示例
let arr:[String] = [Empty,Empty,Full空,全] 让结果:[Int] = arr.indexes(ofItemsNotEqualTo item:Empty) //返回[2,4] $ c $ <$ c $我尝试了一个通用函数: c>扩展数组{ func索引< T:可比较>(ofItemsNotEqualTo item:T) - > [INT]? { var result:[Int] = [] for self.enumerated(){ if elem!= item { result.append(n )} } 返回result.isEmpty? nil:result } }但是这给出了一个警告:二元运算符不能应用于元素和T类型的操作数。
所以我做了这个我投的元素(注意 扩展数组{ func索引< T:可等分>(ofItemsNotEqualTo item:T) - > [INT]? { var result:[Int] = [] $ b $ for(n,elem)in self.enumerated(){ if elem as? T!= item { result.append(n)} } return result.isEmpty? nil:result } }但是现在看起来类型检查有出去了窗口,因为如果我传入一个整数,我得到了错误的结果
let arr:[String] = [ 让结果:[Int] = arr.indexes(ofItemsNotEqualTo item:100) //返回[0,1 ,2,3,4]帮助将不胜感激。
使用 reduce 函数有更好的方法吗?
解决方案您已经定义了一个泛型方法
func索引< T:Equatable>(ofItemsNotEqualTo item :T)→> [INT]?它的参数类型为 T which必须是 Equatable ,但与数组的 Element 类型无关。
因此
let arr = [Empty,Empty,完整,空白,全部] let result = arr.indexes(ofItemsNotEqualTo:100)编译,但 elem as? T 对所有数组给出 nil (这是!= item )元素。
你想要的是一种仅为 Equatable 元素数组定义的方法。这可以通过限制的扩展来实现:
扩展数组其中元素:Equatable { func索引(ofItemsNotEqualTo item:Element) - > [INT]? { var result:[Int] = [] $ b $ for(n,elem)in enumerated(){ if elem!= item { result.append(n)} } 返回result.isEmpty? nil:result } }其实我不会让返回值可选。 如果所有元素都等于给定项目,那么逻辑返回值就是空数组。 >有没有更好的方法来使用reduce函数做到这一点?
好的,你可以 使用 reduce(),但这不是非常有效,因为在每个迭代步骤中都会创建中间数组:
扩展数组其中元素:Equatable { func索引(ofItemsNotEqualTo item:Element) - > [Int] { return enumerated()。reduce([]){ $ 1.element == item? $ 0:$ 0 + [$ 1.offset] } } }你实际上有一个filter + map操作:
扩展数组其中Element:Equatable { func索引(ofItemsNotEqualTo item:Element) - > [Int] { return enumerated()。filter {$ 0.element!= item} .map {$ 0.offset} } }可以使用 flatMap()简化:
扩展数组其中,Element:Equatable { func索引(ofItemsNotEqualTo item:Element) - > [Int] { return enumerated()。flatMap {$ 0.element!= item? $ 0.offset:nil} } }示例:
let arr = [Empty,Empty,Full,Empty,Full] arr .indexes(ofItemsNotEqualTo:Full)// [0,1,3] [1,1,1] .indexes(ofItemsNotEqualTo:1)// [] arr.indexes(ofItemsNotEqualTo:100) //错误:无法将'Int'类型的值转换为期望的参数类型'String'
Swift 3
Trying to write a generic array extension that gets all indexes of items that DON'T equal value
example
let arr: [String] = ["Empty", "Empty", "Full", "Empty", "Full"] let result: [Int] = arr.indexes(ofItemsNotEqualTo item: "Empty") //returns [2, 4]I tried to make a generic function:
extension Array { func indexes<T: Equatable>(ofItemsNotEqualTo item: T) -> [Int]? { var result: [Int] = [] for (n, elem) in self.enumerated() { if elem != item { result.append(n) } } return result.isEmpty ? nil : result } }But that gives a warning: Binary operator cannot be applied to operands of type "Element" and "T".
So then I did this where I cast the element (note the as?)
extension Array { func indexes<T: Equatable>(ofItemsNotEqualTo item: T) -> [Int]? { var result: [Int] = [] for (n, elem) in self.enumerated() { if elem as? T != item { result.append(n) } } return result.isEmpty ? nil : result } }But now it seems the type checking has gone out the window, because if I pass in an integer I get the wrong result
let arr: [String] = ["Empty", "Empty", "Full", "Empty", "Full"] let result: [Int] = arr.indexes(ofItemsNotEqualTo item: 100) //returns [0, 1, 2, 3, 4]Help would be greatly appreciated.
Is there a better way to do this with the reduce function?
解决方案You have defined a generic method
func indexes<T: Equatable>(ofItemsNotEqualTo item: T) -> [Int]?which takes an argument of type T which is required to be Equatable, but is unrelated to the Element type of the array.
Therefore
let arr = ["Empty", "Empty", "Full", "Empty", "Full"] let result = arr.indexes(ofItemsNotEqualTo: 100)compiles, but elem as? T gives nil (which is != item) for all array elements.
What you want is a method which is defined only for arrays of Equatable elements. This can be achieved with a constrained extension:
extension Array where Element: Equatable { func indexes(ofItemsNotEqualTo item: Element) -> [Int]? { var result: [Int] = [] for (n, elem) in enumerated() { if elem != item { result.append(n) } } return result.isEmpty ? nil : result } }Actually I would not make the return value an optional. If all elements are equal to the given item, then the logical return value would be the empty array.
Is there a better way to do this with the reduce function?
Well, you could use reduce(), but that is not very efficient because intermediate arrays are created in each iteration step:
extension Array where Element: Equatable { func indexes(ofItemsNotEqualTo item: Element) -> [Int] { return enumerated().reduce([]) { $1.element == item ? $0 : $0 + [$1.offset] } } }What you actually have is a "filter + map" operation:
extension Array where Element: Equatable { func indexes(ofItemsNotEqualTo item: Element) -> [Int] { return enumerated().filter { $0.element != item }.map { $0.offset } } }which can be simplified using flatMap():
extension Array where Element: Equatable { func indexes(ofItemsNotEqualTo item: Element) -> [Int] { return enumerated().flatMap { $0.element != item ? $0.offset : nil } } }Examples:
let arr = ["Empty", "Empty", "Full", "Empty", "Full"] arr.indexes(ofItemsNotEqualTo: "Full") // [0, 1, 3] [1, 1, 1].indexes(ofItemsNotEqualTo: 1) // [] arr.indexes(ofItemsNotEqualTo: 100) // error: cannot convert value of type 'Int' to expected argument type 'String'
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