Asked by Hollis Williams
on 22 Apr 2019 at 18:05

I have an array of size 3 x 100 and I basically want to create a new array where I take each column vector in the array and compute the cross product with the same vector each time to create another array of size 3 x 100, so in this case I take every vector and form the cross product with [0 0 1]'. What would be the easiest way of doing this?

Answer by James Tursa
on 22 Apr 2019 at 18:45

Edited by James Tursa
on 22 Apr 2019 at 18:46

Accepted Answer

M = your 3xN matrix

v = your 3x1 vector

result = cross(M,repmat(v,1,size(M,2)));

James Tursa
on 22 Apr 2019 at 21:22

This is what I get:

>> M = rand(3,100);

>> v = rand(3,1);

>> size(repmat(v,1,size(M,2)))

ans =

3 100

I have no idea why you would be getting a 3x300 result. Did you actually run the size(M) and size(v) commands at the MATLAB prompt, or are you just reporting to me what you think they are? Actually type this in and see what you get

size(M)

size(v)

Hollis Williams
on 22 Apr 2019 at 22:24

Yes, I was using the size() command but made a typo, it is working now. I have created three 1 x 100 arrays, is it possible to put these together into one 3 x 100 array? So if A,B,C are all 1 x 100 arrays, I would need

A

B

C

James Tursa
on 22 Apr 2019 at 22:26

result = [A;B;C];

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Answer by Matt J
on 22 Apr 2019 at 19:02

Edited by Matt J
on 22 Apr 2019 at 19:05

This way avoids repmatting, which may be desirable when N is large.

M = your 3xN matrix

v = your 3x1 vector

result=xprodmat(v)*M./vecnorm(M);

where

function A=xprodmat(a)

%Matrix representation of a cross product

%

% A=xprodmat(a)

%

%in:

%

% a: 3D vector

%

%out:

%

% A: a matrix such that A*b=cross(a,b)

if length(a)<3, error 'Input must be a vector of length 3'; end

ax=a(1);

ay=a(2);

az=a(3);

A=zeros(3);

A(2,1)=az; A(1,2)=-az;

A(3,1)=-ay; A(1,3)=ay;

A(3,2)=ax; A(2,3)=-ax;

end

Hollis Williams
on 22 Apr 2019 at 20:51

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