[FIXED] Julia: sort two arrays (like lexsort in numpy)

Issue

Python example

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In Numpy there is lexsort to sort one array within another:

Given multiple sorting keys, which can be interpreted as columns in a spreadsheet, lexsort returns an array of integer indices that describes the sort order by multiple columns.

So taking the following example:

import numpy as np

a = np.array([1,1,1,2,2,2])
b = np.array([10,8,11,4,8,0])

sorted_idx = np.lexsort((b,a))
print(b[sorted_idx])
# [ 8 10 11  0  4  8]

So this sorts b within a as we can see like:

1 1  1      2  2  2
8 10 11     0  4  8

I can't find anything similar in Julia so I wonder how this can be achieved? In my case two columns is sufficient

---

Julia

So lets take the same data to figure that out:

a = Vector([1,1,1,2,2,2])
b = Vector([10,8,11,4,8,0])

---

Little benchmark

using StructArrays
using DataFrames
using BenchmarkTools


a = rand(100000)
b = rand(100000)

function f1(a, b)
    return sortperm(StructArray((a, b)))
end

function f2(a, b) 
    return sortperm(DataFrame(a=a,b=b, copycols=false))
end
        
function f3(a, b)
    return sortperm(collect(zip(a, b)))
end



@btime f1(a,b)
@btime f2(a,b)
@btime f3(a,b)

Giving:

6.075 ms (8 allocations: 781.50 KiB)
13.808 ms (8291 allocations: 5.93 MiB)
15.892 ms (7 allocations: 2.29 MiB)

So StructArray is almost twice as fast as the other two and uses less memory

Solution

Similar to the DataFrames solution but a bit more lightweight in terms of dependencies, a nice solution is to use the StructArrays package, which lets you treat a pair of arrays as if it were an array of tuples, without making a copy of the data, which you can then sort (tuples sort lexicographically):

using StructArrays
i = sortperm(StructArray((a, b)))

Instead of finding the permutation array i and doing b[i], you can also do:

sort!(StructArray((a, b)))

which sorts both a and b in-place lexicographically by (a[j], b[j]).

Answered By - SGJ