Hi:

Try this:

do.call(rbind, lapply(split(h, h$file), function(x) x[sample(1:nrow(x), 1),
]))

My test returns
                                   file time_pred distance_1 distance_2
12.03.08_ins_odo_01 12.03.08_ins_odo_01       210     19.003     18.023
12.03.08_ins_odo_02 12.03.08_ins_odo_02        90     13.668     12.950
12.03.08_ins_odo_03 12.03.08_ins_odo_03       120     21.220     26.370
12.03.08_ins_odo_07 12.03.08_ins_odo_07       180     16.301     19.976
                    distance_3
12.03.08_ins_odo_01     14.666
12.03.08_ins_odo_02     13.506
12.03.08_ins_odo_03     23.962
12.03.08_ins_odo_07     25.309

The function does the following:
(1) Splits the data frame into a list, where each component of the list is a
sub-data frame.
(2) Applies the (anonymous) sampling function to each list component
(lapply)
(3) Combines the individual outputs together using the rbind function
(do.call)

Since this is the raison d'etre of the plyr package, one can also use

library(plyr)
> ddply(d, 'file', function(x) x[sample(1:nrow(x), 1), ])
                 file time_pred distance_1 distance_2 distance_3
1 12.03.08_ins_odo_01       270     15.694      9.285      4.135
2 12.03.08_ins_odo_02       270     17.252     18.235     18.661
3 12.03.08_ins_odo_03       240     18.117     19.111     19.870
4 12.03.08_ins_odo_07        90     19.790     23.276     18.678

(Your results may vary, but you do get one row per file as output.)

HTH,
Dennis

On Sun, Jun 27, 2010 at 6:16 PM, Kristiina Hurme
<kristiina.hurme@uconn.edu>wrote:

>
> hello everyone. please bear with me if this is very easy...
>
> I have a data set with many trials, and frames within each trial. I would
> like to pull out one random frame from each trial.
> here is an example. i have 4 unique trials (file), and various frames
> within
> each (time_pred). I would like to randomly sample 4 rows, but 1 from each
> trial (file).
>
> this sample data is called "h"
>                  file               time_pred distance_1 distance_2
> distance_3
> 1  12.03.08_ins_odo_01       210     19.003     18.023     14.666
> 2  12.03.08_ins_odo_01       240     23.905     20.087     17.266
> 3  12.03.08_ins_odo_01       270     15.694      9.285      4.135
> 4  12.03.08_ins_odo_02         0     22.142     16.061     14.776
> 5  12.03.08_ins_odo_02        30      2.968     12.533     19.696
> 6  12.03.08_ins_odo_02        60      6.175     17.701     20.198
> 7  12.03.08_ins_odo_02        90     13.668     12.950     13.506
> 8  12.03.08_ins_odo_02       120      7.098     17.817     22.878
> 9  12.03.08_ins_odo_02       270     17.252     18.235     18.661
> 10 12.03.08_ins_odo_02       300      7.967     15.944      8.130
> 11 12.03.08_ins_odo_03        90     18.724     17.931     21.148
> 12 12.03.08_ins_odo_03       120     21.220     26.370     23.962
> 13 12.03.08_ins_odo_03       150     21.225     24.376     20.194
> 14 12.03.08_ins_odo_03       180     22.298     24.119     24.606
> 15 12.03.08_ins_odo_03       210      8.413     14.464     15.219
> 16 12.03.08_ins_odo_03       240     18.117     19.111     19.870
> 17 12.03.08_ins_odo_07        60     24.063     25.779     24.800
> 18 12.03.08_ins_odo_07        90     19.790     23.276     18.678
> 19 12.03.08_ins_odo_07       120     15.617     23.707     19.545
> 20 12.03.08_ins_odo_07       150     24.818     22.373     24.515
> 21 12.03.08_ins_odo_07       180     16.301     19.976     25.309
> 22 12.03.08_ins_odo_07       210     23.843     24.772     26.025
> 23 12.03.08_ins_odo_07       240      9.029     15.125     20.139
> 24 12.03.08_ins_odo_07       270      6.533     22.833     23.618
>
> here is my code so far...
>
> > random <-for(i in unique(file)){h[sample(1:24,1),]}
> > random
>
> but this only gives me one sample... and if I try to exclude naming it as
> random, then nothing comes up. i'm confused and very new to R. please help!
> many many thanks!
> kristiina
>
>
> --
> View this message in context:
> http://r.789695.n4.nabble.com/sampling-one-random-frame-from-each-unique-trial-tp2270396p2270396.html
> Sent from the R help mailing list archive at Nabble.com.
>
> ______________________________________________
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>

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