Re: Best way to get indices and values at sorting (rows/columns of) a matrix?

[Hakuna M.]

Hi there, I'm new to Julia but have some experience with Matlab.

To sort each column of a matrix M ( r rows, c columns) I can write in Matlab:

sry, there is an error with posting, I cant post the full text:

[v i] = sort (M)

return values are the sorted matrix v and the rowindex i of each value.
Example:

m =  3     2
     1     4
     6     5

[v i] = sort(m)

v =  1     2
     3     4
     6     5

i =  2     1
     1     2
     3     3

What is the best way to get the same results (sorted matrix, and indices) in Julia?

[Hakuna M.]

currently I'm using:

i = mapslices(sortperm, m, 1);
offset=repmat( (0:(c-1))'.*r ,r, 1); #'
v = m[offset+i];

[Hakuna M.]

but I don't like the storage allocation of 'offset'. Is there a better way to do it?
I would like to avoid for-loops.

Is there a way to access the current index of mapslices (ciom) ? And do something like this:

mapslices(x - > m[ x[:, ciom ], ciom], i, 1);

or do i need to do a for?

for ciom=1:c;  v[:,ciom] =m [ i[:, ciom ], ciom] ; end

other ideas?

[Steven G. Johnson]

See https://github.com/JuliaLang/julia/issues/9258

[Steven G. Johnson]

Actually, see https://github.com/JuliaLang/julia/issues/16273

[Hakuna M.]

Am Montag, 9. Mai 2016 15:26:48 UTC+2 schrieb Steven G. Johnson:

Actually, see https://github.com/JuliaLang/julia/issues/16273

Thanks for your answer. Sry, about my strange postings, there was an error, you have seen the other two postings below?.

sortperm(A, dim)

would be nearly the same as:

mapslices(sortperm, A, dim);

or?
But then I still need the values of the sorted matrix.
How can I get these? (maybe sortperm!)




[Steven G. Johnson]

On Monday, May 9, 2016 at 9:53:29 AM UTC-4, Hakuna M. wrote:

would be nearly the same as:

mapslices(sortperm, A, dim);

or?


No, I think sortperm(A, dim) would need to return an array of single-index indices (i.e. to be used with A[i]).  I posted a sample implementation for 2d arrays in the issue linked above, which should suffice for your use.

[Hakuna M.]

Am Montag, 9. Mai 2016 19:03:18 UTC+2 schrieb Steven G. Johnson:

No, I think sortperm(A, dim) would need to return an array of single-index indices (i.e. to be used with A[i]).  I posted a sample implementation for 2d arrays in the issue linked above, which should suffice for your use.

Ah, ok that could be usefull as well. For current task I need the indices without offset for later usage. It could depend on the number of output parameters like Matlab.