swift - 如何在 Swift 中获取数组的中间索引？

Question

我有一个错误获取数组的中间索引。这是数组：

CountedArray = ["Jake","Jacob","Sam","Tom","Ben","Joe","Bob","Chen","sara","Simon"]

Answer 1

关于什么

let index = CountedArray.count == 0 ? 0 : (CountedArray.count / 2) - 1

Answer 2

试试下面的代码：

let middle = CountedArray.count / 2
print(CountedArray[middle])

Answer 3

这里有一些担忧。

• 如果数组包含odd多个元素，则只有一个中间元素
• 如果数组包含多个元素，则中间even有两个元素
• 如果数组只包含一个元素，那么就是中间元素
• 如果数组只包含两个元素，那么它们都是中间元素
• 如果数组是empty，则没有中间元素

考虑到上述用例，我可以使用数组类型的扩展从数组中找到我的中间元素：

extension Array {
    var middleElements: (first: Element, second: Element?)? {
        if count == 0 {
            return nil
        } else if count.isEven {
            // even number of elements
            let middleIndex = count / 2
            let element1 = self[middleIndex - 1]
            let element2 = self[middleIndex]
            return (element1, element2)
        } else {
            // odd number of elements
            let middleIndex = count / 2
            let element = self[middleIndex]
            return (element, nil)
        }
    }
}

// helper extension
extension Int {
    var isEven: Bool {
        return self % 2 == 0
    }
}

用法：

具有 3 个元素的数组：

let array = ["One", "Two", "Three"]
if let first = array.middleElements?.first {
    print(first) // Prints Two
}
if let second = array.middleElements?.second {
    print(second)
}

具有 2 个元素的数组：

let array = ["One", "Two"]
if let first = array.middleElements?.first {
    print(first) // Prints One
}
if let second = array.middleElements?.second {
    print(second) // Prints Two
}

具有 1 个元素的数组：

let array = ["One"]
if let first = array.middleElements?.first {
    print(first) // Prints One
}
if let second = array.middleElements?.second {
    print(second)
}

空数组：

let array: [String] = []
if let first = array.middleElements?.first {
    print(first)
}
if let second = array.middleElements?.second {
    print(second)
}
// Prints nothing