title: How to get value of selectinput and show a variable data in another selectinput with shiny
body: <p>I want to retrieve the selected value by the user, so i put this code in server.R inside the function </p>
  
  <pre><code>selected_value &lt;- input$valtext
  A &lt;-Filter (catalog, gene_name == selected_value)
  B &lt;-select (A,snp)
  </code></pre>
  
  <p>valtext : is the id of my selectizeinput </p>
  
  <p>then i want to show the snp correspondent of selected_value by the user 
  (B) in another selectizeinput in "choices", but to do this i need to retrieve the variable B : </p>
  
  <pre><code>selectizeInput( 
    'snpvalue', 'Choisir le snp a visualiser :', choices = "" ,
    multiple = TRUE, options = list(maxItems = 2)
        )
  </code></pre>
  
  <p>My problem is i can not see B in the data, it seems nothing change !nothing execute, because if i will have my variable B, i can easily show it in my selectizeinput </p>