title: remove all quotation marks from a data frame
body: <p>I have a data frame <code>rep</code> that looks like this:</p>
  
  <pre><code>&gt; head(rep)
       position chrom  value label  
  [1,] "17408"  "chr1" "0"   "miRNA"
  [2,] "17409"  "chr1" "0"   "miRNA"
  [3,] "17410"  "chr1" "0"   "miRNA"
  [4,] "17411"  "chr1" "0"   "miRNA"
  [5,] "17412"  "chr1" "0"   "miRNA"
  [6,] "17413"  "chr1" "0"   "miRNA"
  </code></pre>
  
  <p>How can I remove the quotation marks from all elements?</p>
  
  <p>Note: <code>rep$position</code> and <code>rep$value</code> should be <code>numeric</code> type, <code>rep$chrom</code> and <code>rep$label</code> should be <code>character</code> type.</p>