mirror of
https://github.com/jbranchaud/til
synced 2026-09-01 17:21:46 +00:00
37 lines
1.6 KiB
Markdown
37 lines
1.6 KiB
Markdown
# Use Slice To Reorder Hash Keys
|
|
|
|
I recently ran into some code that was building up a hash keyed by _category_.
|
|
It was a series of chained method calls building and transforming to produce the
|
|
hash. It then ended with `.slice(*CATEGORIES)`.
|
|
|
|
That
|
|
[`Hash#slice`](https://docs.ruby-lang.org/en/master/Hash.html#method-i-slice)
|
|
call at the end was doing two things:
|
|
|
|
1. It was removing any key-value pairs not in `CATEGORIES`
|
|
2. It was reordering the hash keys based on the order they appeared in `CATEGORIES`
|
|
|
|
It was this second behavior that surprised me.
|
|
|
|
Let's look at a minimum example of this:
|
|
|
|
```ruby
|
|
> CATEGORIES = [:books, :clothes, :glassware]
|
|
=> [:books, :clothes, :glassware]
|
|
> items_by_category = { glassware: [1,2,3], media: [:a, :b, :c], books: [:e, 3, :g], clothes: [4, 5] }
|
|
=> {glassware: [1, 2, 3], media: [:a, :b, :c], books: [:e, 3, :g], clothes: [4, 5]}
|
|
> items_by_category.slice(*CATEGORIES)
|
|
=> {books: [:e, 3, :g], clothes: [4, 5], glassware: [1, 2, 3]}
|
|
```
|
|
|
|
- I first define `CATEGORIES` which is an (ordered) array of symbols that
|
|
represent the categories I care about.
|
|
- Then I manufacture a hash of items by category. This is a bunch of dummy data,
|
|
including extra categories, meant to demonstrate what `#slice` can do.
|
|
- Lastly I call `.slice(*CATEGORIES)` on this hash which both pairs it down to
|
|
that set of categories and reorders the hash so that the keys appears in the
|
|
same order they do in the `CATEGORIES` array.
|
|
|
|
This might be useful if I've defined `CATEGORIES` to be in a specific display
|
|
order and I'm preparing this hash for consumption in some UI layer.
|