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

Question

Zuha Yousuf 2020-3-19

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https://ww2.mathworks.cn/matlabcentral/answers/511681-how-to-use-tril-function-on-only-specific-columns-in-a-large-matrix

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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Answer 1

Sriram Tadavarty 2020-3-19

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https://ww2.mathworks.cn/matlabcentral/answers/511681-how-to-use-tril-function-on-only-specific-columns-in-a-large-matrix#answer_420865

编辑：Sriram Tadavarty 2020-3-19

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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

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Zuha Yousuf 2020-3-19

Thank you so much! I really appreciate it.

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Answer 2

David Goodmanson 2020-3-19

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https://ww2.mathworks.cn/matlabcentral/answers/511681-how-to-use-tril-function-on-only-specific-columns-in-a-large-matrix#answer_420871

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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.