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

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Zuha Yousuf
Zuha Yousuf 2020-3-19
评论： Zuha Yousuf 2020-3-19
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??

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采纳的回答

Sriram Tadavarty
Sriram Tadavarty 2020-3-19
编辑：Sriram Tadavarty 2020-3-19
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

更多回答（1 个）

David Goodmanson
David Goodmanson 2020-3-19
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.

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