[R] 2 questions on matrix manipulation in R

[Joshua Wiley]

Hi Lei,

Here are some examples.  Look at the documentation for ?"%*%" ?"%o%"
and ?"[" with the drop = FALSE argument (in case you ever do select
just one column/row of a matrix).

## your first A matrix
(A <- matrix(1:55, ncol = 5))

## you were rather unclear what
## your b vectors dimensions were
## here are some possibilities
(b1 <- 5)
(b2 <- 1:11)
(b3 <- 1:5)

## you were also unclear whether
## what sort of multiplication was going on
## here are some possibilities
A * b1
A * b2
## equivalenet to each column of A %*% b3
A %o% b3

## your second A matrix and B matrix
(A2 <- matrix(1:30, nrow = 2))
(B <- matrix(1:4, 2))

## obviously does a little more work than needed
## but for most cases, still probably easier than a loop
diag(t(A2) %*% B %*% A2)

Cheers,

Josh

BTW, these strike me as more linear algebra questions than really R
questions.  Even without the builtin recycling etc. one could
construct the necessary matrices to do it without loops fairly easily.

On Sun, Aug 7, 2011 at 9:18 PM, Lei Liu <liulei at virginia.edu> wrote:
> Hi there,
>
> I have two questions on matrix manipulation. For the first one, I want to
> calculate the product of each column of a matrix (say A) with another vector
> (say b). That is, if A has 5 columns (a1, a2, a3, a4, a5), I want to obtain
> a matrix with columns (a1*b, a2*b, aA3*b, a4*b, a5*b). Can I do it directly,
> without using "for" loop?
>
> For the second one, I have a matrix A of dimension 2 by n (say columns of
> a1, a2, ..., an), another matrix B of dimension 2 by 2. I want to obtain the
> vector with elements (t(a1) %*% B %*% a1, t(a2) %*% B %*% a2, ..., t(an) %*%
> B %*% an). Can I do it without using "for" loop?
>
> Thanks a lot!
>
> Lei Liu
> Associate Professor
> Division of Biostatistics
> Department of Public Health Sciences
> University of Virginia School of Medicine
>
> http://people.virginia.edu/~ll9f/
>
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>



-- 
Joshua Wiley
Ph.D. Student, Health Psychology
Programmer Analyst II, ATS Statistical Consulting Group
University of California, Los Angeles
https://joshuawiley.com/