title: Concatenate row-wise across specific columns of dataframe
body: <p>I have a data frame with columns that, when concatenated (row-wise) as a string, would allow me to partition the data frame into a desired form.</p>
  
  <pre><code>&gt; str(data)
  'data.frame':   680420 obs. of  10 variables:
   $ A              : chr  "2011-01-26" "2011-01-26" "2011-02-09" "2011-02-09" ...
   $ B              : chr  "2011-01-26" "2011-01-27" "2011-02-09" "2011-02-10" ...
   $ C              : chr  "2011-01-26" "2011-01-26" "2011-02-09" "2011-02-09" ...
   $ D              : chr  "AAA" "AAA" "BCB" "CCC" ...
   $ E              : chr  "A00001" "A00002" "B00002" "B00001" ...
   $ F              : int  9 9 37 37 37 37 191 191 191 191 ...
   $ G              : int  NA NA NA NA NA NA NA NA NA NA ...
   $ H              : int  4 4 4 4 4 4 4 4 4 4 ...
  </code></pre>
  
  <p>For each row, I would like to concatenate the data in columns F, E, D, and C into a string (with the underscore character as separator).  Below is my unsuccessful attempt at this:</p>
  
  <pre><code>data$id &lt;- sapply(as.data.frame(cbind(data$F,data$E,data$D,data$C)), paste, sep="_")
  </code></pre>
  
  <p>And below is the undesired result:</p>
  
  <pre><code>  &gt; str(data)
      'data.frame':   680420 obs. of  10 variables:
       $ A              : chr  "2011-01-26" "2011-01-26" "2011-02-09" "2011-02-09" ...
       $ B              : chr  "2011-01-26" "2011-01-27" "2011-02-09" "2011-02-10" ...
       $ C              : chr  "2011-01-26" "2011-01-26" "2011-02-09" "2011-02-09" ...
       $ D              : chr  "AAA" "AAA" "BCB" "CCC" ...
       $ E              : chr  "A00001" "A00002" "B00002" "B00001" ...
       $ F              : int  9 9 37 37 37 37 191 191 191 191 ...
       $ G              : int  NA NA NA NA NA NA NA NA NA NA ...
       $ H              : int  4 4 4 4 4 4 4 4 4 4 ...
       $ id             : chr [1:680420, 1:4] "9" "9" "37" "37" ...
        ..- attr(*, "dimnames")=List of 2
        .. ..$ : NULL
        .. ..$ : chr  "V1" "V2" "V3" "V4"
  </code></pre>
  
  <p>Any help would be greatly appreciated.</p>