Matlab: repeat every column sequentially n times

Question

I'm pretty much beginner so it's probably possible to do what I want in a simple way. I have a matrix 121x62 but I need to expand it to 121x1488 so every column has to be repeated 24 times. For example, transform this:

   2.2668       2.2667       2.2667       2.2666       2.2666       2.2666       
   2.2582       2.2582       2.2582       2.2582       2.2581       2.2581       
    2.283        2.283        2.283       2.2829       2.2829       2.2829       
   2.2881       2.2881       2.2881       2.2881       2.2881        2.288        
    2.268        2.268       2.2679       2.2679       2.2678       2.2678       
   2.2742       2.2742       2.2741       2.2741       2.2741        2.274

into this:

2.2668     2.2668     2.2668  and so on to 24th     2.2667     2.2667  and again to 24x
2.2582     2.2582     2.2582 ...

with every column.

I've tried to create a vector with these values and then transform with vec2mat and ok I have 121x1488 matrix but repeated by rows:

2.2668   2.2668   2.2668  2.2668  2.2668  2.2668 ...    2.2582   2.2582  2.2582  2.2582 ...

How to do it by columns?

Could You paste the code with vec2mat transformation? – Grzegorz Piwowarek Apr 28 '13 at 19:29 — Grzegorz Piwowarek, Apr 28 '13 at 19:29
GPT2=repmat(ZHD_GPT2(:),1,24); X=vec2mat(GPT2,1488); – papkin Apr 28 '13 at 19:31 — papkin, Apr 28 '13 at 19:31

Oleg · Answer 1 · 2015-03-31T12:44:06.390

23

Suppose you have this simplified input and you want to expand columns sequentially n times:

A   = [1 4
       2 5
       3 6];

szA = size(A); 
n = 3;

There are few ways to do that:

Replicate, then reshape:
```
reshape(repmat(A,n,1),szA(1),n*szA(2))
```
Kronecker product:
```
kron(A,ones(1,n))
```
Using FEX: expand():
```
expand(A,[1 n])
```
Since R2015a, repelem():
```
repelem(A,1,n)
```

All yield the same result:

ans =
     1     1     1     4     4     4
     2     2     2     5     5     5
     3     3     3     6     6     6

edited Mar 31 '15 at 12:44

answered Apr 28 '13 at 23:34

Oleg

10,406
3
29
57

4

+1 for `kron` solution – Shai Apr 29 '13 at 06:58
Thanks for three solutions. Is there any preference regarding performance? Actually, I do this in a huge loop which makes the performance quite important. – Alireza Mirian Mar 20 '14 at 09:52
1

@AlirezaMirian Replicate, reshape should be the fastest. – Oleg Mar 21 '14 at 08:31
Agree. kron is a quite an elegant solution – Camilo Martinez Dec 10 '14 at 15:19

score 1 · Answer 2 · answered Aug 21 '17 at 16:09

1

Just for the sake of completeness. If you want to duplicate along the rows, you can also use rectpulse().

A = [1,2,3;...
     4,5,6];

n = 3;

rectpulse(A, n);

gives you

answered Aug 21 '17 at 16:09

Malte

31
2

Grzegorz Piwowarek · Answer 3 · 2013-05-26T07:35:32.280

0

Here You go:

function [result] = repcolumn(A, n)
    %n - how many times each column from A should be repeated

    [rows columns] = size(A);
    result = repmat(A(:,1),1,n);

    for i = 2:columns
        result = [result,repmat(A(:,i),1,n)];
    end
end

There must be an easier way but it does the job.

edited May 26 '13 at 07:35

answered Apr 28 '13 at 19:51

Grzegorz Piwowarek

13,172
8
62
93

Thank You. It is clean enough and it works – papkin Apr 28 '13 at 20:22

Matlab: repeat every column sequentially n times

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