Matlab: reshape a vector by permuting?

Question

I have the following vector u:

u=[a1,a2,a3,a4,b1,b2,b3,b4,c1,c2,c3,c4];

I want to permute the elements of of u to make the following vector, uNew:

uNew=[a1,b1,c1,a2,b2,c2,a3,b3,c3,a4,b4,c4];

I can think of no way of doing this other than with a for loop:

uNew=[]; for i=1:4 uNew=[uNew,u(i:4:end)]; end

But I'm hoping that a built-in function exits? Thanks!

Do you know before hand how many elements separate each vector? For example, do you know that a, b and c are all the same size? In addition, is there a possibility that there are more vectors?... could there be a d? e? — rayryeng
– rayryeng, Commented Aug 26, 2015 at 19:22
No there can't be d, e, etc. so I know everything about the vector. ai, bi and ci are scalars I used to make the concept clearer in the question. — space_voyager
– space_voyager, Commented Aug 26, 2015 at 19:23

Daniel · Accepted Answer · 2015-08-26 19:23:57Z

3

Reshape it to a matrix which, read column by column, contains the order you want:

n=3 % number of categories (a,b,c) u2=reshape(u,[],n).'

then convert it back to a vector:

u2=u2(:);

answered Aug 26, 2015 at 19:22

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3 Comments

Was seriously just about to put this.

@rayryeng: Is there any alternative? ;)

... not that I can think of currently!

Community · Accepted Answer · 2017-05-23 11:43:57Z

I'd use Daniel's approach. But just to provide an alternative:

m = 4; %// size of each initial "block" [~, ind] = sort(mod(0:numel(u)-1,m)+1); uNew = u(ind);

Note that this works because, as per sort's documentation,

The ordering of the elements in B (output) preserves the order of any equal elements in A (input)

Rafael Monteiro · Accepted Answer · 2015-08-26 20:20:51Z

0

Another approach:

N = 4; uNew = u(mod(0:numel(u)-1, N) * N + floor((0:numel(u)-1)/N) + 1);

answered Aug 26, 2015 at 20:20

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Your code as it stands gives me an error. Do you mean u(mod(0:numel(u)-1, numel(u)/N)*N + floor((0:numel(u)-1)/numel(u)*N) + 1)?