List.forEach无法修改元素？ [英] List.forEach unable to modify element?

问题描述

我正在尝试创建一个列表列表来模仿Dart中2D阵列的功能，并且在弄清楚如何使其工作方面遇到了一段时间。

I'm attempting to create a list of lists to imitate the functionality of a 2D array in Dart, and I was having trouble for a while figuring out how to make it work.

我最初使用 List.forEach 来创建列表并填充列表，但是在每次循环之后，好像我从未创建过列表。代码如下所示：

I originally used List.forEach in order to create the lists and fill them, but after each loop, it's as if I never created the lists. Here's what that code looked like:

currentMap = new List<List<int>>(height);
currentMap.forEach((e) {
    e = new List<int>(width);
    e.forEach((f) {
        f = 1;
    });
});

打印 currentMap 会生成 null 。这就是我现在拥有的东西：

Printing currentMap resulted in a list of null. Here's what I have now:

currentMap = new List<List<int>>(height);
for(int i = 0; i < height; i++) {
    currentMap[i] = new List<int>(width);
    for(int j = 0; j < width; j++) {
        currentMap[i][j] = 1;
    }
}

此功能完全符合我的预期。

This works exactly as I expected it to.

我了解这可能是一个非常基本的问题，我假设 forEach 不允许您修改元素，但是我想对此进行确认，Dart API文档未指定，只是带有短语将函数f附加到每个元素...，这可能对我来说并不明确。

I understand that this is probably a very basic issue, and I am assuming that forEach does not allow you to modify the element, but I wanted some confirmation on that, and the Dart API docs do not specify except with the phrase "apples the function f to each element..." which may just not be clear to me.

此外，我知道类似 List.filled（）构造函数的方法-这就是我开始朝着其他功能。

Also, I am aware of things like the List.filled() constructor -- this is just how I am starting to build towards other functionality.

编辑：好的，我想我现在明白了。参数是 pass-by-sharing（通过共享），这意味着将复制对对象的引用。这意味着您可以修改参数所指向的可变对象的成员（如下所示）：

Okay, I think I understand now. Arguments are "pass-by-sharing" which means that a copy of a reference to the object is made. This means that you can modify a member of a mutable object that is pointed to by the argument (as follows):

void function(MyObject o) {
    o.x = 5;
}

，但尝试更改 o 指向退出函数后不会更改参数，例如：

but trying to change what o points to will not change the argument after exiting the function, such as this:

void function(MyObject o) {
    MyObject p = new MyObject();
    o = p;
}

推荐答案

Dart可以没有变量引用，所有元素都作为对对象而不是变量的引用传递。 （换句话说，Dart就像Java和JavaScript一样，都是共享呼叫。）

Dart does not have variable references, all elements are passed as a reference to an object, not to a variable. (In other words, Dart is purely "call-by-sharing" like both Java and JavaScript).

这意味着 e forEach 回调的code>参数只是普通的局部变量，对其赋值在回调之外没有任何作用。迭代器也是如此：它们返回可迭代对象中的值，但此后没有对可迭代对象的引用。

That means that the e parameter to the forEach callback is just a normal local variable, and assigning to it has no effect outside the callback. The same goes for iterators: they return the value in the iterable, but it has no reference back to the iterable after that.

双循环是您想要的东西做读-修改-写。您从列表中读取了一个值，对其进行了修改，然后使用列表的索引集运算符将其存储回列表中。

The double loop is what you want when you do "read-modify-write". You read a value from the list, modify it and store it back into the list using the list's index-set operator.

我知道您知道 List.filled ，但仅针对其他读者，我将提供一线内容以创建初始化为 1 值的二维数组：

I know you know about List.filled, but just for other readers, I'll give the one-liner to create the two-dimensional array initialized to 1 values:

currentMap = new List.generate(height, (_) => new List.filled(width, 1));

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