Scala 集合包含相同的元素，但 sameElements() 返回 false
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Scala Sets contain the same elements, but sameElements() returns false
提问by DNA
Whilst working through the Scala exercises on Iterables, I encountered the following strange behaviour:
在对 Iterables进行 Scala练习时，我遇到了以下奇怪的行为：
val xs = Set(5,4,3,2,1)
val ys = Set(1,2,3,4,5)
xs sameElements ys // true
val xs = Set(3,2,1)
val ys = Set(1,2,3)
xs sameElements ys // false - WAT?!
Surely these Sets have the same elements, and should ignore ordering; and why does this work as expected only for the larger set?
当然，这些集合具有相同的元素，应该忽略排序；为什么这仅适用于更大的集合？
回答by DNA
The Scala collections library provides specialised implementations for Sets of fewer than 5 values (see the source). The iterators for these implementations return elements in the order in which they were added, rather than the consistent, hash-based ordering used for larger Sets.
Scala 集合库为少于 5 个值的集合提供了专门的实现（请参阅源代码）。这些实现的迭代器按照元素的添加顺序返回元素，而不是用于较大集合的一致的、基于哈希的排序。
Furthermore, sameElements(scaladoc) is defined on Iterables (it is implemented in IterableLike- see the source); it returns true only if the iterators return the same elements in the same order.
此外，sameElements( scaladoc) 是在Iterables上定义的（它是在IterableLike- 参见源代码中实现的）；只有当迭代器以相同的顺序返回相同的元素时，它才返回真。
So although Set(1,2,3)and Set(3,2,1)oughtto be equivalent, their iterators are different, therefore sameElementsreturns false.
所以虽然Set(1,2,3)和Set(3,2,1)应该是等价的，但它们的迭代器是不同的，因此sameElements返回 false。
This behaviour is surprising, and arguably a bug since it violates the mathematical expectations for a Set (but only for certain sizes of Set!).
这种行为令人惊讶，可以说是一个错误，因为它违反了对 Set 的数学期望（但仅限于某些大小的 Set！）。
As I.K. points out in the comments, ==works fine if you are just comparing Sets with one another, i.e. Set(1,2,3) == Set(3,2,1). However, sameElements is more general in that it can compare the elements of any two iterables. For example, List(1, 2, 3) == Array(1, 2, 3)is false, but List(1, 2, 3) sameElements Array(1, 2, 3)is true.
正如 IK 在评论中指出的那样，==如果您只是将 Sets 相互比较，即Set(1,2,3) == Set(3,2,1). 但是，sameElements 更通用，因为它可以比较任意两个可迭代对象的元素。例如，List(1, 2, 3) == Array(1, 2, 3)是假的，但是List(1, 2, 3) sameElements Array(1, 2, 3)是真的。
More generally, equality can be confusing - note that:
更一般地说，平等可能会令人困惑——请注意：
List(1,2,3) == Vector(1,2,3)
List(1,2,3) != Set(1,2,3)
List(1,2,3) != Array(1,2,3)
Array(1,2,3) != Array(1,2,3)
I have submitted a fixfor the Scala exercisesthat explains the sameElementsproblem.
