title: Interconnect slider and numeric input in Shiny
body: <p>I want a slider for quick, visual input, but also a corresponding numeric input for exact calculations. I've managed to tie the slider <code>value</code> to <code>input$</code>, so they will follow whatever is written in the numeric inputs. However, I can't seem to make it work the other way around.</p>
  
  <p>ui</p>
  
  <pre><code>    numericInput("num_l",
                   label = "Beam length in m.",
                   value = 10),
  
      numericInput("num_a",
                   label = "Choose position, where to apply force, starting from left, in m.",
                   value = input$slider_a), # THIS DOESN'T WORK
  
      numericInput("num_x",
                   label = "Calculate the deflection, at position starting from left, in m.",
                   value = input$slider_x), # NEITHER DOES THIS
  </code></pre>
  
  <p>server</p>
  
  <pre><code>output$slider &lt;- renderUI({
      tagList( # Need this for multiple reactive sliders
          sliderInput("slider_a",
                      label = "Load force position:",
                      min = 0,
                      max = input$num_l,
                      value = input$num_a, # THIS WORKS
                      step = 0.1),
  
          sliderInput("slider_x",
                      label = "Deflection calculation position:",
                      min = 0,
                      max = input$num_l,
                      value = input$num_x, # THIS ALSO WORKS
                      step = 0.1)
      )
  })
  </code></pre>