R help - Feb 2010 - How do I use "tapply" in this case ?

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.

for (i in 1:P)
{
  tmp = matrix(Z[i, index], nrow=1)
  tmpr[[i]]=tmp
}

any help is highly appreciated!!

Thank you all

Carrie

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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
>
> 	[[alternative HTML version deleted]]
>
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