On Feb 6, 2011, at 3:25 PM, B77S wrote:
Hello,
How can I randomly sample individuals within a sites from a site
(row) X
species abundance (column) data frame or matrix? As an example, the
matrix
"abund2" made below.
##### (sorry, Im a newbie and this is the only way I know to get an
example
on here)
abund1 <- c(150, 300, 0, 360, 150, 300, 0, 240, 150,
0, 60,
0, 150, 0, 540, 0, 0, 300, 0, 240, 300, 300, 0, 360, 300, 0, 600, 0)
abund2 <- matrix(data=abund1, nrow=4, ncol=7)
colnames(abund2) <- c("spA", "spB", "spC", "spD", "spa", "spF", "spG")
rownames(abund2)<-c("site1", "site2", "site3", "site4")
Perfect. Best submission of an example by a newbie in weeks.
#####
abund2
spA spB spC spD spa spF spG
site1 150 150 150 150 0 300 300
site2 300 300 0 0 300 300 0
site3 0 0 60 540 0 0 600
site4 360 240 0 0 240 360 0
How can I make a random subsample of 100 individuals from the
abundances
given for each site?
samptbl <- apply(abund2, 1, function(x) sample(colnames(abund2), 100,
prob=x, replace=TRUE) )
samptbl
site1 site2 site3 site4
[1,] "spG" "spa" "spD" "spF"
[2,] "spF" "spF" "spG" "spB"
[3,] "spF" "spB" "spC" "spA"
[4,] "spD" "spa" "spG" "spA"
[5,] "spF" "spa" "spD" "spa"
[6,] "spA" "spB" "spD" "spF"
[7,] "spA" "spF" "spD" "spA"
[8,] "spG" "spF" "spG" "spa"
[9,] "spF" "spF" "spG" "spa"
[10,] "spG" "spB" "spD" "spA"
Snipped
apply() always transposes the results when called with row margins.
The t() function would "fix" this if it needed to be arranged with
rows by site. You could check by further apply-(cation) of table to
the columns:
> apply(samptbl, 2, table)
$site1
spA spB spC spD spF spG
8 13 6 13 32 28
$site2
spa spA spB spF
25 31 25 19
$site3
spC spD spG
9 51 40
$site4
spa spA spB spF
22 27 19 32
This is probably really easy.
Thanks.
Bubba
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David Winsemius, MD
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