I bumped into this example trying to convert some julia to python and was surprised. In the example here, python and matlab give the same answer, while julia differs… If you start with… X=rand(3,100) (and do X=mat(X) if in python) and then do the following… matlab/octave: X(1,:)*X(1,:)' julia…

Yup, there absolutely is a reason and it’s a doozy. It turns out that the embedding of one-dimensional containers into the algebra of matrices is not obvious. There are a number of choices you must make in how they behave, impacting everything from indexing to transposes. It’s one of the few desi…

[image] Why column major? General Usage As I understand it, it was mainly historical precedent, driven by the fact that most linear-algebra libraries (LAPACK etc.) are column-major (because Fortran is column-major, and hence Matlab is column-major), though it would have be…

Yes, it’s expected. X[1,:] gives a flat vector. In case you want an 1xn array, do X[1:1,:].

[image] mbauman: The resulting dimensionality of an indexing expression is the sum of the dimensionalities of the indices. Or even more specifically, the resulting axes of an indexing expression is the concatenation of all the axes of all the indices used. I hope this is in a very visible pla…

It appears here: Multi-dimensional Arrays · The Julia Language , which is where we talk about all things indexing.

no but your summary here can beat 3 paragraphs and lengthy example in the actual manual I think.

[image] Brian_Blais: while julia returns a column vector (Nx1) One thing to keep in mind, that may make this less weird, is that Julia does not return an Nx1 array, but simply an N vector (no ‘x1’). The distinction makes a difference.

[image] Brian_Blais: matlab/octave: X(1,:)*X(1,:)' julia: X[1,:]*X[1,:]' python: X[1,:]*X[1,:].T what size should the result be and why? in python and matlab (and julia), X[1,:] will select a row. python and matlab however return a row vector (or a 1xN matrix) This is actually not quite ri…

[image] Brian_Blais: (and do X=mat(X) if in python) I noticed this just now, and it’s not something I’ve seen before. Does it alter the behavior of numpy arrays?

Yes, we need to pester @jiahao to make his draft paper public; if I remember right he details the “universes” of reasonable ways you can get vectors to work with matrices. None are perfect.

very interesting. I can see the reason. I caught me by surprise because of the difference with matlab and python with matrix operations. python uses two data types (array and matrix) so the operations for matrix are a bit different. matlab only had matrix operations. at least now I can understa…

Unintuitive Julia result: selecting a row from a matrix

New to Julia

jling August 11, 2021, 7:26pm 2

julia> x[1,:] # this returns a Vector, which is a column

julia> x[1,:]' # this traspose the Vector into a row-vector

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Unintuitive Julia result: selecting a row from a matrix

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