How to expand a Matrix ?

I need to expand a matrix of size 505*3 to a size of 30300*3. Basically 30300/505 is 60. My requirement is to copy the same values of every row to the next 59 rows and thereby increase the matrix size. The whole idea is to convert the values of some data we receive hourly to mins. The data changes hourly however the computation is required to be in mins.Thus the values would remain the same for every hour. Any help would be appreciated.

Answer 1

[Walter Roberson]

NewMatrix = kron(YourMatrix, ones(60,1));

You should also consider using retime()

[Stephen23]

+1 kron is the simplest solution to this... or perhaps repelem is.

[John D'Errico]

Yes, it is.

[alper cicek]

Thanks :)

Answer 2

[John D'Errico]

One answer is to just use indexing, using a carefully designed index vector. Or, you can use tools like repmat & permute & reshape, and think about how data is stored in memory. Both work...

A = rand(505,3);

indexing:

ind = ceil((1:(505*60))/60);
B = A(ind,:);

If you want to understand why it works, look at the vector ind. Think about how it was created.

repmat:

B = reshape(permute(repmat(A,[1 1 60]),[3 1 2]),505*60,3);

To understand this one, you need to take apart each piece. First, think about what the repmat does. Then look at permute. Understand what that does in memory. Finally, look at how reshape works. So, effectively, look at what each of these steps do:

B1 = repmat(A,[1 1 60]);
B2 = permute(B1,[3 1 2]);
B = reshape(B2,505*60,3);

Read the help for these things if you don't understand them. This is the only way you will learn MATLAB.

Answer 3

[Stephan]

Hi, this would work i think:

A = [1 2 3; 4 5 6]
n = 3 % Number of repeated values --> 60 for your case

for k = 1:size(A,2)
    B(:,n*k-(n-1) : n*k) = repmat(A(:,k),1,n);
end

For example:

A =

       1     2     3
       4     5     6

n =

       3

B =

       1     1     1     2     2     2     3     3     3
       4     4     4     5     5     5     6     6     6

Maybe there are more elegant ways to do this.

Best regards

Stephan

[John D'Errico]

While an idea like this can work, be careful. If you do it without preallocation, it can be terribly slow when you start creating large arrays.

Otherwise, MATLAB will need to reallocate memory each time, for a slightly larger array. Not a problem for a matrix that ends up growing only 3 times. But what if you wanted to do it 60 times? What if you wanted to replicate it 3600 times to get data by the second?

This is why you preallocate such arrays. If B is already the correct size in advance, then MATLAB only needs to stuff data into the proper place in memory. Without preallocation, you have an array that grows, taking quadratically increasing time. Again, no real problem if you do it 3 times,

If you want to see the difference, test the time for different amounts of replication, comparing what happens. This is perhaps the number one reason why people ask questions about how to speed up their code on Answers, and it is the easiest problem to fix. Just preallocate B in advance.