Julia 0.5 introduced support for any indexing scheme you care to invent.

1-based 0-based 20-based

https://docs.julialang.org/en/latest/devdocs/offset-arrays/

Should array indices start at 0 or 1? My compromise of 0.5 was rejected without, I thought, proper consideration.

-Stan Kelly-Bootle

That is an ugly hack, mainly because it was not in the original language design.

it is in the original language design which is all about type-genericness and separation of implementation from interface with zero-cost abstractions. Creating an array with a different `getindex` dispatch is a great example of what Julia's type system was made to do! The standard library chose to use 1-base indexed contiguous fixed-dimesion dynamic arrays, but that's not the right choice for every problem.

As evidence of this, check out JuliaArrays (https://github.com/JuliaArrays) which is a whole Github organization devoted to the development of alternative array types, like StaticArrays (which are stack-allocated immutable arrays) or CatViews (arrays which are non-contiguous and constructed from views of multiple different arrays). The nice thing about Julia is that, if packages are written to work with generic types, they can natively (and efficiently) work with these "non-standard" array types, making them easy to integrate into the scientific ecosystem.

Why ugly? It looks similar to what I know in the Pascal family, where indexes can be ranges or enumerations.

It is ugly because 1) 1-based indexing and x-based indexing are treated differently; 2) x-based indexing has to use more complex syntax, which in effect discourages the use of non-1 indexing; 3) this strategy sets potential pitfalls (e.g. implementing length and size for non-1 indexing arrays). A cleaner design, I guess, would be to specify index range on declaration like pascal static arrays and use low(A):high(A) for iteration rather than 1:length(A). This, however, complicates 1-based use cases.

Generally, I don't think there is a good way to achieve flexible indexing without causing troubles somewhere, so I don't think Julia has really solved the problem.

1) No, they are just different dispatches to getindex. 2) No, iteration is through indices(A) or eachindex(A), etc., which are the preferred way of iterating anyways. You shouldn't do 1:length(A) which is a MATLABism that works but I would say isn't good Julia. 3) Defining new dispatches for length and size is a pretty standard use of the language?

"Non-standard" arrays with non-standard indexing already work in lots of packages. It could be better (that's one of the things that I am advocating for), but it's not a language tooling issue whenever it's a problem, it was the developer going `::Array` and thus requiring a contiguous 1-base index array where other AbstractArrays would actually work.

@attractivechaos I don't know how to reply to your last reply, so I'll do it here. 1:length(A) is bad because it's using a standard construction for intervals of numbers, but using it for indices. We don't want to get rid of it because 1:5 or 0:0.2:1 is something that is very common and necessary, but I don't see how to tell one that they should instead use eachindex(A) except through proper docs. 1:length(A) is so common in MATLAB though that I am sure people will carry it over, and I'll PR to their library to fix it. I'm not sure how to fix a knowledge issue like that.

You're not understanding generic types and its relation to (1). There's only one way to access an array: getindex. That's the function that's called with A[i]. However, you can use an immutable to put a thin (zero-cost) wrapper over an array, and define dispatches to getindex to do whatever you need it to do. So it's both implicit syntactically because the user just does A[i], but it's explicit because the user has to choose a different type. getindex is then usually inlined and then compiled according to the type, making it a thin abstraction over the implementation.

There are iterators which don't have a size or length. You can write generic algorithms which require an AbstractArray which HasLength and query at compile time for things like that and throw appropriate errors (those are called traits).

There is still a lot of development to do here, but the basics like this are pretty much solved except when new users treat Julia like MATLAB, but I'm not sure how anyone could control for that.

You can click on the timestamp, and there will be a reply link there. I think reply links are hidden for a little bit of time after posting, but I'm not sure why.

This is an anti-flame war feature, designed to let people cool off before replying (fast paced discussions were often contentious before that was introduced).

I guess so. This is a well thought feature. I like it.

On 2), if you think 1:length(A) is bad, why not forbid it from beginning (e.g. use low(A):high(A) instead)? To find an x-based array length, why not just length(A), instead of length(linearindices(A))? Decisions like such are remedies of immature early design. Also, what if I use length() on an x-based array? Abort or a wrong number silently? On 1), having two different ways to access array, the most fundamental data type, is already worrying enough. On 3), the page says "don't implement size or length". That is very uncommon in most other mainstream languages.

Julia has potential to become a great general-purpose programming language, but this indexing issue will practically limit it to the numerical computing community. Perhaps achieving that is already good enough.

The `length(linearindices)` and no `size` etc are for a short transitory period while packages are ported to the indexing-based-non-reliant framework.

One of the most mindboggling thing about the recurrent 0- vs 1-based indexing discussion is how incredibly rarely that difference is ever used programmatically in Julia. Most of the large julia packages are programmed in a way that doesn't care whether the array is 0 or 1 based. It is important in some other languages, and then I just think people are happy that this is something that everybody can agree to disagree on. I don't think the discussion is very productive though.

Hmm... I wanted to learn more about 0-indexed arrays, but after looking around, I still have not figured out how to declare 0-indexed arrays without extending the AbstractArray interface or using another package.

I don't have any particular feelings toward one or the other (it is a convention, get over it), but I think that zero-based indexing is just an artifact of C that stuck around.

In C, the array syntax is "mostly" just syntactic sugar for pointer arithmetic.

When you do "a[n]=value;" this is equivalent to "*(a+n) = value;". To get the nth cell of an array, you just add "n" to your base pointer "a".

Array indexing, therefore, is consistent with the pointer arithmetic.

That said, and funnily enough, Fortran, which is much older than C, has 1-based indexing (by default, but you can configure 0 based indexing, if I remember correctly).

Zero based indexing is not a C artifact. Here's Dijkstra writing about it in '82:

https://www.cs.utexas.edu/users/EWD/transcriptions/EWD08xx/E...

Note how, in the PDF version [0], Dijkstra numbers the pages starting on zero (handwriting, upper right corner), but whoever created the PDF disregarded its message and did numbering starting on one (lower right corner). :-)

[0] https://www.cs.utexas.edu/users/EWD/ewd08xx/EWD831.PDF

Edit: Fixed link. Hacker News apparently doesn't understand the delimiting of a URL with "<" and ">". >-( https://tools.ietf.org/html/rfc3986#appendix-C

There's the very real possibility that zero based indexing is in fact a Yacht Racing artifact.

http://exple.tive.org/blarg/2013/10/22/citation-needed/

> The social reason is that we had to save every cycle we could, because if the job didn’t finish fast it might not finish at all and you never know when you’re getting bumped off the hardware because the President of IBM just called and fuck your thesis, it’s yacht-racing time.

I don't buy it. Wouldn't people want their programs to run fast regardless of this?

In C arrays are given an offset to the pointer, not an index, that is why they start at 0.

Which happens to common across many languages outside C universe.

Some languages, like the Algol ones, even have user defined indexing.

So it is not neither 0 or 1, rather whatever the min value of the index happens to be.

Like every other language-design decision?