title: compute variance per row in data.table
body: <p>as it is said in the title, I want to compute the variance per row in my dataset, all columns are continuous :
  I have tried to use the rowVars function from the package matrixStats, but it doesn't work </p>
  
  <pre><code>x[, variance := rowVars(.SD), .SDcols=varQuant]
  </code></pre>
  
  <p>I have the following error :</p>
  
  <pre><code>Error in rowVars(.SD): Argument 'x' must be a matrix or a vector.
  </code></pre>