On May 23, 2010, at 10:00 AM, Kang Min wrote:
Hi,
I have a dataset that looks like the one below.
data
plot plantno. species
H 31 ABC
D 2 DEF
Y 54 GFE
E 12 ERF
Y 98 FVD
H 4 JKU
J 7 JFG
A 55 EGD
. . .
. . .
. . .
I want to select rows belonging to 7 random plots for 100 times.
So you should be thinking about a function that will do what you want
exactly once and then wrapping it in replicate().
(There are 50 plots in total)
So I created a list of 100 vectors, each vector has 7 elements.
samp <- lapply(1:100, function(i) sample(LETTERS))
Please. "Minimal"!!! 5 samples should be enough for testing.
samp2 <- lapply(samp2, "[", 1:7)
How can I select the 26 plots from 'data' using 'samp'?
samp3 <- sample(LETTERS, 7)
You do not want to sample from LETTERS but rather from the vector of
data named "plot". Otherwise you will not be creating a representative
sample. And ... "plot" is a really crappy name for a column. Try to
avoid naming your columns with names that are common functions.
Confusion of the humans reading your code is the predictable result,
and occasional "confusion" of the R interpreter also may occur.
[After reading your reply to Holtman.... Or maybe you do want to
sample from LETTERS. The fix would be obvious.]
samp4 <- subset(data, plot %in% samp3) # this works
So this is what you want to do once:
samp1 <- function() subset(data, plot %in% sample(data$plot, 7) )
samp15 <- replicate(10, samp1())
samp5[,1] will be one sampled subset. (samp10 is now an array of lists.)
Unforfunately, I noticed that even with minimal "data" example you
provided (not in reproducible form unfortunately) that I was getting 7
or 8 samples and realized that using letters to subset was creating
some overlaps whenever "H" was sampled. So this is safer:
samp1 <- function() data[ sample(1:nrow(data), 7 ),]
samp5 <- replicate(5, samp1() )
for(1 in 1:5) print(samp5[,i])
Then I noticed your reply to Holtman, so perhaps you do really wnat
the first solution. Just so you understand it might not be
statistically correct.
--
David.
samp5 <- subset(data, plot %in% samp2[[1]]) # this works as well, but
I used a for loop to get it to select 7 plots 100 times.
for (i in nrow(samp2)) {
samp6 <- subset(data, plot %in% samp2[[i]])
} # this doesn't work
Am I missing something, or is there a better solution?
Thanks.
Kang Min
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David Winsemius, MD
West Hartford, CT
______________________________________________
R-help@r-project.org mailing list
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PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
and provide commented, minimal, self-contained, reproducible code.
