# [R] How do I use "tapply" in this case ?

David Winsemius dwinsemius at comcast.net
Fri Feb 5 06:24:39 CET 2010

```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 <dwinsemius at 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)
>
> # 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
>
>        [[alternative HTML version deleted]]
>
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> and provide commented, minimal, self-contained, reproducible code.
>
> David Winsemius, MD
> Heritage Laboratories
> West Hartford, CT
>
>

David Winsemius, MD
Heritage Laboratories
West Hartford, CT

```