R help - Feb 2024 - [External] converting MATLAB -> R | element-wise operation

So, trying to convert a very long, somewhat technical bit of lin alg 
MATLAB code to R. Most of it working, but raninto a stumbling block that 
is probaably simple enough for someone to explain.

Basically, trying to 'line up' MATLAB results from an element-wise 
division of a matrix by a vector with R output.

Here is a simplified version of the MATLAB code I'm translating:

NN = [1, 2, 3; 4, 5, 6];? % Example matrix
lambda = [2, 3, 4];? % Example vector
result_matlab = NN ./ lambda;

which yields

 ?0.50000?? 0.66667?? 0.75000
 ?2.00000?? 1.66667?? 1.50000


So, the only way I have stumbled onto in R to generate the same results 
is to use 'sweep'. The following 'works', but I'm hoping
someone can
explain why I need something as convoluted as this seems (to me, at least).

NN <- matrix(c(1, 2, 3, 4, 5, 6), nrow = 2, byrow = TRUE)? # Example matrix
lambda <- c(2, 3, 4)? # Example vector
sweep(NN, 2, lambda, "/")


 ???? [,1]????? [,2] [,3]
[1,]? 0.5 0.6666667 0.75
[2,]? 2.0 1.6666667 1.50

First tried the more 'obvious' NN/lambda, but that yields 'the wrong
answer' (based solely on what I'm trying to accomplish):


 ?????? [,1] [,2] [,3]
[1,] 0.500000? 0.5? 1.0
[2,] 1.333333? 2.5? 1.5

So, why 'sweep'?

	[[alternative HTML version deleted]]

> t(t(NN)/lambda)
     [,1]      [,2] [,3]
[1,]  0.5 0.6666667 0.75
[2,]  2.0 1.6666667 1.50
>
R  matrices are column-based. MATLAB matrices are row-based.
> On Feb 27, 2024, at 14:54, Evan Cooch <evan.cooch at gmail.com>
wrote:
>
> So, trying to convert a very long, somewhat technical bit of lin alg
> MATLAB code to R. Most of it working, but raninto a stumbling block that
> is probaably simple enough for someone to explain.
>
> Basically, trying to 'line up' MATLAB results from an element-wise
> division of a matrix by a vector with R output.
>
> Here is a simplified version of the MATLAB code I'm translating:
>
> NN = [1, 2, 3; 4, 5, 6];  % Example matrix
> lambda = [2, 3, 4];  % Example vector
> result_matlab = NN ./ lambda;
>
> which yields
>
>  0.50000   0.66667   0.75000
>  2.00000   1.66667   1.50000
>
>
> So, the only way I have stumbled onto in R to generate the same results
> is to use 'sweep'. The following 'works', but I'm
hoping someone can
> explain why I need something as convoluted as this seems (to me, at least).
>
> NN <- matrix(c(1, 2, 3, 4, 5, 6), nrow = 2, byrow = TRUE)  # Example
matrix
> lambda <- c(2, 3, 4)  # Example vector
> sweep(NN, 2, lambda, "/")
>
>
>      [,1]      [,2] [,3]
> [1,]  0.5 0.6666667 0.75
> [2,]  2.0 1.6666667 1.50
>
> First tried the more 'obvious' NN/lambda, but that yields 'the
wrong
> answer' (based solely on what I'm trying to accomplish):
>
>
>        [,1] [,2] [,3]
> [1,] 0.500000  0.5  1.0
> [2,] 1.333333  2.5  1.5
>
> So, why 'sweep'?
>
> [[alternative HTML version deleted]]
>
> ______________________________________________
> R-help at r-project.org mailing list -- To UNSUBSCRIBE and more, see
> 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.

Why anything but sweep?

The fundamental data type in Matlab is a matrix... they don't have vectors,
they have Nx1 matrices and 1xM matrices.

Vectors don't have any concept of "row" vs. "column".
Straight division is always elementwise with recycling as needed, and matrices
are really vectors in row-major order:

1 2 3
4 5 6

is really

1 4 2 5 3 6

and when you do straight division NN / lambda then lambda is repeated:

1 4 2 5 3 6
2 3 4 2 3 4

to get

0.5 1.3 0.5 2.5 1.0 1.5

but if you transpose first

1 4
2 5
3 6

then that corresponds to an underlying vector:

1 2 3 4 5 6

which lines up with lambda in t(NN)/lambda as:

1 2 3 4 5 6
2 3 4 2 3 4

to obtain:

0.50 0.67 0.75 2.0 1.67 1.50

and inherits the dimensions of t(NN):

0.50 2.00
0.67 1.67
0.75 1.50

which can be transposed back as in t( t( NN ) / lambda ):

0.50 0.67 0.75
2.00 1.67 1.50

but that requires a lot of moving elements around while sweep does not.

Operators are not necessarily "better" than named functions... they
just look different.


On February 27, 2024 11:54:26 AM PST, Evan Cooch <evan.cooch at gmail.com>
wrote:
>So, trying to convert a very long, somewhat technical bit of lin alg 
>MATLAB code to R. Most of it working, but raninto a stumbling block that 
>is probaably simple enough for someone to explain.
>
>Basically, trying to 'line up' MATLAB results from an element-wise 
>division of a matrix by a vector with R output.
>
>Here is a simplified version of the MATLAB code I'm translating:
>
>NN = [1, 2, 3; 4, 5, 6];? % Example matrix
>lambda = [2, 3, 4];? % Example vector
>result_matlab = NN ./ lambda;
>
>which yields
>
> ?0.50000?? 0.66667?? 0.75000
> ?2.00000?? 1.66667?? 1.50000
>
>
>So, the only way I have stumbled onto in R to generate the same results 
>is to use 'sweep'. The following 'works', but I'm hoping
someone can
>explain why I need something as convoluted as this seems (to me, at least).
>
>NN <- matrix(c(1, 2, 3, 4, 5, 6), nrow = 2, byrow = TRUE)? # Example
matrix
>lambda <- c(2, 3, 4)? # Example vector
>sweep(NN, 2, lambda, "/")
>
>
> ???? [,1]????? [,2] [,3]
>[1,]? 0.5 0.6666667 0.75
>[2,]? 2.0 1.6666667 1.50
>
>First tried the more 'obvious' NN/lambda, but that yields 'the
wrong
>answer' (based solely on what I'm trying to accomplish):
>
>
> ?????? [,1] [,2] [,3]
>[1,] 0.500000? 0.5? 1.0
>[2,] 1.333333? 2.5? 1.5
>
>So, why 'sweep'?
>
>	[[alternative HTML version deleted]]
>
>______________________________________________
>R-help at r-project.org mailing list -- To UNSUBSCRIBE and more, see
>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.
-- 
Sent from my phone. Please excuse my brevity.

... and here is a more or less direct translation of the Matlab code that
should now be obvious given your previous responses:
> m <- matrix(1:6, nr=2, byrow = TRUE) ## Matlab order
> m
     [,1] [,2] [,3]
[1,]    1    2    3
[2,]    4    5    6
> sweep(m, 2, 2:4, "/")
     [,1]      [,2] [,3]
[1,]  0.5 0.6666667 0.75
[2,]  2.0 1.6666667 1.50

Cheers,
Bert

On Tue, Feb 27, 2024 at 1:03?PM Evan Cooch <evan.cooch at gmail.com>
wrote:
> So, trying to convert a very long, somewhat technical bit of lin alg
> MATLAB code to R. Most of it working, but raninto a stumbling block that
> is probaably simple enough for someone to explain.
>
> Basically, trying to 'line up' MATLAB results from an element-wise
> division of a matrix by a vector with R output.
>
> Here is a simplified version of the MATLAB code I'm translating:
>
> NN = [1, 2, 3; 4, 5, 6];  % Example matrix
> lambda = [2, 3, 4];  % Example vector
> result_matlab = NN ./ lambda;
>
> which yields
>
>   0.50000   0.66667   0.75000
>   2.00000   1.66667   1.50000
>
>
> So, the only way I have stumbled onto in R to generate the same results
> is to use 'sweep'. The following 'works', but I'm
hoping someone can
> explain why I need something as convoluted as this seems (to me, at least).
>
> NN <- matrix(c(1, 2, 3, 4, 5, 6), nrow = 2, byrow = TRUE)  # Example
matrix
> lambda <- c(2, 3, 4)  # Example vector
> sweep(NN, 2, lambda, "/")
>
>
>       [,1]      [,2] [,3]
> [1,]  0.5 0.6666667 0.75
> [2,]  2.0 1.6666667 1.50
>
> First tried the more 'obvious' NN/lambda, but that yields 'the
wrong
> answer' (based solely on what I'm trying to accomplish):
>
>
>         [,1] [,2] [,3]
> [1,] 0.500000  0.5  1.0
> [2,] 1.333333  2.5  1.5
>
> So, why 'sweep'?
>
>         [[alternative HTML version deleted]]
>
> ______________________________________________
> R-help at r-project.org mailing list -- To UNSUBSCRIBE and more, see
> 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.
>
	[[alternative HTML version deleted]]

On Tue, 27 Feb 2024 14:54:26 -0500
Evan Cooch <evan.cooch at gmail.com> wrote:
> So, trying to convert a very long, somewhat technical bit of lin alg 
> MATLAB code to R. Most of it working, but raninto a stumbling block
> that is probaably simple enough for someone to explain.
On 

https://cran.r-project.org/other-docs.html

the documents:

?Matlab? / R Reference? by David Hiebeler (PDF, 2010-05-25, 52 pages)

and

?R and Octave? by Robin Hankin (Text), a reference sheet translating
between the most common Octave (or Matlab) and R commands

might be useful.  IIRC, I once used them when I had to do something in
Matlab/Octave and used them as reverse-lookup (I know how to do it in
R, how is it done in Matlab??). :-)

Cheers,

	Berwin