title: Use mutate_at to change multiple column types
body: <p>I'm trying to use <code>dplyr</code> to tidy a dataset. The columns I want to change have a character string that's really a double but with comma instead of a decimal point. So far I got this:</p>
  
  <pre><code>presupuesto_2016 &lt;- read_csv( "http://datos.gob.ar/dataset/89f1a2dd-ad79-4211-87b4-44661d81ac0d/resource/84e23782-7d52-4724-a4ba-2f9621fa5f4e/download/presupuesto-2016.csv")
  
  names(presupuesto_2016) &lt;- str_replace(names(presupuesto_2016), "\uFEFF", "")
  
  presupuesto_2016 %&gt;%
    mutate_at(starts_with("monto_"),
              str_replace, pattern = ",", replacement = "\\.") %&gt;% 
    mutate_at(starts_with("monto_"), funs(as.numeric))
  </code></pre>
  
  <p>But this manages to change every column to numeric. What am I doing wrong here?</p>