CODEWITHSUNDEEP
julia
Shayan
Shayan

Reputation: 6395

getindex for specific dims in Julia

Suppose I want to write a dynamic function that gets an object subtype of AbstractMatrix and shuffles the values along a specified dimension. Surely there can be various approaches and ways to do this, but suppose the following way:

import Random.shuffle

function shuffle(data::AbstractMatrix; dims=1)
  n = size(data, dims)
  shuffled_idx = shuffle(1:n)
  data[shuffled_idx, :] #This line is wrong. It's not dynamic

A wrong way is to use several (actually indefinite) if-else statements like if dims==1 do... if dims==2 do. But it isn't the way to do these kinds of things. I could write data::AbstractArray then the input could have various dimensions. So this came to my mind that this can be possible if I can do something like getindex(data, [idxs]; dims). But I checked for the dims keyword argument (or even positional one) in the dispatches of getindex, but there isn't such a definition. So how can I get values by specified indexes and along a dim?

Upvotes: 0

Views: 278

Answers (1)

DNF
DNF

Reputation: 12664

You are looking for selectdim:

help?> selectdim
search: selectdim

  selectdim(A, d::Integer, i)

  Return a view of all the data of A where the index for dimension d equals i.    
  Equivalent to view(A,:,:,...,i,:,:,...) where i is in position d.

Here's a code example:

function myshuffle(data::AbstractMatrix; dim=1)
    inds = shuffle(axes(data, dim))
    return selectdim(data, dim, inds)
end

Make sure not to use 1:n as indices for AbstractArrays, as they may have non-standard indices. Use axes instead.

BTW, selectdim apparently returns a view, so you may or may not need to use collect on it.

Upvotes: 4

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