What is the best way in Julia to vectorize a function along a specific axis? For example sum up all the rows of a matrix. Is it possible with the dot notation?
sum.(ones(4,4))
Does not yield the desired result.
2 Answers
Try using the dims argument on a lot of functions that deal with sets of values.
sum([1 2; 3 4], dims=2)
2×1 Matrix{Int64}:
3
7
# or
using Statistics
mean([1 2; 3 4], dims=1)
1×2 Matrix{Float64}:
2.0 3.0
There is already a standard function called mapslices, looks like exactly what you need.
julia> mapslices(sum, ones(4, 4), dims = 2)
4-element Vector{Float64}:
4.0
4.0
4.0
4.0
You can find the documentation here or by typing ? followed by mapslices in REPL.
If in your example you want to use the dot notation you should pass an array of rows, not the array itself. Otherwise, sum is applied to each element resulting in the same matrix. It can be done with eachrow and eachcol for rows and columns respectively.
julia> sum.(eachrow(ones(4, 4)))
4-element Vector{Float64}:
4.0
4.0
4.0
4.0
EDIT: I tried to suggest a more general solution, but if you have this option I would recommend using Andre's answer.
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