How to conditionally append a list using action buttons

[Andres Fonseca]

Hey everyone, I am successfully iterating through a list of lists using a 'yes' and 'no' action button. Like so...

inc <- reactive({input$yes + input$no + 1}) ##where input$yes and input$no are the buttons in question

The problem I'm having is that while (with some safety precautions) I can move through and display each elements of my list of lists 

by clicking either button, what I really want is to generate another list of list that represents those elements for which I clicked yes instead of no.

As a simplified example, imagine a page with a side bar that say 'is this letter a vowel?' and a button for yes and no underneath.

In the main panel we see the letters[[inc()]] (or 'a' initially). I want to be able to build a list of those elements I clicked yes for.

I hope that makes sense. Please let me know if you want me to elaborate on anything.

Thanks!

[Andres Fonseca]

Here is some sample code for the alphabet example

##############

SERVER.R

library(shiny)

shinyServer(function(input, output) {

  inc <- reactive({input$yes + input$no + 1}) 

  i <- reactive({ifelse(inc() <= 26, inc(), -1)})

  add <- reactive({

    if (input$yes == 0) return(invisible())

    isolate({ letters[i()] })

  })

  alphabet <- reactive({

    if (i() < 0) return('No more letters') 

    else return(letters[i()] )

  })

  

  output$testing <- renderPrint({ alphabet() })

  output$lastAdd <- renderPrint({ add() })

  

})

###########

UI.R

library(shiny)

shinyUI(fluidPage(

  titlePanel("Iteratively build a list?"),

  sidebarLayout(

    sidebarPanel(

      h3('Is this a vowel?'),

      actionButton("yes", label = "Yes"),

      actionButton("no",  label = "No" )

      

    ),

    mainPanel(

      verbatimTextOutput('lastAdd'),

      verbatimTextOutput('testing')

    )

  )

))

What I want is to have an object that gains a letter every time I click yes. I've been doing some research into reactiveValues but can't 

seem to figure out how to append anything to them.

Hope this clarifies my question.

[Joe Cheng]

In your server function (note, I haven't tested this code but it seems like it should work):

values <- reactiveValues(answers = logical(0))

observe({

  if (input$yes == 0) return()

  isolate({

    values$answers <- c(values$answers, TRUE)

  })

})

observe({

  if (input$no == 0)

    return()

  isolate({

    values$answers <- c(values$answers, FALSE)

  })

})

Then refer to values$answers wherever you want to get at the answers. (Unlike with a reactive expression, you don't need to do values$answers(), just values$answers will do--like an input.)

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