[R] conditionally merging adjacent rows in a data frame

[Titus von der Malsburg]

Hi, I have a data frame and want to merge adjacent rows if some condition is
met.  There's an obvious solution using a loop but it is prohibitively slow
because my data frame is large.  Is there an efficient canonical solution for
that?

> head(d)
     rt dur tid  mood roi  x
55 5523 200   4  subj   9  5
56 5523  52   4  subj   7 31
57 5523 209   4  subj   4  9
58 5523 188   4  subj   4  7
70 4016 264   5 indic   9 51
71 4016 195   5 indic   4 14

The desired result would have consecutive rows with the same roi value merged.
dur values should be added and x values averaged, other values don't differ in
these rows and should stay the same.

> head(result)
     rt dur tid  mood roi  x
55 5523 200   4  subj   9  5
56 5523  52   4  subj   7 31
57 5523 397   4  subj   4  8
70 4016 264   5 indic   9 51
71 4016 195   5 indic   4 14

There's also a solution using reshape.  It uses an index for blocks

  d$index <- cumsum(c(TRUE,diff(d$roi)!=0))

melts and then casts for every column using an appropriate fun.aggregate.
However, this is a bit cumbersome and also I'm not sure how to make sure that
I get the original order of rows.

Thanks for any suggestion.

  Titus