title: How to use shiny conditional panel
body: <p>Hi currently I've got a sample data set like this</p>
  
  <pre><code> df&lt;- data.frame(site = c('A1', 'A1', 'A1', 'B1', 'B1','B1','B1','C1','C1'), 
                   part = c('a1', 'a1', 'a2', 'b1', 'b1', 'b2','b3','c1','c2'),
                   value = c(2,3,4,5,6,7,8,9,10))
  
  &gt; df
  site part value
    A1   a1     2
    A1   a1     3
    A1   a2     4
    B1   b1     5
    B1   b1     6
    B1   b2     7
    B1   b3     8
    C1   c1     9
    C1   c2    10
  </code></pre>
  
  <p>I would like to put a side bar in shiny user interface with two select boxes. One is "choose" site and the other one is "part".</p>
  
  <pre><code>shinyUI(
  
  fluidPage(    
  
  titlePanel("Choose Site, part"),
  
  sidebarLayout(      
  
  
    sidebarPanel(
      selectInput("select_site", label = "Select Site", 
                  choices = unique(df$site), 
                  selected = unique(df$site)[1]),
  
        selectInput("select_part", label = "Select Part", 
                    choices = unique(df$part), 
                    selected = unique(df$part)[1])
  
    ),
  
    mainPanel(
      plotOutput("example")  
    )
  
  )
  )
  )
  </code></pre>
  
  <p>The objective is when I choose A1 in "select_site", I can only choose a1 or a2 in "select_part". However this code right now can choose a1, a2, b1, b2, b3, c1, c2 in "select_part" no matter how I choose in "select_site".</p>
  
  <p>In the real data set there are a lot more sites and parts than this example so I'm looking for a generic way to identify the parts within each site rather than type in the names myself.</p>
  
  <p>Can anyone help me with some modification of this code?</p>
  
  <p>Thanks in advance</p>