In the matrix below, the first, second, and third columns represent the height, area, and volume of rectangular objects, respectively. I need to sort the items in non-ascending order of height (from the highest to the lowest). After sorting, the associated area and volume for each item should remain…

Try: julia> sortslices(A, dims=1, rev=true) 5×3 Matrix{Float64}: 29.58 273.83 840.17 18.99 89.68 683.06 10.77 269.75 1928.6 4.27 122.62 102.83 2.18 178.34 214.79

Another option: julia> @view A[sortperm(A[:,1]; rev=true),:] 5×3 view(::Matrix{Float64}, [1, 2, 3, 4, 5], :) with eltype Float64: 29.58 273.83 840.17 18.99 89.68 683.06 10.77 269.75 1928.6 4.27 122.62 102.83 2.18 178.34 214.79 This doesn’t materialize the sorted matrix which …

for this small matrix the first expression seems more efficient (but perhaps for larger matrices the others work better) julia> using BenchmarkTools julia> @btime stack(sort(eachrow(A),by=first, rev=true),dims=1) 375.879 ns (3 allocations: 464 bytes) 5×3 Matrix{Float64}: 29.58 273.83 840.17 …

[image] rocco_sprmnt21: @view A[sortperm(A[:,1]; rev=true),:] Oooh, I forgot another @view: @view A[sortperm(@view A[:,1]; rev=true),:] (maybe this is better)

[image] Dan: @view A[sortperm(@view A[:,1]; rev=true),:] Cleaner to use @views A[sortperm(A[:,1]; rev=true),:]

@Dan when you state that this does not materialize, could you explain it a bit more? Does this mean that instead of sorting an array/matrix/whatever, one can get a view of the element in a sorted order? And if I update elements of this view, then it should propagate correctly to the initial eleme…

Seems you got it right: [image] Ahmed_Salih: Does this mean that instead of sorting an array/matrix/whatever, one can get a view of the element in a sorted order? Yes [image] Ahmed_Salih: And if I update elements of this view, then it should propagate correctly to the initial element? …

[image] Dan: “does not materialize” means does not have to allocate memory and set it to these values, but the required values are calculated on the CPU from the inputs as needed. Just note that this is not free. Each array access now requires an additional load. If you plan to access the sor…

in fact it seemed strange that it could be an “original” topic :grin: I tried various combinations of the parameters of the various expressions. This seems to be the most efficient @btime @views $A[sortperm($A[:,1]; lt=(x,y)->y<x),:]

Sorting matrix according to the first column

General Usage

rafael.guerra March 28, 2024, 7:36am 10

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Sorting matrix according to the first column

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