title: R Shiny: Reactive Error
body: <p>I'm building my first Shiny app with the intent of creating a mortgage calculator and adjustable amoritization schedule.  I am able to get the following code to render with runApp(), but it is not functional (i.e., doesn't output any values, nor display the graph). Further, it generates the following error in RStudio's console:</p>
  
  <p>"Error in .getReactiveEnvironment()$currentContext() : 
    Operation not allowed without an active reactive context. (You tried to do something that can only be done from inside a reactive expression or observer.)"</p>
  
  <p>For background, I'm running:
  Win 7, 64-bit OS | R v3.1.1 | RStudio v0.98.944</p>
  
  <p>And have tried implementing the procedures defined here with no luck:
  <a href="https://stackoverflow.com/questions/17002160/shiny-tutorial-error-in-r">Shiny Tutorial Error in R</a>
  <a href="https://stackoverflow.com/questions/24960407/r-shiny-numeric-input-without-selectors">R Shiny - Numeric Input without Selectors</a></p>
  
  <p>ui.R</p>
  
  <pre><code>library(shiny)
  shinyUI(
        pageWithSidebar(
        headerPanel(
              h1('Amoritization Simulator for Home Mortgages'), 
              windowTitle = "Amoritization Simulator"
        ),
        sidebarPanel(
              h3('Mortgage Information'),
              h4('Purchase Price'),
              p('Enter the total sale price of the home'),
              textInput('price', "Sale Price ($USD)", value = ""),
              h4('Percent Down Payment'),
              p('Use the slider to select the percent of the purchase price you 
                intend to pay as a down payment at the time of purchase'),
              sliderInput('per.down', "% Down Payment", value = 20, min = 0, max = 30, step = 1),
              h4('Interest Rate (APR)'),
              p('Use the slider to select the interest rate of the loan expressed
                as an Annual Percentage Rate (APR)'),
              sliderInput('apr', "APR", value = 4, min = 0, max = 8, step = 0.125),
              h4('Term Length (Years)'),
              p('Use the buttons to define the term of the loan'),
              radioButtons('term', "Loan Term (Years)", choices = c(15, 30), selected = 30),
              submitButton('Calculate')
              ),
        mainPanel(
              h3('Payment and Amoritization Simulation'),
              p('Use this tool to determine your monthly mortgage payment, 
                how much interest you will owe over the life of the loan, and how 
                you can reduce that amount with additional payment'),
              h4('Monthly Payment (Principal and Interest)'),
              p('This is the amount (in $USD) you would pay each month for a 
                mortgage under the terms you defined'),
              verbatimTextOutput("base.monthly.payment"),
              h4('Total Interest Over Life of Loan'),
              p('If paying just that amount per month, this is the total amount 
                in $USD you will spend on interest for that loan'),
              verbatimTextOutput("base.total.interest"),
              h4('Additional Principal Simulation'),
              p('One way to reduce the interest expense is to pay more principal 
                each month. Use the slider below to select an additional amount to
                include with your payment and see the reduction in interest expense
                for the life of the loan.'),
              sliderInput('add', "Additional Principal ($USD)", value = 250, min = 0, max = 1000, step = 25),
              p('Interest costs saved with this additional principal (in $USD)'),
              verbatimTextOutput("savings"),
              p('You will also pay the loan off the loan this many months early'),
              verbatimTextOutput("early"),
              plotOutput('plot')
              )
        )
  )
  </code></pre>
  
  <p>server.R</p>
  
  <pre><code>library(shiny)
  library(ggplot2)
  library(scales)
  shinyServer(
  function(input, output) {
  ## determine baseline payment and interest total
  price &lt;- reactive({as.numeric(input$price)})
  per.down &lt;- reactive({input$per.down / 100})
  int &lt;- reactive({input$apr / 1200})
  n &lt;- reactive({input$term * 12})
  base.monthly.payment &lt;- (int() * price() * (1 - per.down()) * ((1 + int())^n())) / (((1 + int())^n()) - 1)
  output$base.monthly.payment &lt;- renderPrint({base.monthly.payment})
  base.total.interest &lt;- (base.monthly.payment * n()) - (price() * (1 - per.down()))
  output$base.total.interest &lt;- renderPrint({base.total.interest})
  ## create dataframe to populate with increments of additional payment
  schedule &lt;- data.frame(matrix(data = NA, nrow = 41, ncol = 6, 
                           dimnames = list(1:41, c("add", "add.n",
                                                  "prin", "add.total.interest", 
                                                  "savings", "early"))))
  ## initialize 'for' loop to populate possible amoritization totals
  c &lt;- 1
  for (i in seq(0, 1000, 25)) {
        schedule$add[c] &lt;- i
        schedule$add.n[c] &lt;- log(((base.monthly.payment + i) / int()) / (((base.monthly.payment + i) / int()) - (price() * (1 - per.down())))) / log(1 + int())
        schedule$prin[c] &lt;- round(price() * (1 - per.down()), digits = 2)
        schedule$add.total.interest[c] &lt;- round(((base.monthly.payment + i) * schedule$add.n[c]) - schedule$prin[c], digits = 2)
        schedule$savings[c] &lt;- round(base.total.interest - schedule$add.total.interest[c], digits = 2)
        schedule$early[c] &lt;- round(n() - schedule$add.n[c], digits = 0)
        c &lt;- c + 1
  }
  add &lt;- reactive({input$add})
  output$savings &lt;- renderPrint({schedule$savings[which(schedule$add == add())]})
  output$early &lt;- renderPrint({schedule$early[which(schedule$add == add())]})
  ## create data.frame suitable for plotting
  graph.data &lt;- data.frame(matrix(data = NA, nrow = 82, ncol = 3, 
                                  dimnames = list(1:82, c("add", "amount", "type"))))
  c &lt;- 1
  for (i in seq(0, 1000, 25)) {
        graph.data$add[c] &lt;- i
        graph.data$add[c + 1] &lt;- i
        graph.data$amount[c] &lt;- schedule$prin[which(schedule$add == i)]
        graph.data$amount[c + 1] &lt;- schedule$add.total.interest[which(schedule$add == i)]
        graph.data$type[c] &lt;- "Principal" 
        graph.data$type[c + 1] &lt;- "Interest"
        c &lt;- c + 2
  }
  ## create plot of amoritization with line for additional principal amount
  output$plot &lt;- renderPlot({
  ggplot(graph.data, aes(x = add, y = amount), color = type)
  + geom_area(aes(fill = type), position = 'stack', alpha = 0.75)
  + geom_vline(xintercept = add(), color="black", linetype = "longdash", size = 1)
  + labs(x = "Additional Principal/Month", y = "Total Cost")
  + scale_fill_manual(values=c("firebrick3", "dodgerblue3"), name = "Payment Component")
  + theme(axis.title.x = element_text(face = "bold", vjust = -0.7, size = 16), 
          axis.title.y = element_text(face = "bold", vjust = 2, size = 16),
          axis.text.x = element_text(size = 14), 
          axis.text.y = element_text(size = 14), 
          panel.margin = unit(c(5, 5, 5, 5), "mm"),
          plot.margin = unit(c(5, 5, 5, 5), "mm"),
          panel.background = element_blank(),
          panel.grid.major.y = element_line(colour = "gray"),
          panel.grid.minor.y = element_line(colour = "gray86"),
          panel.grid.major.x = element_blank(),
          panel.grid.minor.x = element_blank())
  + scale_x_continuous(labels = dollar)
  + scale_y_continuous(labels = dollar)
  })
  })
  </code></pre>
  
  <p>Thanks in advance for any help!</p>