Hi all I have seen many examples of converting an array of arrays into a matrix, but I would like to go the other way. What I have cobbled together works, but may not be the most elegant or fast solution. I would appreciate any comments and builds. function sliceMatrix(A) m, n = size(A) B …

I don’t think you gain much from preallocating in this situation, but here is one way where I only modified your functions a little bit: julia> function slicematrix(A::AbstractMatrix{T}) where T m, n = size(A) B = Vector{T}[Vector{T}(undef, n) for _ in 1:m] for i in…

Thank you all! The suggestions are much appreciated.

While trying this myself, I found a faster solution than the simple suggestion above. Iterate over the columns with eachcol or eachrow: A = rand(5,10000) @btime [c[:] for c in eachcol(A)] @btime [A[:,c] for c in 1:size(A,2)] julia> 382.300 μs (20005 allocations: 1.75 MiB) 2.788 ms (38985 alloca…

Note that your benchmarking produced false inferences due to accessing global variables. If you interpolate the variables into the expressions (using $). The results are much more similar. I also include an even faster version, if you can work with static vectors: using StaticArrays A = rand(5,100…

I think @MatthijsCox approx with eachcol is in fact faster (but still slower than StaticArrays), the problem is how columns are collected. A = rand(5,10000); @btime collect(eachcol($A)); @btime [c[:] for c in eachcol($A)]; @btime [$A[:,c] for c in 1:size($A,2)]; [A[:,c] for c in 1:size(A,2)] == col…

[image] BMO: @btime collect(eachcol($A)); @btime [c[:] for c in eachcol($A)]; @btime [$A[:,c] for c in 1:size($A,2)]; Note that the latter two create copies, while the first one returns views: julia> @btime [c[:] for c in eachcol($A)]; 535.007 μs (20005 allocations: 1.75 MiB) julia> @bt…

Converting a matrix into an array of arrays

General Usage

baggepinnen November 1, 2018, 8:48am 3

A one liner, likely not as efficient as the suggestion above, is
mapslices(x->[x], randn(5,5), dims=2)[:]

Iterating over the columns of a matrix

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Converting a matrix into an array of arrays

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