how to "multiply" list of matrices by list of vectors

Try:

n <- length(mlist)
result <- lapply(1:n,function(i,m,v){m[[i]]%*%v[[i]]},m=mlist,v=vlist)

These days the use of lapply is unlikely to be much, if at all, faster than
the use of a for loop.

     cheers,

         Rolf Turner

Hi everyone,

I'd like to be able to apply lda to each 2D matrix slice of a 3D array, and
then use the scalings to obtain the corresponding lda scores.

I can use 'apply' to get a list of the lda output for each 2D slice, and
can create a list of the resulting scalings, but I'm not sure how to
multiply them in a vectorized way.


Here's how I made a list of 2D matrices (suggestion on improving this would
be welcome, too!):

aa <- array(1:24,c(4,2,3))
mlist <- apply(aa,2,list)
mlist <- lapply(mlist, unlist)
mlist <- lapply(mlist, function(x) matrix(x,4,2))

and here's how I made a list of vectors:

mm <- matrix(1:6,2,3)
vlist <- apply(mm, list)
vlist <- lapply(vlist, unlist)

Now I'd like to make the list whose i-th element is mlist[[i]]%*%vlist[[i]]
without having to loop through the indices.

Any help would be appreciated!

Hi,

I got an error message with:
vlist <- apply(mm, list)

Error in match.fun(FUN) : argument "FUN" is missing, with no default
#assuming that
vlist <- apply(mm,2,list)

mapply("%*%",mlist,vlist[1:2],SIMPLIFY=FALSE)
#[[1]]
?# ? ?[,1]
#[1,] ? 19
#[2,] ? 22
#[3,] ? 25
#[4,] ? 28
#
#[[2]]
?# ? ?[,1]
#[1,] ? 67
#[2,] ? 74
#[3,] ? 81
#[4,] ? 88

A.K.
----- Original Message -----
From: David Romano <dromano at stanford.edu>
To: r-help at r-project.org
Cc: 
Sent: Wednesday, February 6, 2013 12:50 AM
Subject: [R] how to "multiply" list of matrices by list of vectors

Hi everyone,

I'd like to be able to apply lda to each 2D matrix slice of a 3D array, and
then use the scalings to obtain the corresponding lda scores.

I can use 'apply' to get a list of the lda output for each 2D slice, and
can create a list of the resulting scalings, but I'm not sure how to
multiply them in a vectorized way.


Here's how I made a list of 2D matrices (suggestion on improving this would
be welcome, too!):

aa <- array(1:24,c(4,2,3))
mlist <- apply(aa,2,list)
mlist <- lapply(mlist, unlist)
mlist <- lapply(mlist, function(x) matrix(x,4,2))

and here's how I made a list of vectors:

mm <- matrix(1:6,2,3)
vlist <- apply(mm, list)
vlist <- lapply(vlist, unlist)

Now I'd like to make the list whose i-th element is mlist[[i]]%*%vlist[[i]]
without having to loop through the indices.

Any help would be appreciated!

Thanks,
David

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