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Is there an alternative way to perform the following matrix transformation?

Asked by Erivelton Gualter on 22 Nov 2019 at 7:25
Latest activity Edited by Stephen Cobeldick on 22 Nov 2019 at 9:17
Is there a better way to perform the following operation.
For example, for the following matrix:
A =
1 13 25
2 14 26
3 15 27
4 16 28
5 17 29
6 18 30
7 19 31
8 20 32
9 21 33
10 22 34
11 23 35
12 24 36
I am looking to transform to:
A_new =
1 13 25 2 14 26 3 15 27
2 14 26 3 15 27 4 16 28
3 15 27 4 16 28 5 17 29
Note, I am combining groups of three rows in a row for the entired matrix.
The following piece of code is already able to perform this. However, I have a huge matrix, and I believe there is an optimal way to do this. Also, I would like to play aroung with the number of rows to create the new matrix.
% Matrix A
A=1:36; A = reshape(A,12,3)
j=1;
for i=1:3
A_new(i,:) = [A(j,:) A(j+1,:) A(j+2,:)];
j = j +1;
end
A_new

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2 Answers

Answer by Andrei Bobrov
on 22 Nov 2019 at 8:54
Edited by Andrei Bobrov
on 22 Nov 2019 at 9:08
 Accepted Answer

i = 3;
[m,n] = size(A);
j = hankel(1:i,i:m);
out = A(sub2ind([m,n],kron(j,ones(1,n)),repmat(1:n,i,m-n+1)));
or in your case
j = hankel(1:i,i:2*i-1);
out = A(sub2ind([m,n],kron(j,ones(1,i)),repmat(1:n,i,i)));
or for out:
out = reshape(permute(reshape(A(j,:),i,n,[]),[1,3,2]),i,[]);

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Answer by Stephen Cobeldick on 22 Nov 2019 at 9:03
Edited by Stephen Cobeldick on 22 Nov 2019 at 9:17

>> N = 3;
>> [R,C] = size(A); % specify R to change the number of rows of A used.
>> X = hankel(1:N,N:R);
>> B = reshape(A(X,:).',N*C,[]).'
B =
1 13 25 2 14 26 3 15 27
2 14 26 3 15 27 4 16 28
3 15 27 4 16 28 5 17 29
4 16 28 5 17 29 6 18 30
5 17 29 6 18 30 7 19 31
6 18 30 7 19 31 8 20 32
7 19 31 8 20 32 9 21 33
8 20 32 9 21 33 10 22 34
9 21 33 10 22 34 11 23 35
10 22 34 11 23 35 12 24 36

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