Displaying an updated variable after clicking an action button in Shiny

Question

I'm new to shiny and trying to accomplish rather a simple task using an action button:

• User clicks a button and a function is called
• This function does some calculations using input variables and updates/creates several global variables (reactiveValues, probably inside an observe block?)
• I'd like to display those values back on the UI (using render* function)
• Whenever user changes input values, the UI is automatically updated

Relevant code bits are:

server.R

...
rv <- reactiveValues() 
observe({
 if(input$run){
  rv$a <- someFunc(input$aa)
 }
})
output$msg = renderText({ rv$a })
...

ui.R

...
selectInput("aa", ...)
...
actionButton("run", "Run")
...
textOutput("msg")

How can I change msg based on the input aa each time user clicks the button?

Answer 1

I am not convinced I understood what you want, but I imagine it to be something like this:

library(shiny)
u <- fluidPage(
  titlePanel("Simple Selectable Reactive Function"),
    sidebarLayout(
      sidebarPanel(
        sliderInput("vv", "Choose a value",min=-3.14,max=3.14,value=0),
        selectInput("aa", "Choose a function", choices=c("sin","cos","exp")),
        actionButton("run", "Change Function and Run")
      ),
     mainPanel(
      h2("Results"),
      verbatimTextOutput("msg")
 )))
s <- function(input,output){

  rv <- reactiveValues(func=NULL) 

  observeEvent(input$run,{   rv$func <- input$aa })

  funcval <- reactive({
    v <- 0
    if (rv$func=="sin") v <- sin(input$vv)
    if (rv$func=="cos") v <- cos(input$vv)
    if (rv$func=="exp") v <- exp(input$vv)
    v
  })
  output$msg = renderPrint({
       if (is.null(rv$func)) return("not running")
       fv <- funcval()
       sprintf("%s(%.3f)=%.3f",rv$func,input$vv,fv)
    })
}
shinyApp(ui=u,server=s)

Yielding this:

Note that the slider input value formats its current value rather badly when the min and max values are not even. Not sure what one can do about this.