错误：通过reduce函数传递不可变值 [英] Error: Immutable value passed on reduce function

问题描述

我正在尝试执行以下代码，将一组元组转换为字典，但是我收到一个编译错误：

类型'[String：String]的不可变值只有名为'updateValue'的变异成员

  var array = [（key0，value0），（key1，value1）] 
 var initial = [String：String]（）
 var final = 
 dictionary.updateValue（tuple.0，forKey：tuple.1）
返回字典
} 
  

为什么如果初始已被声明为 var ？这是否与缩小版签名中的 @noescape 有关？

  func reduce< U>（初始化：U，组合：@noescape（U，T）→U） - > U 
  

解决方案

您可以简单地将字典参数可变，前缀为 var ：

  var final = array.reduce（initial）{（var dictionary，tuple）in 
 ^^^ 
  

请注意，使用减少每个迭代创建一个新的字典，使算法效率低下。您可能需要考虑使用传统 foreach 循环

I'm trying to do the following code that transforms an array of tuples into a dictionary but I'm receiving a compile error saying:

Immutable value of type '[String : String]' only has mutating members named 'updateValue'

var array = [("key0", "value0"), ("key1", "value1")]
var initial = [String: String]()
var final = array.reduce(initial) { (dictionary, tuple) in
    dictionary.updateValue(tuple.0, forKey: tuple.1)
    return dictionary
}

Why is that if initial was declared as var? Does it have to do with @noescape on reduce's signature?

func reduce<U>(initial: U, combine: @noescape (U, T) -> U) -> U

解决方案

You can simply make the dictionary parameter mutable by preceding it with var:

var final = array.reduce(initial) { (var dictionary, tuple) in
                                     ^^^

Note however that using reduce a new dictionary is created at each iteration, making the algorithm very inefficient. You might want to consider using a traditional foreach loop

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