title: calculate median from data.table columns in R
body: <p>I am trying to calculate a median value across a number of columns, however my data is a bit funky. It looks like the following example.</p>
  
  <pre><code>library(data.table)
  
  dt &lt;- data.table("ID" = c(1,2,3,4),"none" = c(0,5,5,3), 
                   "ten" = c(3,2,5,4),"twenty" = c(0,2,3,1))
  
  
     ID none ten twenty
  1:  1    0   3      0
  2:  2    5   2      2
  3:  3    5   5      3
  4:  4    3   4      1
  </code></pre>
  
  <p>In the table to column represents the number of occurrences of that value. I am wanting to calculate the median occurrence.</p>
  
  <p>For example for ID = 1</p>
  
  <pre><code>median(c(10, 10, 10))
  </code></pre>
  
  <p>is the calculation I am wanting to create.</p>
  
  <p>for ID = 2</p>
  
  <pre><code>median(c(0, 0, 0, 0, 0, 10, 10, 20, 20))
  </code></pre>
  
  <p>I have tried using <code>rep()</code> and <code>lapply()</code> with very limited success and am after some clear guidance on how this might be achieved. I understand for the likes of <code>rep()</code> I would be having to hard code my value to be repeated (e.g. <code>rep(0,2)</code> or <code>rep(10,2)</code>) and this is what I expect. I am just struggling to create a list or vector with the repetitions from each column.</p>