title: R: Melt with data.table into two columns
body: <p>I would like to convert a data.table from wide to long format.
  A "normal" melt works fine, but I would like to melt my data into two different columns.</p>
  
  <p>I found information about this here:
  <a href="https://cran.r-project.org/web/packages/data.table/vignettes/datatable-reshape.html" rel="nofollow noreferrer">https://cran.r-project.org/web/packages/data.table/vignettes/datatable-reshape.html</a> (Point 3)</p>
  
  <p>I tried this with my code but somehow it is not working and until now I could not figure out the problem.
  It would be great if you could explain me the mistake in the following example.</p>
  
  <p>Thanks</p>
  
  <pre><code>#Create fake data
  a = c("l1","l2","l2","l3")
  b = c(10, 10, 20, 10)
  c = c(30, 30, 30, 30)
  d = c(40.2, 32.1, 24.1, 33.0)
  e = c(1,2,3,4)
  f = c(1.1, 1.2, 1.3, 1.5)
  
  df &lt;- data.frame(a,b,c,d,e,f)
  colnames(df) &lt;- c("fac_a", "fac_b", "fac_c", "m1", "m2.1", "m2.2")
  
  #install.packages("data.table")
  require(data.table)
  TB &lt;- setDT(df)
  
  #Standard melt - works
  TB.m1 = melt(TB, id.vars = c("fac_a", "fac_b", "fac_c"), measure.vars = c(4:ncol(TB)))
  
  #Melt into two columns
  colA = paste("m1", 4, sep = "")
  colB = paste("m2", 5:ncol(TB), sep = "")
  DT = melt(TB, measure = list(colA, colB), value.name = c("a", "b"))
  #Not working, error: Error in melt.data.table(TB, measure = list(colA, colB), value.name = c("a",  : One or more values in 'measure.vars' is invalid.
  </code></pre>