# [R] Multiplying a list of matrices with a vector

Mendolia, Franco fmendolia at mcw.edu
Thu Sep 29 17:02:28 CEST 2011

```Thank you!

Franco
________________________________________
From: David Winsemius [dwinsemius at comcast.net]
Sent: Thursday, September 29, 2011 8:09 AM
To: R. Michael Weylandt <michael.weylandt at gmail.com>
Cc: Mendolia, Franco; r-help at r-project.org
Subject: Re: [R] Multiplying a list of matrices with a vector

On Sep 28, 2011, at 6:26 PM, R. Michael Weylandt wrote:

> Untested:
>
> mapply('*', LL, vec)
>
> Those should be backticks but I can't type them on my phone

Actually mapply does not require backticks since it is expecting its
functions to be quoted anyway, but it didn't seem to work as you (and
I) expected.

> mapply('*', LL, vec)
[,1] [,2] [,3]
[1,]    1    2    3
[2,]    0    0    0
[3,]    0    0    0
[4,]    0    0    0
[5,]    1    2    3
[6,]    0    0    0
[7,]    0    0    0
[8,]    0    0    0
[9,]    1    2    3

What is needed is to add SIMPLIFY=FALSE

> mapply('*', LL, vec, SIMPLIFY=FALSE)
\$A
[,1] [,2] [,3]
[1,]    1    0    0
[2,]    0    1    0
[3,]    0    0    1

\$B
[,1] [,2] [,3]
[1,]    2    0    0
[2,]    0    2    0
[3,]    0    0    2

\$C
[,1] [,2] [,3]
[1,]    3    0    0
[2,]    0    3    0
[3,]    0    0    3

--
David.

>
> Michael
>
> On Sep 28, 2011, at 6:04 PM, "Mendolia, Franco" <fmendolia at mcw.edu>
> wrote:
>
>> Hi all!
>>
>> I have a list of matrices and I want to multiply the ith element of
>> the list with the ith element of a another vector. That is,
>>
>>> LL <- list(A=diag(3),B=diag(3),C=diag(3))
>>> vec <- 1:3
>>> for(i in 1:3)
>> + {
>> +   LL[[i]] <- LL[[i]]*vec[i]
>> + }
>>> LL
>> \$A
>>    [,1] [,2] [,3]
>> [1,]    1    0    0
>> [2,]    0    1    0
>> [3,]    0    0    1
>>
>> \$B
>>    [,1] [,2] [,3]
>> [1,]    2    0    0
>> [2,]    0    2    0
>> [3,]    0    0    2
>>
>> \$C
>>    [,1] [,2] [,3]
>> [1,]    3    0    0
>> [2,]    0    3    0
>> [3,]    0    0    3
>>
>> Is there another (more efficient) way of doing this without using
>> loops? I found an older entry in this list with a similar problem,
>> where the list elements were always multiplied with the same
>> number, e.g., lapply(LL, function(x) x*3) and I was looking for
>> something similar.
>>
>> Best,
>> Franco
>> ______________________________________________
>> R-help at r-project.org mailing list
>> https://stat.ethz.ch/mailman/listinfo/r-help
>> and provide commented, minimal, self-contained, reproducible code.
>
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