On Feb 4, 2010, at 11:53 PM, Carrie Li wrote:
Thank you, David
Here is an example :
Z=matrix(rnorm(20), nrow=4)
index=replicate(4, sample(1:5, 3))
P=4
tmpr=list()
for (i in 1:P)
{
tmp = Z[i,index[,i]]
tmpr[[i]]=tmp
}
So, I am trying to pull out the values in Z for each row, but now I
only want to pull out the values indexed by the matrix "index"
And, since I have large P, so I would like to avoid loop.
My understanding (subject to correction):
the notion that loops are inefficient for this sort of activity
is a misconception. If your count of desired elements to be extracted
from Z are always the same, then a loop is probably reasonably
efficient.
You could accelerate the process by not making an assignment to the
intermediate 'tmp"
for (i in 1:P) { tmpr[[i]] <- Z[i,index[,i]] }
-- David.
Any quick way to do this ?
Thanks again!!
Carrie
On Thu, Feb 4, 2010 at 10:39 PM, David Winsemius <dwinsem...@comcast.net
> wrote:
On Feb 4, 2010, at 9:51 PM, Carrie Li wrote:
Dear R-helpers ,
I have a simple loop as follows, however, to be more efficient, I
would like
to use any apply functions (tapply, I suppose)
But how can I do this ? I am not very clear about this.
# Z is a P * Q matrix
# so for each row of Z, I would like to pull out only some of the
elements,
and save as a separate matrix under a list
# index is a vector with length smaller than Q, but the index could be
different for each row of Z.
An example would help. Sounds like you need a list structure to hold
the index and I don't see that you have constructed one.
for (i in 1:P)
{
tmp = matrix(Z[i, index], nrow=1)
Seems as though you would need to "index" in a verb sense (with "i")
the index (in a noun sense) structure. Might be better to call it
"idx".
Perhaos
tmp = matrix(Z[i, index[[i]] ], nrow=1) # the nrow=1 appears
superfluous
tmpr[[i]]=tmp
}
any help is highly appreciated!!
Thank you all
Carrie
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David Winsemius, MD
Heritage Laboratories
West Hartford, CT
David Winsemius, MD
Heritage Laboratories
West Hartford, CT
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