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