You want to select two subplots for each DL value. Try:
df - data.frame( DL=gl(3,4), subplot=rep(1:4,3) )
df$index - 1:nrow(df)
ind - tapply( df$index, df$DL, function(x) sample(x,2) )
df[ unlist(ind), ]
You could also have used rownames(df) instead of creating df$index.
OR
tmp - lapply( split(df, df$DL), function(m) m[sample(1:nrow(m),2),] )
do.call(rbind, tmp)
Regards, Adai
Christoph Scherber wrote:
Dear all,
I have a tricky problem here:
I have a dataframe with biodiversity data in which suplots are a
repeated sequence from 1 to 4 (1234,1234,...)
Now, I want to randomly pick two subplots each from each diversity level
(DL).
The problem is that it works up to that point - but if I try to subset
the whole dataframe, I get stuck:
DL=gl(3,4)
subplot=rep(1:4,3)
diversity.data=data.frame(DL,subplot)
subplot.sampled=NULL
for(i in 1:3)
subplot.sampled=c(subplot.sampled,sort(sample(4,2,replace=F)))
subplot.sampled
[1] 3 4 1 3 1 3
subplot[subplot.sampled]
[1] 3 4 1 3 1 3
## here comes the tricky bit:
diversity.data[subplot.sampled,]
DL subplot
31 3
41 4
11 1
3.1 1 3
1.1 1 1
3.2 1 3
How can I select those rows of diversity.data that match the exact
subplots in subplot.sampled?
Thank you very much for your help!
Best wishes,
Christoph
(I am using R 2.4.1 on Windows XP)
##
Christoph Scherber
DNPW, Agroecology
University of Goettingen
Waldweg 26
D-37073 Goettingen
+49-(0)551-39-8807
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