How to normalize the columns of a matrix in Julia

Question

Given a matrix A of dimensions m,n, how would one normalize the columns of that matrix by some function or other process in Julia (the goal would be to normalize the columns of A so that our new matrix has columns of length 1)?

Answer 1

mapslices seems to have some issues with performance. On my computer (and v1.7.2) this is 20x faster:

x ./ norm.(eachcol(x))'

This is an in-place version (because eachcol creates views), which is faster still (but still allocates a bit):

normalize!.(eachcol(x))

And, finally, some loop versions that are 40-70x faster than mapslices for the 5x3 matrix:

# works in-place:
function normcol!(x)
    for col in eachcol(x)
        col ./= norm(col)
    end
    return x
end
# creates new array:
normcol(x) = normcol!(copy(x))

Edit: Added a one-liner with zero allocations:

foreach(normalize!, eachcol(x))

The reason this does not allocate anything, unlike normalize!., is that foreach doesn't return anything, which makes it useful in cases where output is not needed.