Assign a value to each page of a 3D Matrix

Hello everyone

I got a 3D Matrix:

A=ones[3,3,3]

and a Vector of values:

x=[10 20 30]

How can I assign the values of x to all values of a whole page of A?

My goal is to get A to be:

A( :, :, 1) = 10

A( :, :, 2) = 20

A( :, :, 3) = 30

I want it to be vectorised - also the number of elements of x (and the number of pages in A - respectively) can change in my program)

Can anyone help? Much appreciated!

Best Regard

DS

Answer 1

[Bjorn Gustavsson]

If we stick to the first rule of programming (KISS) this should get the job done:

A = zeros([3,4,5]);
x = [10 20 30 2^.5 exp(1)];
for i1 = 1:numel(x),
  A(:,:,i1) = x(i1);
end

Unless you have very specific needs I see no reason to mess about with anything else - unless you can present profiling-results showing that this is a time-critical step in a program...

HTH

[DS]

Hey Bjorn,

thanks a lot for your answer.

I posted a simple example here. My programm contains a Matrix with dimensions

10 000x 100x 10 000. My supervisor asked me to vectorise this step - unfortunately I'm new to programming so I have no idea if this can get any time critical or not.

[Bjorn Gustavsson]

Hello Daniel,

Then you should look into that. Simply do a couple of tests of your program with the profiler (check the help and documentation to profile - that should show you how to do that). Try that with a few sizes of your variables this will show you what lines of code takes time. The thing I think you will struggle with is the sheer size of the 10^4-by100-by-10^4 array - to my QD estimate that will require some 80 GB of space to store. Maybe there's ways to do this assignment faster, but loops over pre-allocated loops aren't that bad - since a long time ago.

[Matt J]

My programm contains a Matrix with dimensions 10000 x 100 x 10000

That would be 37 GB in single float precision. Do you really have that much RAM to work with?

[DS]

That, in fact, is a pretty valid point!

I'll try your solution and see how it goes.

Thanks a lot for your Time

[Bjorn Gustavsson]

My pleasure.

[Stephen23]

While vectorization of this might be possible with some effort, it would be more complex, obfuscated, and require large intermediate arrays (i.e. be slow). This answer is perfectly good MATLAB code, whatever your supervisor might think.