R help - Jul 2011 - Select Random Rows from a dataframe

Hi all,
I have a dataframe of behavioral observations from 360 fish, each with 241
observation points(rows), which looks like this:
> head(d)
  	fish	treatment tank trial video tid pid   ang.chg    abs.ac           t      
len        vel     d2p           x         y
1	  1	         3     1      1       1      1   1        NA        NA           
0.0   0.000    NA    NA       5.169   9.617
2 	  1	         3     1      1       1      1   2        NA        NA           
0.5   0.203  0.405   0.203    5.254   9.433
3  	  1	         3     1      1       1      1   3  -78.69660  78.69660    1.0  
0.321  0.238   0.119    5.184    9.337
4  	  1	         3     1      1       1      1   4  -29.58347  29.58347    1.5  
0.648  0.653   0.327    5.147    9.013
5 	  1	         3     1      1       1      1   5  140.96235 140.96235  2.0  
0.988  0.680   0.340    5.434    8.830
6 	  1	         3     1      1       1      1   6  11.52867  11.52867     2.5  
1.463  0.949   0.474    5.877    8.660

I have divided it into subsets of "treatment" and "video"
types (i.e. dsub.r1 has all treatment 1 and video 1, dsub.s2 all treatment 2,
video 2, etc for treatments 1:3 and video1:2)

First: I want to randomly sample a block of 40 consecutive rows from each fish
(1-360),
BUT: I can only use that sample if the sum of the "d2p" column for
that set of 40 rows is at least 5, otherwise I need to re-sample for another
40-row block, until I get one with a d2p sum of at least 5, if available.

I know how to sample one row, or multiple random rows, but can't figure out
how to sample a block of consecutive rows...let alone how to check if the sum of
the d2p column is at least 5 and then re-sample if not.

 Sadly this is beyond my current knowledge level in R. Please help!!

Thanks in advance!

Hi Sean,

Here is one option that I believe does what you want.  The logic is simple,

1) instatiate a test value that does not meet your criteria
2) use a while loop that will keep looping until your criteria is met
(be sure you set a meetable criteria or this will continue ad
nauseum!!!),
3) randomly select a starting point from all rows except the last 40
(because if it started there, you could not have a consecutive block
of 40, unless you allow wrapping to the beginning in which case you
need to say that).


n <- nrow(yourdata)
test <- 0

while (test < 5) {
  i <- sample(1:(n - 40), 1)
  x <- yourdata[seq(from = i, to = i + 40), ]
  test <- sum(x[, "dp2"])
}

Hope this helps,

Josh

On Thu, Jul 21, 2011 at 1:16 PM, Sean Bignami <bignami83 at gmail.com>
wrote:
> Hi all,
> I have a dataframe of behavioral observations from 360 fish, each with 241
observation points(rows), which looks like this:
>
>> head(d)
> ? ? ? ?fish ? ?treatment tank trial video tid pid ? ang.chg ? ?abs.ac ? ? ?
? ? t ? ? ? len ? ? ? ?vel ? ? d2p ? ? ? ? ? x ? ? ? ? y
> 1 ? ? ? ? 1 ? ? ? ? ? ? ?3 ? ? 1 ? ? ?1 ? ? ? 1 ? ? ?1 ? 1 ? ? ? ?NA ? ? ?
?NA ? ? ? ? ? ?0.0 ? 0.000 ? ?NA ? ?NA ? ? ? 5.169 ? 9.617
> 2 ? ? ? ? 1 ? ? ? ? ? ? ?3 ? ? 1 ? ? ?1 ? ? ? 1 ? ? ?1 ? 2 ? ? ? ?NA ? ? ?
?NA ? ? ? ? ? ?0.5 ? 0.203 ?0.405 ? 0.203 ? ?5.254 ? 9.433
> 3 ? ? ? ? 1 ? ? ? ? ? ? ?3 ? ? 1 ? ? ?1 ? ? ? 1 ? ? ?1 ? 3 ?-78.69660
?78.69660 ? ?1.0 ? 0.321 ?0.238 ? 0.119 ? ?5.184 ? ?9.337
> 4 ? ? ? ? 1 ? ? ? ? ? ? ?3 ? ? 1 ? ? ?1 ? ? ? 1 ? ? ?1 ? 4 ?-29.58347
?29.58347 ? ?1.5 ? 0.648 ?0.653 ? 0.327 ? ?5.147 ? ?9.013
> 5 ? ? ? ? 1 ? ? ? ? ? ? ?3 ? ? 1 ? ? ?1 ? ? ? 1 ? ? ?1 ? 5 ?140.96235
140.96235 ?2.0 ? 0.988 ?0.680 ? 0.340 ? ?5.434 ? ?8.830
> 6 ? ? ? ? 1 ? ? ? ? ? ? ?3 ? ? 1 ? ? ?1 ? ? ? 1 ? ? ?1 ? 6 ?11.52867
?11.52867 ? ? 2.5 ? 1.463 ?0.949 ? 0.474 ? ?5.877 ? ?8.660
>
> I have divided it into subsets of "treatment" and
"video" types (i.e. dsub.r1 has all treatment 1 and video 1, dsub.s2
all treatment 2, video 2, etc for treatments 1:3 and video1:2)
>
> First: I want to randomly sample a block of 40 consecutive rows from each
fish (1-360),
> BUT: I can only use that sample if the sum of the "d2p" column
for that set of 40 rows is at least 5, otherwise I need to re-sample for another
40-row block, until I get one with a d2p sum of at least 5, if available.
>
> I know how to sample one row, or multiple random rows, but can't figure
out how to sample a block of consecutive rows...let alone how to check if the
sum of the d2p column is at least 5 and then re-sample if not.
>
> ?Sadly this is beyond my current knowledge level in R. Please help!!
>
> Thanks in advance!
>
> ______________________________________________
> R-help at 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
> and provide commented, minimal, self-contained, reproducible code.
>


-- 
Joshua Wiley
Ph.D. Student, Health Psychology
University of California, Los Angeles
https://joshuawiley.com/