How to use tril function on only specific columns in a large matrix?

So I have a very large matrix (31413 rows, 950 columns). I want to use the tril function on columns 2 till 10, and then from 12 to 20, and then from 22 to 30, and so on until the end of columns.

I need something like this

[col1 col2...col10 col11 col12...col20 col21 col22...col30 and so on]

where the columns in between have tril function used on them.

Is there any way I can do this??

Answer 1

[Sriram Tadavarty]

Hi Zuha,

You can use a for loop as below and it does what is required:

m = rand(31413,950);
for i = 2:10:size(m,2)
    m(:,i:i+8) = tril(m(:,i:i+8));
end

Hope this helps.

Regards,

Sriram

[Zuha Yousuf]

Thank you so much! I really appreciate it.

Answer 2

[David Goodmanson]

Hi Zuha,

here's another, more vectorized version

m = rand(31413,950);
[r c] = size(m);
[jj ii] = meshgrid(1:c,1:8);  % ii row  jj col
ind = ii<mod(jj-1,10);
m18 = m(1:8,:);
m18(ind) = 0;
m(1:8,:) = m18;

It's faster than the for loop version, which is not a big deal since Sriram's code only takes 0.2 sec, and that code is more self-explanatory.

[Zuha Yousuf]

Thanks so much David! This approach makes sense to me too.