R help - Jan 2007 - Matrix operations in a list

I have matrices stored within a list like something as follows:

a <- list(matrix(rnorm(50), ncol=5), matrix(rnorm(50), ncol=5))
b <- list(matrix(rnorm(50), nrow=5), matrix(rnorm(50), nrow=5))

I don't recall how to perform matrix multiplication on each list element
such that the result is a new list 

result <- list(a[[1]]%*%b[[1]], a[[2]]%*%b[[2]])

I think I'm close with mapply(), but I'm doing something silly

mapply('%*%', a,b)

Thanks.
Harold


	[[alternative HTML version deleted]]

On Tue, 2007-01-23 at 10:21 -0500, Doran, Harold wrote:
> I have matrices stored within a list like something as follows:
> 
> a <- list(matrix(rnorm(50), ncol=5), matrix(rnorm(50), ncol=5))
> b <- list(matrix(rnorm(50), nrow=5), matrix(rnorm(50), nrow=5))
> 
> I don't recall how to perform matrix multiplication on each list
element
> such that the result is a new list 
> 
> result <- list(a[[1]]%*%b[[1]], a[[2]]%*%b[[2]])
> 
> I think I'm close with mapply(), but I'm doing something silly
> 
> mapply('%*%', a,b)
> 
> Thanks.
> Harold
Harold,

That should basically be working.  Just note that by default, each
resultant matrix is put into a column (vector) format, rather than as a
matrix.

 Res <- mapply("%*%", a, b)
 Res1 <- a[[1]] %*% b[[1]]
 Res2 <- a[[2]] %*% b[[2]]

> str(Res)
 num [1:100, 1:2]  0.1713  0.8290 -0.0864  3.5420 -1.4638 ...
 - attr(*, "dimnames")=List of 2
  ..$ : NULL
  ..$ : NULL
> all.equal(Res[, 1], as.vector(Res1))
[1] TRUE
> all.equal(Res[, 2], as.vector(Res2))
[1] TRUE

If you want the results to be a list of two matrices, you would do
something like:

Res <- mapply("%*%", a, b, SIMPLIFY = FALSE)
> str(Res)
List of 2
 $ : num [1:10, 1:10]  0.1713  0.8290 -0.0864  3.5420 -1.4638 ...
 $ : num [1:10, 1:10]  0.220 -2.048 -0.135 -2.121 -0.399 ...
> all.equal(Res1, Res[[1]])
[1] TRUE
> all.equal(Res2, Res[[2]])
[1] TRUE


HTH,

Marc Schwartz

Try,

mapply('%*%', a, b, SIMPLIFY=FALSE)

-Christos 

-----Original Message-----
From: r-help-bounces at stat.math.ethz.ch
[mailto:r-help-bounces at stat.math.ethz.ch] On Behalf Of Doran, Harold
Sent: Tuesday, January 23, 2007 10:22 AM
To: r-help at stat.math.ethz.ch
Subject: [R] Matrix operations in a list

I have matrices stored within a list like something as follows:

a <- list(matrix(rnorm(50), ncol=5), matrix(rnorm(50), ncol=5)) b <-
list(matrix(rnorm(50), nrow=5), matrix(rnorm(50), nrow=5))

I don't recall how to perform matrix multiplication on each list element
such that the result is a new list 

result <- list(a[[1]]%*%b[[1]], a[[2]]%*%b[[2]])

I think I'm close with mapply(), but I'm doing something silly

mapply('%*%', a,b)

Thanks.
Harold


	[[alternative HTML version deleted]]

______________________________________________
R-help at stat.math.ethz.ch mailing list
https://stat.ethz.ch/mailman/listinfo/r-help
PLEASE do read the posting guide http://www.R-project.org/posting-guide.html
and provide commented, minimal, self-contained, reproducible code.

you need the 'SIMPLIFY' argument of mapply(), i.e.,

mapply("%*%", a, b, SIMPLIFY = FALSE)


I hope it helps.

Best,
Dimitris

----
Dimitris Rizopoulos
Ph.D. Student
Biostatistical Centre
School of Public Health
Catholic University of Leuven

Address: Kapucijnenvoer 35, Leuven, Belgium
Tel: +32/(0)16/336899
Fax: +32/(0)16/337015
Web: http://med.kuleuven.be/biostat/
     http://www.student.kuleuven.be/~m0390867/dimitris.htm

----- Original Message ----- 
From: "Doran, Harold" <HDoran at air.org>
To: <r-help at stat.math.ethz.ch>
Sent: Tuesday, January 23, 2007 4:21 PM
Subject: [R] Matrix operations in a list

>I have matrices stored within a list like something as follows:
>
> a <- list(matrix(rnorm(50), ncol=5), matrix(rnorm(50), ncol=5))
> b <- list(matrix(rnorm(50), nrow=5), matrix(rnorm(50), nrow=5))
>
> I don't recall how to perform matrix multiplication on each list 
> element
> such that the result is a new list
>
> result <- list(a[[1]]%*%b[[1]], a[[2]]%*%b[[2]])
>
> I think I'm close with mapply(), but I'm doing something silly
>
> mapply('%*%', a,b)
>
> Thanks.
> Harold
>
>
> [[alternative HTML version deleted]]
>
> ______________________________________________
> R-help at stat.math.ethz.ch mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide 
> http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
> 

Disclaimer: http://www.kuleuven.be/cwis/email_disclaimer.htm