[R] Select Random Rows from a dataframe

[Sean Bignami]

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!