readline equivalent behavior in Shiny

[KrzyGajow]

Let's consider the following function:

foo <- function(n){
  out <- double(n)
  for( i in 1:n ){
    # Here an external algorithm is run. We need to make a decision regarding its possible outputs.
    ANSWER <- readline("Provide number and press Enter: ")
    out[i] <- as.numeric( ANSWER )
  }
  return(out)
}

When providing 5 as an input, inner loop has 5 iterations. Each time the user has to provide a particular number. As a result we would have a vector with 5 elements. The question is how to emulate this behaviour in Shiny. My suggestion is below:

foo <- function(input, session){
  out <- double(input$n)
  for( i in 1:input$n ){
    # Force to stop a loop
    # Provide a decision
    out[i] <- ( input$decision )
    # Press Enter
  }
  return(out)
}

ui <- fluidPage(
  numericInput( "n", "nIter", value = 5, min = 1, max = 10, step = 1),
  numericInput( "decision", "Provide number and press Enter", value = 1, min = 1, max = 10, step = 1),
  actionButton("enter","Enter"),
  verbatimTextOutput("out")
)

server <- function(input, output, session){
  out <- reactive({
    input$n
    foo( input, session )
  })
  output$out <- renderPrint({
    out()
  })
}

shinyApp(ui = ui, server = server)

The problem is that, now an entire function is executed with all iterations and output vector has the same values.

Best
Chris

[nirgrahamuk]

I recommend you study how to make dynamic ui, uiOutput()/renderUI
The solution would be to use input$n to produce n number of numericInputs dynamically.

[KrzyGajow]

Thank you for the answer. However it is an important requirement to behave like readline. Readline allows to communicate with the foo function while running. I have to provide a number and use this value during the next iteration, without escaping the function. I have an external foo function and I need to put the input from the Shiny into a certain state of my function. Maybe there is a functionality forces the user to make a decision, i.e. observeEvent() and eventReactive() are triggered by input$click or changes in reactiveValue(). I need to observe an event inside my function (to be precise force to observe), I need to stop the current iteration and wait for the decision.