# StringIndex idea (Julia 2.0)

**URL:** <https://discourse.julialang.org/t/stringindex-idea-julia-2-0/107185>\
**Category:** Internals & Design\
**Tags:** strings, indexing\
**Created:** [December 5, 2023, 8:23pm UTC](https://discourse.julialang.org/t/stringindex-idea-julia-2-0/107185 "2023-12-05T20:23:43Z")\
**Posts on this page:** 1\
**Showing post:** 63

<div class="post-metadata">

**Author:** ![uniment](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/uniment/32/24532_2.png) [@uniment](https://discourse.julialang.org/u/uniment)\
**Post date:** [February 8, 2024, 9:57pm UTC](https://discourse.julialang.org/t/stringindex-idea-julia-2-0/107185/63 "2024-02-08T21:57:15Z")

</div>

This idea can be made more ergonomic by merging it with another idea discussed [previously](https://discourse.julialang.org/t/ordinal-indexing-as-a-language-feature/91970):

Something similar to @jishnub’s [`OrdinalIndexing`](https://github.com/jishnub/OrdinalIndexing.jl/) could be added to the language—this is almost identical to this thread’s OP, but with a nicer user interface that can generalize to other indexable types. The purpose of `OrdinalIndexing` is to guarantee 1-based indexing even for `OffsetArray`s. For example:

```julia
my_array = OffsetArray(1:10, -10)
my_array[4th] == 4

```

Applying this concept here, per-character string indexing could go like:

```julia
my_string[6th:10th]

```

(This is reminiscent of the `my_string.nth(i)` function in Rust)

The `4th` object (let’s call it `OrdinalIndex(4)`) would store a single integer representing an _ordinal_ index instead of a _positional_ index. Arithmetic and ranges of `OrdinalIndex` would work as intuitively expected.

When an `OrdinalIndex` is added to an `Integer`, it’s promoted to a `PositionOrdinalIndex`, which could contain both `position_offset` and `ordinal_index` fields like the OP of this thread. For example, `0+6th` would construct a `PositionOrdinalIndex(0,6)` and `6:7th` could be `PositionOrdinalIndex(6,0):PositionOrdinalIndex(0,7)`. (for 6th to 7th, use `(6:7)th` or `6th:7th`). Adding `PositionOrdinalIndex` objects would add both fields, similar to a `Complex`.

Then, indexing from the end should work like this:

```julia
my_string[end-9th:end-5th]

```

It would also be really awesome for ordered dictionaries, wherein it could be imposed that `::OrdinalIndex` and `::PositionOrdinalIndex` hashkeys are not permitted, so that the dictionary entries can be accessed in insertion order by `::OrdinalIndex` and `::PositionOrdinalIndex`-typed indices:

```julia
my_ordered_dict[1th:100th] # first 100 entries

```

Maybe it could also be interesting for accessing sparse matrices—I’m not sure.

I’m agnostic on names btw, and as @StefanKarpinski ponders above it might not be worthwhile to maintain a `OrdinalIndex` type separate from `PositionOrdinalIndex` since the position will often be const-propped anyway: using only the latter would make the implementation of this idea analogous to the representation of complex numbers (wherein all imaginary numbers are represented as `Complex` even with zero real component; similar to `im ≡ Complex(false,true)` we would define `const th = PositionOrdinalIndex(false,true)`).

---

_[View the full topic](https://discourse.julialang.org/t/stringindex-idea-julia-2-0/107185)._