Here is a way of doing it:

> x
   block trial   x   y
1      1     1 605 150
2      1     1 603 148
3      1     1 604 140
4      1     1 600 140
5      1     1 590 135
6      1     1 580 135
7      1     2 607 148
8      1     2 605 152
10     1     2 600 158
> do.call(rbind, lapply(split(x, list(x$block, x$trial), drop=TRUE), head,
2))
      block trial   x   y
1.1.1     1     1 605 150
1.1.2     1     1 603 148
1.2.7     1     2 607 148
1.2.8     1     2 605 152


On Mon, May 11, 2009 at 7:49 AM, Jens Bölte <boelte@psy.uni-muenster.de>wrote:

> Hello,
>
> I have been struggling for quite some time to find a solution for the
> following problem. I have a data frame which is organized by block and
> trial. Each trial is represented across several rows in this data frame. I'd
> like to extract the first x rows per trial and block.
>
> For example
>        block   trial   x       y
> 1       1       1       605     150
> 2       1       1       603     148
> 3       1       1       604     140
> 4       1       1       600     140
> 5       1       1       590     135
> 6       1       1       580     135
> 7       1       2       607     148
> 8       1       2       605     152
> 10      1       2       600     158
> .....
>
> Selecting only the the first two rows per trial should result in
> block trial x y 1       1       605     150
> 1       1       603     148
> 1       2       607     148
> 1       2       605     152
>
> The data I am dealing with a x-y coordinates (samples) from an eye-tracking
> experiment. I receive the data in this format and need to eliminate unwanted
> samples.
>
> Thanks Jens Bölte
>
> ______________________________________________
> R-help@r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
> http://www.R-project.org/posting-guide.html<http://www.r-project.org/posting-guide.html>
> and provide commented, minimal, self-contained, reproducible code.
>
>


-- 
Jim Holtman
Cincinnati, OH
+1 513 646 9390

What is the problem that you are trying to solve?

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