title: R: Summing a sequence of columns row-wise with dplyr
body: <p>In the spirit of similar questions along these lines <a href="https://stackoverflow.com/questions/28873057/sum-across-multiple-columns-with-dplyr">here</a> and <a href="https://stackoverflow.com/questions/28095526/summarise-over-all-columns">here</a>, I would like to be able to sum across a sequence of columns in my <code>data_frame</code> &amp; create a new column:</p>
  
  <pre><code>df_abc = data_frame(
    FJDFjdfF = seq(1:100),
    FfdfFxfj = seq(1:100),
    orfOiRFj = seq(1:100),
    xDGHdj = seq(1:100),
    jfdIDFF = seq(1:100),
    DJHhhjhF = seq(1:100),
    KhjhjFlFLF = seq(1:100),
    IgiGJIJFG= seq(1:100),
  )
  
  # this does what I want
  df_abc %&gt;% 
    mutate(
      sum_1 = orfOiRFj + xDGHdj + jfdIDFF + DJHhhjhF
    ) 
  </code></pre>
  
  <p>Clearly, if there are a lot of variables in this sequence, typing them out is not feasible. Also, the names of the variables are not regex-friendly, so cannot be selected by a rule, other than the fact that they occur in a sequence. </p>
  
  <p>I am hoping that there exists an abstraction in the tidyverse, that allows something like: </p>
  
  <pre><code>df_abc %&gt;% 
    mutate(
      sum_1 = sum(orfOiRFj:DJHhhjhF)
    )
  </code></pre>
  
  <p>Thanks.</p>