How to Store Monte Carlo Simulation Outputs Within a reactive() Function In Shiny

Question

I have been working on a side project that involves a simple shiny app that allows users to input the parameters of a dice roll for a board game then have the code preform 10,000 rolls with those parameters and present the average of the rolls. I have a basic code that successfully makes this happen but I am struggling how to make it into a shiny app to make accessible to others.

The issue I face is that in the server part of the shiny code I do not know how to store the intermediate results within a single reactive() function. Is there a local storage option that works with a repeating function?

The code I am using is:

# Define UI for application that draws a histogram
ui <- fluidPage(

   # Application title
   titlePanel("10,000 Roll Simulator"),

   # Sidebar with a slider input for number of bins 
   sidebarLayout(
      sidebarPanel(
         numericInput(inputId = "num_tac", label = "TAC", 
                      value =1 , min = 1, max = 20),
         numericInput(inputId = "num_def", label = "DEF", 
                      value =1 , min = 1, max = 10),
         numericInput(inputId = "num_arm", label = "ARM", 
                      value =0 , min = 0, max = 10)
      )
   )
)

server <- function(input, output){
 data()<- reactive({
   One_roll<- function(){dice <- sample(1:6, size = input$num_tac, replace = TRUE)
   return(sum((dice >= input$num_def)-input$num_arm))
   sims<-replicate(10000, One_roll()}
output$stats <- renderPrint({mean(data())})
}
# Run the application 
shinyApp(ui = ui, server = server)

Any help would be greatly appreciated, thank you!

Answer 1

A few issues with your code :

• data()<- is not allowed. Use data<- instead then call it with data()

• Using input$ inside a function is definitely not the right way to pass parameters

This is a modified server function where the One_roll function is defined outside the reactive, but called inside, with input passed as parameters:

server <- function(input, output){
  One_roll<- function(num_tac,num_def,num_arm){
    dice <- sample(1:6, size = num_tac, replace = TRUE)
    sum((dice >= num_def)-num_arm)
  }
  data<- reactive(replicate(10000, One_roll(input$num_tac,input$num_def, input$num_arm )))
  output$stats <- renderPrint(mean(data()))
}

And also you need a textOutput in the ui function to call the renderText for example:

ui <- fluidPage(

  # Application title
  titlePanel("10,000 Roll Simulator"),

  # Sidebar with a slider input for number of bins 
  sidebarLayout(
    sidebarPanel(
      numericInput(inputId = "num_tac", label = "TAC", 
                   value =1 , min = 1, max = 20),
      numericInput(inputId = "num_def", label = "DEF", 
                   value =1 , min = 1, max = 10),
      numericInput(inputId = "num_arm", label = "ARM", 
                   value =0 , min = 0, max = 10)
    ), mainPanel = textOutput("stats")
  )
)