Suppose I am given an mXn matrix A, and I have a permutation of {1,…,m}, for example by p = sortperm(A[:,1]); I now wish to rearrange the wors of A based on p. How do I do that ? The permute!() function only applies to vectors, as shown here . I can always loop over the columns, as in for j in 1:…

[image] iamsuddhasattwa: But there is no way anyone can possibly understand from the documentation alone that lt=(x,y)->isless(x[2],y[2])) is supposed to mean ``by second column". You can also write sortslices(A, dims=1, by=row->row[2]), although I agree it’s not super-obvious what’s going to…

[image] iamsuddhasattwa: permute!(A[:,j], p); Note that this permutes a copy of the column, unless you switch to using views (e.g. wrapping the code block or the line in a @views macro call). For an alternative approach, you could write something similar to the Base.permutecols!! function (w…

[image] stevengj: (It has a double !! because it also overwrites the permutation array p .) Is this a (new) convention? Is it meant to signify that more than one passed argument is being mutated, or is its meaning more specific?

[image] anon94023334: Is this a (new) convention? Is it meant to signify that more than one passed argument is being mutated, or is its meaning more specific? A couple of Base functions use it to indicate that multiple arguments are mutated, but I don’t think it’s a very widespread convention…

You can use sortslices, it sorts by rows, columns, or any row/column specified by the user. julia> A = rand(0:9,5,5) 5×5 Array{Int64,2}: 4 6 4 6 3 2 7 5 5 2 7 7 0 4 3 4 5 2 5 6 6 5 8 1 0 julia> sortslices(A, dims=1) 5×5 Array{Int64,2}: 2 7 5 5 2 4 5 2 5 6 4 6 …

Thank you. Although I was looking for an in-place algorithm, this function is a very useful to learn. From the examples that you provided, it seems that the sorting is only according to the first column or row. But what if I try to rearrange the rows in in increasing order of the second column ? You…

Thank you, however the function permutecolums itself did not work. julia> permutecols!!(A,p) ERROR: UndefVarError: permutecols!! not defined Stacktrace: [1] top-level scope at none:0 In fact, it seems that Base does not have any such function defined. julia> Base.permute permute! permute!! …

Yes, as I said you can sort by any row/column you want which can be very useful. Just override the lt function to specify how the new comparison is done. Searching for documentation inside the REPL is so easy, just type ?sortslices and you get good examples. julia> A = rand(0:9,5,5) 5×5 Array{Int64…

Thanks. I did search for sortslices. But there is no way anyone can possibly understand from the documentation alone that lt=(x,y)->isless(x[2],y[2])) is supposed to mean ``by second column".

[image] iamsuddhasattwa: In fact, it seems that Base does not have any such function defined. It was added in Julia 1.2.

Permuting the rows of a matrix based on given a given permutation

General Usage

stevengj Split this topic May 22, 2025, 12:08am 14

2 posts were split to a new topic: Constructing a permutation matrix

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Can't get sortperm to return the sorting order of columns of a matrix General Usage question , sort , sortperm	11	1468	July 16, 2022
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Fastest way to permute Array, given some permutation Performance sortperm , arrays	13	4770	September 12, 2021
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Permuting the rows of a matrix based on given a given permutation

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