Access jth component of stacked 1D array

General Usage
1

Hello,

I would like to access the jth component of stacked 1D arrays, what is the best option? Is it possible to create a view of this subarray?

e.g.:

Nx = 50
Ne = 20

A = map(i->randn(Nx,1:Ne)

# Access jth component
j  = 10
[A[i][j] for i=1:Ne]
2

You can broadcast getindex.(A,j)

2 Likes
3

However, if what you want is a view in the sense that you can replace an element in this new array B to replace an element of a vector in A (i.e., B[i] = x reflects in A[i][j] value) then this will not work. You can broadcast setindex too if necessary.

4

You can try to concatenate views into one, using any of these… only the last seems to allow setindex!:

using LazyArrays
Vcat(view.(A,1))

using LazyStack
stack(view.(A,1))

using JuliennedArrays
Align(view.(A,Ref(1:1)),2)

But it might be easier to work with a matrix, not a vector of vectors.

5

Thank you for your answers,

A related question:
Can we sort in-place component by component an array of 1D array?

Here is my code but it creates an extra array

Nx = 50
Ne = 20

A = map(i->randn(Nx,1:Ne))
B =sort!(hcat(A...),dims = 2)
A = map(i-> B[:,i],1:Ne)
6

Note that you’re missing a bracket, your code doesn’t run. But sure, you can do this:

julia> A = [rand(Int8,3) for _ in 1:5]
5-element Array{Array{Int8,1},1}:
 [17, 2, 83]     
 [65, -71, -30]  
 [-119, -127, -9]
 [-90, 22, 122]  
 [22, -26, 63]   

julia> sort!(Align(A,2), dims=1);

julia> A
5-element Array{Array{Int8,1},1}:
 [-119, -127, -30]
 [-90, -71, -9]   
 [17, -26, 63]    
 [22, 2, 83]      
 [65, 22, 122]
7

The following function seems to work:

import Base: sort!

function sort!(A::Array{Array{Float64,1},1})
    Nx = size(A[1],1)
    for i = 1:Nx
        setindex!.(A,sort!(getindex.(A,i)), i)
    end
end

  Ne = 10
A = [[Ne-i+2; i+5.0; i] for i=1:Ne]
sort!(A)
A

Thank you for the reference to package for lazy arrays. What is the advantage to use lazy arrays compared to regular arrays?

I don’t know how to create a view that contains the first N components of each array.