title: Shiny: How to make reactive value initialize with default value
body: <p>Consider the following actionButton demo: 
  <a href="http://shiny.rstudio.com/gallery/actionbutton-demo.html" rel="noreferrer">http://shiny.rstudio.com/gallery/actionbutton-demo.html</a></p>
  
  <p><strong>server.R:</strong></p>
  
  <pre><code>shinyServer(function(input, output) {
  
    # builds a reactive expression that only invalidates 
    # when the value of input$goButton becomes out of date 
    # (i.e., when the button is pressed)
    ntext &lt;- eventReactive(input$goButton, {
      input$n
    })
  
    output$nText &lt;- renderText({
      ntext()
    })
  })
  </code></pre>
  
  <p><strong>ui.R:</strong></p>
  
  <pre><code>shinyUI(pageWithSidebar(
    headerPanel("actionButton test"),
    sidebarPanel(
      numericInput("n", "N:", min = 0, max = 100, value = 50),
      br(),
      actionButton("goButton", "Go!"),
      p("Click the button to update the value displayed in the main panel.")
    ),
    mainPanel(
      verbatimTextOutput("nText")
    )
  ))
  </code></pre>
  
  <p>In this example, prior to the action button being pressed, the right-hand side panel is empty. I would instead like the text with default value "50" to be rendered by default.</p>
  
  <p>How to I get the output to display with default inputs if the action button has not yet been pressed? </p>