title: Applying gsub to various columns
body: <p>What is the most efficient way to apply <code>gsub</code> to various columns?
  The following does not work</p>
  
  <pre><code>x1=c("10%","20%","30%")
  x2=c("60%","50%","40%")
  x3 = c(1,2,3)
  x = data.frame(x1,x2,x3)
  per_col = c(1,2)
  x = gsub("%","",x[,per_col])
  </code></pre>
  
  <p>How can I most efficiently drop the "%" sign in specified columns.
  Can I apply it to the whole dataframe? This would be useful in the case where I don't know where the percentage columns are.</p>