CODEWITHSUNDEEP
indexingjulia
DNF
DNF

Reputation: 12664

Dropping indices with Julia non-standard arrays

In an answer I wrote previously: https://stackoverflow.com/a/41966459/2749865 I tried to make the code general enough to handle arrays with non-standard indexing. To that end used the construct indices in place of 1:size(...):

[C[i+1, j]/C[i, j]-1 for i in 1:size(C, 1)-1, j in indices(C, 2)]

As you can see, though, I could not figure out how to use indices when I want to drop the last element.

I can skip the first n elements by writing drop(indices(C, 1), n), but cannot find any way to drop the last n indices.

How can I accomplish this?

Edit: Just to head this one off. In my specific example I could have written

[C[i, j]/C[i-1, j]-1 for i in drop(indices(C, 1), 1), j in indices(C, 2)]

but I'm still interested in hearing if there is a general solution.

Upvotes: 1

Views: 138

Answers (2)

tholy
tholy

Reputation: 12179

Try something like

[C[i+1, j]/C[i, j]-1 for i in indices(C, 1)[1:end-1], j in indices(C, 2)]

In other cases I've used constructs like

inds_interior = map(r->first(r)+1:last(r)-1, indices(A))

to skip the first and last element of each dimension.

Upvotes: 2

Dan Getz
Dan Getz

Reputation: 18227

If you use Iterators package:

using Iterators

for i in imap(first,partition(indices(C,1),n+1,1))
  @show i
end

does the trick. Specifically partition takes seqeunces of length n+1 and first takes only the first element, and the last n+1 sequence up to the end of the indices has the first element n elements before the last.

But perhaps a function would make things look better:

Base.chop(itr,n) = imap(first,partition(itr,n+1,1))

and now:

julia> chop(1:10,3)|>collect
7-element Array{Any,1}:
 1
 2
 3
 4
 5
 6
 7

chop is already defined for strings, removing the last character. It seems the semantics are so close to make overloading it appropriate.

Upvotes: 1

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