title: Summing across rows of a data.table for specific columns
body: <p>I have a large data table (from the package <a href="http://cran.r-project.org/web/packages/data.table/index.html" rel="noreferrer">data.table</a>) with over 60 columns (the first three corresponding to factors  and the remaining to response variables, in this case different species) and several rows corresponding to the different levels of the treatments and the species abundances.
  A very small version looks like this:</p>
  
  <pre><code>library(data.table)
  TEST &lt;- data.table(Time=c("0","0","0","7","7","7","12"),
               Zone=c("1","1","0","1","0","0","1"),
               quadrat=c(1,2,3,1,2,3,1),
               Sp1=c(0,4,29,9,1,2,10),
               Sp2=c(20,17,11,15,32,15,10),
               Sp3=c(1,0,1,1,1,1,0))
  
  setkey(TEST,Time)
  TEST
  
  #    Time Zone quadrat Sp1 Sp2 Sp3
  # 1:    0    1       1   0  20   1
  # 2:    0    1       2   4  17   0
  # 3:    0    0       3  29  11   1
  # 4:   12    1       1  10  10   0
  # 5:    7    1       1   9  15   1
  # 6:    7    0       2   1  32   1
  # 7:    7    0       3   2  15   1
  </code></pre>
  
  <p>I first want to calculate the mean abundances of each species across Time for each Zone x quadrat combination and that's fine:</p>
  
  <pre><code>Abundance = TEST[ , lapply(.SD, mean), by = "Zone,quadrat"]
  Abundance
  #    Zone quadrat Time       Sp1  Sp2       Sp3
  # 1:   Z1       1   NA  6.333333 15.0 0.6666667
  # 2:   Z1       2   NA  2.500000 24.5 0.5000000
  # 3:   Z0       1   NA 15.500000 13.0 1.0000000  
  </code></pre>
  
  <p>Then I want to calculate rowwise sum for the 'species' columns, in the example from Sp1 to Sp3. I have tried the following code with no success:  </p>
  
  <pre><code>Abundance$SumAbundance &lt;- rowSums(Abundance[ , c(4:6)])  
  </code></pre>
  
  <p>I get the error message:  </p>
  
  <pre><code># Error in rowSums(Abundance[, c(4:6)]) : 
  # 'x' must be an array of at least two dimensions
  </code></pre>
  
  <p>How can I compute row sums for specific columns of a <code>data.table</code>?</p>