title: Reshape2 melt error "One or more values in 'id.vars' is invalid"
body: <p>I'm trying to melt two columns and use rownames as id's by doing:</p>
  
  <pre><code>fst = fread("file.fst")
  colnames(fst) = c("fst2", "snp", "chr", "pos", "fst")
  d &lt;- melt(fst[,c("fst","fst2")], id.vars="as.numeric(row.names(fst))")
  </code></pre>
  
  <p>but I'm getting the error:</p>
  
  <pre><code>Error in melt.data.table(fst[, c("fst", "fst2")], id.vars = "as.numeric(row.names(fst))") :
    One or more values in 'id.vars' is invalid.
  </code></pre>
  
  <p>The data looks like</p>
  
  <pre><code> fst2          snp chr      pos      fst
  1: 0.265838        CLIC6   1    25001 0.150339
  2: 0.390470        RUNX1   1   115001 0.259316
  3: 0.126332        SETD4   1   635001 0.128946
  4: 0.236400 LOC100222525   1   645001 0.117627
  5: 0.181189       DOPEY2   1   705001 0.190456
  
  &gt; class(fst)
  [1] "data.table" "data.frame"
  &gt; typeof(fst)
  [1] "list"
  &gt; str(fst)
  Classes ‘data.table’ and 'data.frame':  10066 obs. of  5 variables:
   $ fst2: num  0.266 0.39 0.126 0.236 0.181 ...
   $ snp : chr  "CLIC6" "RUNX1" "SETD4" "LOC100222525" ...
   $ chr : int  1 1 1 1 1 1 1 1 1 1 ...
   $ pos : int  25001 115001 635001 645001 705001 735001 745001 955001 985001 1105001 ...
   $ fst : num  0.15 0.259 0.129 0.118 0.19 ...
   - attr(*, ".internal.selfref")=&lt;externalptr&gt;
  </code></pre>
  
  <p>What's wrong?</p>