When to use a set rather than an array
Swift version: 5.1
Sets and arrays both store objects of your choosing, but they have four important differences:
- Sets do not store objects in the order they add them.
- Instead, they are stored in a way to make them fast to find, which means finding items in sets is extremely efficient.
- Sets store each item precisely once.
- All items you want to store in a set must conform to
Hashable.
As a result, you should use a set rather than an array if all the following criteria are true:
- You intend to add each item only once. Sets never allow duplicates.
- You don’t care about the order of the items in the set.
- You don’t need to use APIs that require arrays.
- You’re storing
Hashabletypes, either your own or one of Swift’s built-in types likes strings and integers. Sets use hash values for fast look up of items.
You can switch between an array and a set simply enough:
let array = [1, 2, 3]
let set = Set(array)
let array2 = Array(set)Using contains() on a set takes the same amount of time if you have one item as it does if you have one thousand items – it’s called an O(1) operation.
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