
but I get the error:
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.)

Answer 1

besides what MLavoie already mentioned, you want to use updateSelectInput for things like this. Below is working example.
shinyApp(
ui = fluidPage(
column(
width = 3,
selectInput(
inputId = "variable",
label = "Variable:",
choices = colnames(mtcars)),
actionButton(
inputId = "doExtractor",
label = "do Extractor"
)
),
column(
width = 9,
dataTableOutput("data")
)
),
server = function(input, output,session) {
RV <- reactiveValues(data = mtcars)
observeEvent(input$doExtractor, {
browser()
RV$data$cyl2<-mtcars$cyl*10
})
observe(
updateSelectInput(
session = session,
inputId = "variable",
choices = colnames(RV$data),
selected = input$variable # only necessary if you want to keep the selection
)
)
output$data <- renderDataTable({
browser()
DT::datatable(RV$data)
})
}
)
Hope this helps!