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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