[R] sampling one random frame from each unique trial?

[Kristiina Hurme]

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


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