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sortingmatrixjuliaranking

Ranking over each matrix column's sort in julia

I have a matrix (m) of scores for 4 students on 3 different exams.

4 3 1
3 2 5
8 4 6
1 5 2

I want to know, for each student, the exams they did best to worse on. Desired output:

1 2 3
2 3 1
1 3 2
3 1 2

Now, I'm new to the language (and coding in general), so I read GeeksforGeeks' page on sorting in Julia and tried

mapslices(sortperm, -m; dims = 2)

However, this gives something subtly different: a matrix of each row being the index of the sorting.

1 2 3
3 1 2
1 3 2
2 3 1

Perhaps it was obvious, but I now realize this is not actually what I want, but I cannot find a built-in function/fast way to complete this operation. Any ideas? Preferably something which doesn't iterate through items in the matrix/row, as in reality my matrix is very, very large. Thanks!

Solution

  • Such functionality is provided by StatsBase.jl. Here is an example:

    julia> using StatsBase

julia> m = [4 3 1
            3 2 5
            8 4 6
            1 5 2]
4×3 Array{Int64,2}:
 4  3  1
 3  2  5
 8  4  6
 1  5  2

julia> mapslices(x -> ordinalrank(x, rev=true), m, dims = 2)
4×3 Array{Int64,2}:
 1  2  3
 2  3  1
 1  3  2
 3  1  2

    You might want to use other rank, depending on how you want to split ties, see here for details.