一种通过唯一词典值过滤词典数组的干净方法 [英] Clean way to filter an array of dictionaries by unique dictionary values

问题描述

说我有一个对象:

struct Foo {
    let id: Int
    let bar: Int
}

现在我在数组中有5个这些对象:

Now I have 5 of these objects in an array:

let foo1 = Foo(id: 1, bar: 1)
let foo2 = Foo(id: 2, bar: 1)
let foo3 = Foo(id: 3, bar: 2)
let foo4 = Foo(id: 4, bar: 3)
let foo5 = Foo(id: 5, bar: 3)

let fooArray = [foo1, foo2, foo3, foo4, foo5] 

什么是过滤具有唯一bar值的foo对象的干净方法?

What would be a clean way of filtering foo objects that have unique bar values?

// Desired output
let filteredArray = [foo1, foo3, foo4]

假设要迭代的对象从几百个到几千个.

Assume there's anywhere from a few hundred to a few thousand objects to iterate upon.

推荐答案

一种可能的方法是使用Set来跟踪哪个 bar值已经被看到:

One possible approach is to use a Set which keeps track of which bar values have already been seen:

var seenBarValues = Set<Int>()
let filteredArray = fooArray.filter { foo in
    if seenBarValues.contains(foo.bar) {
        // We already had a `Foo` with this `bar` value: skip.
        return false 
    } else {
        // First `Foo` with this `bar` value: remember and include.
        seenBarValues.insert(foo.bar)
        return true
    }
}

正如@Hamish正确指出的那样，可以将其缩短为

As @Hamish correctly pointed out, this can be shortened to

var seenBarValues = Set<Int>()
let filteredArray = fooArray.filter { 
    seenBarValues.insert($0.bar).inserted
}

利用以下事实

public mutating func insert(_ newMember: Element) -> (inserted: Bool, memberAfterInsert: Element)

返回一个元组，其第一个成员指示元素是否相等 新插入的对象已经存在于集合中.

returns a tuple whose first member indicates if an element equal to the newly inserted one was already present in the set.

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