Add matlab-like tril method for two-dimensional arrays

[fedemagnani]

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Adding trill method for two-dimensional arrays in order to create a lower triangular matrix from a given matrix. This function reproduces the behavior of the Matlab function with same name: it returns the elements on and below the kth diagonal of A

[nilgoyette]

I'm not against adding this feature to ndarray and I think the other maintainers would agree. This being said

• I'm not a fan of the name. tril seems to be the standard (Matlab and Numpy), but it means nothing (maybe it's just me?)
• I didn't take the time to think about it, but the implementation looks like it could be "better". For example, numpy does it in 3 lines (I know we can't and shouldn't do it like Numpy)

[adamreichold]

I'm not a fan of the name. tril seems to be the standard (Matlab and Numpy), but it means nothing (maybe it's just me?)

I guess we could just spell it out, e.g. triangular_lower?

[fedemagnani]

I wrote this method trying to reproduce the behaviour of a matlab script which was invoking this function.

• I'm not a fan of the name. tril seems to be the standard (Matlab and Numpy), but it means nothing (maybe it's just me?)

Like you I found the name a bit confusing at first glance, but since it generates a lower triangular from a given matrix I thought that "tri" was standing for "triangular" and "l" as "lower". I decided to keep the same name for consistency with the matlab function thinking that it could have been useful for people with my similiar use case.

• I didn't take the time to think about it, but the implementation looks like it could be "better". For example, numpy does it in 3 lines (I know we can't and shouldn't do it like Numpy)

Regarding the length of the code, this is the way I wrote it and I think that adding a method which expands the functionalities of the package is better than not doing it :)

I don't think that this method will be used that much but if it will surge the need of optimizing it I bet that it is going to be optimized in the future

[nilgoyette]

I don't think that this method will be used that much but if it will surge the need of optimizing it I bet that it is going to be optimized in the future

There's no need to procrastinate, we can do it properly right now. If you're still interested, of course.

• Direct indexing is "slow" in ndarray and that's what you use
• You could test using a Zip::indexed(m.rows_mut()), then something like row.slice(s![?..]).fill(0.0). If I'm right, this could be done in 4-5 lines.
• Once this is done, you might as well code triu. It doesn't make much sense to have one without the other!