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

[David Winsemius]

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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>
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David Winsemius, MD
Heritage Laboratories
West Hartford, CT