I often need to loop through the upper/lower triangular section of a matrix and usually end up doing something like: A = reshape(1:25,5,5) for j in axes(A,2) for i in axes(A,1) if i>j @show A[i,j] end end end This is fine but can lead to code with way too many …

Another way could be: lowTriIter2(A::AbstractMatrix) = Iterators.flatten(Iterators.map(j->(j,i), i+1:size(A,1)) for i=1:size(A,2)) julia> collect(lowTriIter2(A)) 10-element Vector{Tuple{Int64, Int64}}: (2, 1) (3, 1) (4, 1) (5, 1) (3, 2) (4, 2) (5, 2) (4, 3) (5, 3) (5, 4) This avoids …

Oh I don’t think I would have ever thought of making a generator of maps, interesting. I guess you could skip right to the generator as well: lowTriIter3(A::AbstractMatrix) = ((i,j) for j in axes(A,2) for i in j+1:size(A,1)) I do like that these methods skip over the additional iterations but …

[image] RobertGregg: This is fine but can lead to code with way too many indents. You know you can write both iterations on one line? for j in axes(A,2), i in j+1:size(A,1) @show A[i,j] end

[image] Iterating over elements of upper triangular matrix, but cartesian indices are needed New to Julia Basically what I (think) I need is this function: https://stackoverflow.com/questions/27086195/linear-index-upper-triangular-matrix Of course I can just implement it…

I did, but that’s a fair point! It’s wild how many ways you can accomplish the same task. I think I liked my version because it abstracted away the need to think about indices. Like if I wanted to change this nested loop to iterate over the upper triangle it would take me a minute to figure out ver…

Ah i didn’t see this. Sorry for the repeated topics. That formula for the ut2ci(idx) function is both amazing and headache inducing :rofl:

Just some additional thoughts … First, you could also use iterator comprehensions and the definition of your iterator looks just like your for loop: lowertriag(A::AbstractMatrix) = ((i,j) for j in axes(A, 2) for i in axes(A, 1) if i > j) for (i, j) in lowertriag(A) @show A[i, j] end Further,…

julia> (A[ci] for ci in CartesianIndices(A) if first(ci.I) ≤ last(ci.I)) |> collect 15-element Vector{Int64}: 1 6 7 11 12 13 16 17 18 19 21 22 23 24 25 julia> (A[ci] for ci in CartesianIndices(A) if first(ci.I) ≥ last(ci.I)) |> collect 15-element Vector{Int64}: 1 2 3 4 5…

Oh I did see that Iterators.flatmap was new to v1.9. It definitely looks useful but I’m not sure how I would apply it to this situation.

I’m not sure how this i > j comparison or i = j + 1 works on non-monotonic increasing array indices like StarWarsArrays ?

Nice way to iterate through a triangular matrix

General Usage

rocco_sprmnt21 July 16, 2023, 2:28pm 5

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Nice way to iterate through a triangular matrix

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