Matrix element-by-element multiplication

It looks like pdf is not a "scalar" (that term actually has no meaning in R but I know what you mean) but is rather a 1x1 matrix, as attested by the fact it has dimensions. If you give dnorm() a matrix it will return one, as it did here.

Perhaps you should look at the is.matrix() and as.vector() functions rather than abusing a side-effect of c(), which makes it much more difficult to see R's internal logic, which, while quirky, is useful at the end of the day.

Michael

PS - It's good form to cc the list at each step so others can follow along and contribute when I say something wrong. It also helps you get quicker answers.

On Nov 6, 2011, at 1:06 AM, Steven Yen <syen at utk.edu> wrote:

I am trying to multiply what I know is a scalar (pdf(xb)) to a column vector of coefficient (bb).
In the following, pdf is a scalar and bb is 5 x 1. I first show what worked and then what did not work.
If my pdf is a scalar, why would I need c(pdf) to be able to pre-multiply it by a 5 x 1 vector?

---

x ? ? ?<- as.matrix(colMeans(x))
xb ? ? <- t(x)%*%bb
pdf ? ?<- dnorm(xb)

dim(bb)

[1] 5 1

cpdf ?<- c(pdf)
dim(cpdf)

NULL

cpdf

[1] 0.304201

(dphat <- cpdf*bb)

? ? ? ? ? ? ? ? ? ?[,1]
(Intercept) ?0.32744753
xrage ? ? ? -0.00599225
xryr ? ? ? ? 0.01758431
xrrate ? ? ?-0.08217250
xrrel ? ? ? -0.05695434

pdf ? ?<- dnorm(xb)
dim(pdf)

[1] 1 1

pdf

? ? ? ? ?[,1]
[1,] 0.304201

(dphat <- ?pdf*bb)

Error in pdf * bb : non-conformable arrays

At 12:21 AM 11/6/2011, you wrote:

There are a few (nasty?) side-effects to c(), one of which is
stripping a matrix of its dimensionality. E.g.,

x <- matrix(1:4, 2)
c(x)
[1] 1 2 3 4

So that's probably what happened to you. R has a somewhat odd feature
of not really considering a pure vector as a column or row vector but
being willing to change it to either:

e.g.

y <- 1:2

x %*% y
y %*% x
y %*% y

while matrix(y) %*% x throws an error, which can also trip folks up.
You might also note that x * y and y*x return the same thing in this
problem.

Getting back to your problem: what are v and b and what are you hoping
to get done? Specifically, what happened when you tried v*b (give the
exact error message). It seems likely that they are non-conformable
matrices, but here non-conformable for element-wise multiplication
doesn't mean the same thing as it does for matrix multiplication.
E.g.,

x <- matrix(1:4,2)
y <- matrix(1:6,2)

dim(x)
[1] 2 2

dim(y)
[1] 2 3

x * y -- here R seems to want matrices with identical dimensions, but
i can't promise that.

x %*% y does work.

Hope this helps and yes I know it can seem crazy at first, but there
really is reason behind it at the end of the tunnel,

Michael


On Sun, Nov 6, 2011 at 12:11 AM, Steven Yen <syen at utk.edu> wrote:

My earlier attempt

? ?dp <- v*b

did not work. Then,

? ?dp <- c(v)*b

worked.

Confused,

Steven

At 09:10 PM 11/4/2011, you wrote:

Did you even try?

a <- 1:3
x <- ?matrix(c(1,2,3,2,4,6,3,6,9),3)
a*x

? ? ?[,1] [,2] [,3]
[1,] ? ?1 ? ?2 ? ?3
[2,] ? ?4 ? ?8 ? 12
[3,] ? ?9 ? 18 ? 27

Michael

On Fri, Nov 4, 2011 at 7:26 PM, Steven Yen <syen at utk.edu> wrote:

is there a way to do element-by-element multiplication as in Gauss
and MATLAB, as shown below? Thanks.

---
a

? ? ? ?1.0000000
? ? ? ?2.0000000
? ? ? ?3.0000000
x

? ? ? ?1.0000000 ? ? ? ?2.0000000 ? ? ? ?3.0000000
? ? ? ?2.0000000 ? ? ? ?4.0000000 ? ? ? ?6.0000000
? ? ? ?3.0000000 ? ? ? ?6.0000000 ? ? ? ?9.0000000
a.*x

? ? ? ?1.0000000 ? ? ? ?2.0000000 ? ? ? ?3.0000000
? ? ? ?4.0000000 ? ? ? ?8.0000000 ? ? ? ?12.000000
? ? ? ?9.0000000 ? ? ? ?18.000000 ? ? ? ?27.000000


--
Steven T. Yen, Professor of Agricultural Economics
The University of Tennessee
http://web.utk.edu/~syen/
? ? ? ?[[alternative HTML version deleted]]

--
Steven T. Yen, Professor of Agricultural Economics
The University of Tennessee
http://web.utk.edu/~syen/

--
Steven T. Yen, Professor of Agricultural Economics
The University of Tennessee
http://web.utk.edu/~syen/

? ? ? ?[[alternative HTML version deleted]]