title: Shiny: how to choose a dataframe as input in a Rmd document Shiny
body: <p>I'm having trouble choosing a dataframe variable in Shiny as input.</p>
  <pre><code>inputPanel(
    selectInput(&quot;a&quot;, label = &quot;N:&quot;,
                choices = c(1,2, 3, 4), selected = 1),
    selectInput(&quot;dataframe&quot;, label = &quot;dataframe&quot;,
                choices = c(A,B), selected = A)
  )
  </code></pre>
  <p>In renderPlot I have input$k working fine, as it is a simple numer.</p>
  <p>However, for input$dataframe it is not working.
  I am getting the error: <code>non-numeric argument to binary operator</code>.</p>
  <p>Thanks in advance for your help.</p>