Combining one matrix into another that are of different sizes
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Dear all
I am trying to create a numbers of matrices so that I can use and manipulate them.
First I have:
n = 36;
Tx = zeros(6,n)
Then I have this matrix:
V = [0 1 0 0 0 0;
-1 0 0 0 0 0;
0 0 1 0 0 0;
0 0 0 0 1 0;
0 0 0 -1 0 0;
0 0 0 0 0 1]
I want to insert the matrix 'V' into Tx at certain places.
For example, I want a collection of:
Tx(:,[1:6],1) = V;
Tx(:,[4:9],2) = V;
Tx(:,[7:12],3) = V;
Tx(:,[10:15],4) = V;
clear,clc
Etc.........
Now I have tried using a loop to save all this computational time, but it seems to be counting the previous iteration and adding it on to the matrix Tx. Its saying after one iteration that the left hand side is 6 by 9. I need the left hand side to remain at 6 by 6.
n = 36;
Tx = zeros(6,n);
V = [0 1 0 0 0 0;
-1 0 0 0 0 0;
0 0 1 0 0 0;
0 0 0 0 1 0;
0 0 0 -1 0 0;
0 0 0 0 0 1];
for k = 1:3:30
for j = 6:3:36
for i = 1:4
Tx(:,[k:j],i) = V
end
end
end
I am hoping someone can help?
Many thanks,
Scott
4 Comments
"I need the left hand side to remain at 6 by 6."
How can Tx "remain" 6x6 when you define it to be 6x36 ? It does not shrink.
Not that the indexing overlaps and you have not specified what you want to occur in the overlap: do you want to add the new values within that overlap, or completely replace all elements (which is what you seem to be attempting with that code) ?
Your array sizes are very confusing: although you preallocate Tx to have size 6x36 in the example indexing you also specify the 3rd dimension:
Tx(:,1:6,1) = V;
Tx(:,4:9,2) = V;
Tx(:,7:12,3) = V;
% ^ 3rd dimension
which you do not mention in you question at all. Confusing. What is the actual size of Tx that you expect at the end?
Info: note that the colon operator returns a vector. You then use square brackets to concatenate that vector with nothing else.... which thus returns exactly the same vector as the colon operator returns. In other words, get rid of the totally superfluous misleading square brackets.
Paul
on 2 Aug 2025
At some point you'll get k = 1 and j = 9 and i = 1.
Then the assignment would be Tx(:,1:9,1) = V
The LHS of that statement references 9 columns, but V only has 6 columns, hence the error message.
Scott Banks
on 2 Aug 2025
A simple loop would be:
n = 36;
Tx = zeros(6,n,4);
V = [0 1 0 0 0 0;
-1 0 0 0 0 0;
0 0 1 0 0 0;
0 0 0 0 1 0;
0 0 0 -1 0 0;
0 0 0 0 0 1];
for ii = 0:3
Tx(:,(1:6)+ii*3,ii+1) = V;
end
Accepted Answer
Given
V =[ 0 1 0 0 0 0;
-1 0 0 0 0 0;
0 0 1 0 0 0;
0 0 0 0 1 0;
0 0 0 -1 0 0;
0 0 0 0 0 1];
"I want a collection of:"
%Tx(:,[1:6],1) = V;
%Tx(:,[4:9],2) = V;
%Tx(:,[7:12],3) = V;
%Tx(:,[10:15],4) = V;
Tx=nan(6,15,4); % so can see where write; replace w/ zeros for real
c1=1; % first column
delta=3; % offset in column
w=width(V); % how wide is V
for p=1:4 % over planes
c2=c1+w-1; % column two location
Tx(:,c1:c2,p)=V; % put the V array at this location in this plane
c1=c1+delta; % increment the first column for next plane
end
Tx
Alternatively, since the source is always the same
[h,w]=size(V);
Tx=nan(h,15); % so can see where write; replace w/ zeros for real
delta=3; % offset in column
Tx(:,1:w)=V; % put the V array at start location
Tx=repmat(Tx,1,1,4); % replicate to the four planes
for p=2:4 % over remaining planes
Tx(:,:,p)=circshift(Tx(:,:,p-1),delta,2); % shift to desired location
end
Tx
4 Comments
Scott Banks
on 2 Aug 2025
As you can see, the nans are there only to make the positioning of the blocks more visible and verifiable. How about you take the extra step of converting the NaNs to zeros and Accept @dpb's answer if that's all that is left to do.
Scott Banks
on 2 Aug 2025
dpb
on 2 Aug 2025
As the comment says, use zeros instead of nan to create the original array of whatever final dimensions desired.
In reality, @Stephen23's use of @doc:kron is the elegant solution; I knew it would work, I just always have to scratch my head over it so took the easy way out above.
More Answers (3)
n = 36;
V = kron( eye(2) , [ 0 1 0; -1 0 0; 0 0 1]) ;
V(end,n)=0;
K=numel(1:3:32);
Tx=cell(1,1,K);
Tx{1}=V;
for k = 2:K
Tx{k}=circshift(Tx{k-1},[0,3]);
end
Tx=cell2mat(Tx)
whos Tx
n=36;
V = [0 1 0 0 0 0;
-1 0 0 0 0 0;
0 0 1 0 0 0;
0 0 0 0 1 0;
0 0 0 -1 0 0;
0 0 0 0 0 1];
E=eye(n); E=E(1:3:n-4,:); E=permute(E,[3,2,1]);
Tx=convn(E,V);
Tx=Tx(:,1:n,:)
Scott Banks
on 2 Aug 2025
Edited: Scott Banks
on 2 Aug 2025
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