Stream: helpdesk (published)

Topic: length(x) == length(eachindex(x)) ?

Leandro Martínez (Sep 28 2022 at 14:40):

Is it always true that

length(x) == length(eachindex(x))

?

Expanding Man (Sep 28 2022 at 14:42):

I think so, but I usually do prod(size(x)) for arrays just because I never remember what length is supposed to be on them.

Michael Fiano (Sep 28 2022 at 14:44):

length and eachindex are generic, so no?

Gunnar Farnebäck (Sep 28 2022 at 15:14):

According to the docstrings:

• length: Return the number of elements in the collection.
• eachindex: Create an iterable object for visiting each index of an AbstractArray.

I would interpret this as yes, assuming the methods follow the documentation.

Michael Fiano (Sep 28 2022 at 15:20):

Well that is only part of the description of eachindex. It can be defined for anything iterable, including unbounded sequences.

Michael Fiano (Sep 28 2022 at 15:21):

Just today I implemented it for a non-AbstractArray type as recommended by @Sukera. There's nothing wrong with that.

Sukera (Sep 28 2022 at 15:22):

do note that I just recommended it due to being a convenient name, not for implying your Rect was an AbstractArray

Michael Fiano (Sep 28 2022 at 15:22):

Exactly.

Sukera (Sep 28 2022 at 15:22):

(though the docstring does support "other iterables" as well)

Michael Fiano (Sep 28 2022 at 15:23):

Right, that is my point, that it is not exclusive to arrays.

Leandro Martínez (Sep 28 2022 at 15:52):

So, if the sequence is bounded, then "yes"? Or do you see any other pathological case?

Jakob Nybo Nissen (Sep 28 2022 at 19:11):

I think they should be the same in all cases.

Mason Protter (Sep 28 2022 at 19:18):

Michael Fiano said:

Right, that is my point, that it is not exclusive to arrays.

What does that have to do with it though? The question isn't about arrays in particular, it's about whether length of an object should agree with the length of it's indices, and I'd say that in every case where indices exist, they should absolutely be the same.

Mason Protter (Sep 28 2022 at 19:20):

Of course one can always just define a method that does something whacky, but I think defining an object where it's length doesn't agree with the length of it's indices is far enough beyond the pale that it can be ignored.

Michael Fiano (Sep 28 2022 at 19:21):

Gunnar Farnebäck said:

According to the docstrings:

• length: Return the number of elements in the collection.
• eachindex: Create an iterable object for visiting each index of an AbstractArray.

I would interpret this as yes, assuming the methods follow the documentation.

That wasn't in response to the original question, moreso than it was in response to the partial documentation quoting here.

Michael Fiano (Sep 28 2022 at 19:24):

I agree with that assumption. I did however implement eachindex on a non-array (without an accompanying length implementation) to make iteration on a type more concise, so in that particular case, which satisfied my needs, they are technically different, seeing how one is a MethodError.

Leandro Martínez (Sep 29 2022 at 00:47):

Thank you all!

Last updated: Oct 02 2023 at 04:34 UTC