关于 Objective-C 中 NSArray 循环的术语问题

发布于 2024-08-05 02:09:53 字数 603 浏览 1 评论 0原文

当您有一个 NSArray 并且想要评估和更改元素时，您无法从循环内部更改数组。因此，您创建了一个可以更改的可变副本。

代码示例：

NSMutableArray *bin = [NSMutableArray arrayWithObjects:@"0", @"1", @"2", @"3", @"4", @"5", @"6", @"7", nil];
NSMutableArray *list = [NSMutableArray arrayWithObjects:@"a1", @"b2", @"c3", @"e4", nil];

NSMutableArray *listHolder = list; // can't mutate 'list' within loop so create a holder

for (int i = 0; i < [list count]; i++) {
    [listHolder replaceObjectAtIndex:i withObject:[bin objectAtIndex:i]];
}

第二个数组 listHolder 叫什么？我的意思是，在这种情况下用什么术语来指代数组。

原文

When you have an NSArray and you want to evaluate and change the elements, you can't change the array from inside the loop. So, you create a mutable copy that can be changed.

code example:

NSMutableArray *bin = [NSMutableArray arrayWithObjects:@"0", @"1", @"2", @"3", @"4", @"5", @"6", @"7", nil];
NSMutableArray *list = [NSMutableArray arrayWithObjects:@"a1", @"b2", @"c3", @"e4", nil];

NSMutableArray *listHolder = list; // can't mutate 'list' within loop so create a holder

for (int i = 0; i < [list count]; i++) {
    [listHolder replaceObjectAtIndex:i withObject:[bin objectAtIndex:i]];
}

What is that second array listHolder called? I mean, what term is used to refer to an array in this context.

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dawn曙光 2024-08-12 02:09:53

这是完全有效的：

NSMutableArray *bin = [NSMutableArray arrayWithObjects:@"0", @"1", …, @"7", nil];
NSMutableArray *list = [NSMutableArray arrayWithObjects:@"a1", …, @"e4", nil];

// NSInteger should be used instead of int
for (NSInteger i = 0; i < [list count]; i++) {
    [list replaceObjectAtIndex:i withObject:[bin objectAtIndex:i]];
}

您不允许在 for … in 或 NSEnumerate 循环内更改数组，但使用索引是完全有效的。

令我困扰的是你对指针的误解。
如果这是一个不允许您改变数组的循环，则不会复制数组，而只会复制指向数组的指针，从而有效地修改您不允许的数组。 （我什至不确定这是否有效。）

不要只是复制指针，而是

// can't mutate 'list' within loop so create a holder
NSMutableArray *listHolder = list;

制作一个真实的副本：

NSMutableArray *copy = [[list mutableCopy] autorelease];

如果我真的必须制作一个副本，我会尝试根据其内容来命名它。例如：

NSMutableArray *views;
NSMutableArray *reorderedViews = [views mutableCopy];

// reorder reorderedViews

有时很难找到一个足够好的名称，那么我通常只使用nameCopy。

This is perfectly valid:

NSMutableArray *bin = [NSMutableArray arrayWithObjects:@"0", @"1", …, @"7", nil];
NSMutableArray *list = [NSMutableArray arrayWithObjects:@"a1", …, @"e4", nil];

// NSInteger should be used instead of int
for (NSInteger i = 0; i < [list count]; i++) {
    [list replaceObjectAtIndex:i withObject:[bin objectAtIndex:i]];
}

You're not allowed to change the array inside a for … in or NSEnumerate loop, but using an index is perfectly valid.

What troubles me is your misunderstanding of pointers.
If it were a loop in which you weren't allowed to mutate the array this wouldn't copy the array but only the pointer to the array, effectively modifying the array you're not allowed to. (I'm not even sure if this works.)

Instead of just copying the pointer

// can't mutate 'list' within loop so create a holder
NSMutableArray *listHolder = list;

make a true copy:

NSMutableArray *copy = [[list mutableCopy] autorelease];

In case I really have to make a copy I try to name it according to its content. For example:

NSMutableArray *views;
NSMutableArray *reorderedViews = [views mutableCopy];

// reorder reorderedViews

Sometimes it's hard to find a good enough name, then I usually just use nameCopy.

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蓝礼 2024-08-12 02:09:53

在这种情况下，listHolder 将被称为副本。

不过你的代码有一个错误。这行代码实际上并没有进行复制，它只是让 listHolder 和 list 都引用同一个数组对象：

NSMutableArray *listHolder = list;

这将是一个实际的副本：

NSMutableArray *listHolder = [list mutableCopy];

请确保您使用 <如果您希望副本是可变的，则 code>mutableCopy 而不仅仅是 copy 。 copy 方法将返回所有可变类的不可变变体，例如 NSMutableSet、NSMutableDictionary 等。

另外，正如其他人所指出的，只有在 for (集合中的项目) 循环内，枚举集合才能被改变。在正常的 for (;;) 中，突变是完全可以的，但如果集合中的项目数量发生变化，可能会导致奇怪的结果。

In this context listHolder would be called a copy.

Your code has a bug though. This line is not actually making a copy, it is only letting listHolder and list both reference the same array object:

NSMutableArray *listHolder = list;

This would be an actual copy:

NSMutableArray *listHolder = [list mutableCopy];

Make sure that you use mutableCopy and not just copy if you want the copy to be mutable. The copy method will return immutable variants on all mutable classes such as NSMutableSet, NSMutableDictionary, and so forth.

Also as others have noted it is only inside the for (item in collection) loop that the enumerated collection can not be mutated. In a normal for (;;) mutation is perfectly ok, but can lead to strange result if the number of items in the collection changes.

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