title: problems with select input in r shiny leaflet app
body: <p>I am using <em>R 3.2.3 through RStudio Version 0.99.491, on Windows 10 64bit</em>... I am creating a <strong><em>leaflet shiny</em></strong> app, using graduated <strong><em>circlemarkers</em></strong>.  I want to display different months to show the change in data using <strong><em>selectInput()</em></strong>, but i don't know how to connect it to the 'radius =' argument of <strong><em>addCirclemarker()</em></strong> to make it dynamic.  I know I'm just making it up with the 'radius =' argument of <strong><em>addCirclemarker()</em></strong> but I can't tell if I have <strong><em>selectInput()</em></strong> wrong too. here's the <a href="https://docs.google.com/spreadsheets/d/13Dz_vBKN0l5m503RN7RlkA2OrxcFXXZZ9UJY2tXSo8Y/edit?usp=sharing" rel="nofollow">data</a> I'm using. The result shows no error message and the map worked when the radius argument was given a single column assignment, ie a static map.</p>
  
  <p>ui.r:</p>
  
  <pre><code>library(shiny)
  library(leaflet)
  
  shinyUI(fluidPage(
  titlePanel("CAT Rider Count Map"),
  sidebarLayout(
  sidebarPanel(
    selectInput("var", label = "1. Select the Month", 
                choices = c("April" = 3, "May" = 4, "June" = 5),
                selected = 4)),
  mainPanel(leafletOutput('crossact.map')
  
  ))))
  </code></pre>
  
  <p>server.r</p>
  
  <pre><code>library(shiny)
  library(googlesheets)
  library(leaflet)
  gs_auth()
  ttt &lt;- gs_auth()
  saveRDS(ttt, "ttt.rds")
  gs_auth(token = ttt)
  gs_auth(token = "ttt.rds")
  crossact &lt;- gs_title("crossact")
  crossact &lt;- crossact%&gt;% gs_read_csv()
  
  
  
  shinyServer(
    function(input, output, session){
  
  colm &lt;- reactive({
    as.numeric(input$var)
  })
  
  output$crossact.map &lt;-  renderLeaflet({
  ################################################################## 
  #RADIUS SECTION  
  ##################################################################  
    crossact.map &lt;- leaflet(crossact) %&gt;% 
      addTiles('http://{s}.basemaps.cartocdn.com/dark_all/{z}/{x}/{y}.png')
    crossact.map%&gt;% setView(-71.43381, 42.48649, zoom = 17)
    crossact.map %&gt;% ***addCircleMarkers(~lng, ~lat, popup=~crossact$name, weight =1,
      radius=~(crossact[,colm()]), 
                                      color="#ffa500", stroke = TRUE,     fillOpacity = 0.6)
  })
  })
  </code></pre>
  
  <p>thanks!</p>