Swift 3 - 通过 Int 属性减少对象集合 [英] Swift 3 - Reduce a collection of objects by an Int property

问题描述

我有一个包含 3 个对象的数组，如下所示:

I have an array containing 3 objects like so:

class AClass {
    var distance: Int?
}

let obj0 = AClass()
obj0.distance = 0

let obj1 = AClass()
obj1.distance = 1

let obj2 = AClass()
obj2.distance = 2

let arr = [obj0, obj1, obj2]

当我减少数组并将其分配给变量时，我只能对数组中的最后一个元素求和.

When I reduce the array and assign it to a variable, I can only sum the last element in the array.

let total = arr.reduce(0, {$1.distance! + $1.distance!})  //returns 4

如果我尝试 $0.distance！它的错误是表达式不明确，没有更多上下文".

If I try $0.distance! it errors with "expression is ambiguous without more context".

我尝试更明确:

var total = arr.reduce(0, {(first: AClass, second: AClass) -> Int in
    return first.distance! + second.distance!
})

但是这个带有'Int'的错误与上下文类型'_'不兼容"如何将其减少到距离的总和?

But this errors with "'Int' is incompatible with contextual type '_'" How do I reduce it to the sum of the distances?

推荐答案

var total = arr.reduce(0, {$0 + $1.distance!})

第一个参数是累加器，它已经是一个整数.

The first argument is the accumulator, it is already an integer.

请注意，这将在没有距离的元素上崩溃.你可以解决这个问题，例如使用:

Note that this will crash on elements without distance. You could fix that e.g. using:

let total = arr.reduce(0, {$0 + ($1.distance ?? 0)})

或

let total = arr.compactMap { $0.distance }.reduce(0, +)

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