Scala 集合包含相同的元素，但 sameElements() 返回 false [英] Scala Sets contain the same elements, but sameElements() returns false

问题描述

在使用 Scala Iterables 练习 时，我遇到了以下奇怪的行为:

Whilst working through the Scala exercises on Iterables, I encountered the following strange behaviour:

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?!

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?

Surely these Sets have the same elements, and should ignore ordering; and why does this work as expected only for the larger set?

推荐答案

Scala 集合库为少于 5 个值的集合提供专门的实现(请参阅 源).这些实现的迭代器按照元素添加的顺序返回元素，而不是用于较大集合的一致的、基于哈希的排序.

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.

此外，sameElements(scaladoc) 在 Iterable 上定义(它在 IterableLike - 请参阅源);仅当迭代器以相同的顺序返回相同的元素时，它才返回 true.

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.

所以尽管 Set(1,2,3) 和 Set(3,2,1) 应该是等价的，但它们的迭代器不同，因此 sameElements 返回 false.

So although Set(1,2,3) and Set(3,2,1) ought to be equivalent, their iterators are different, therefore sameElements returns false.

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！).

This behaviour is surprising, and arguably a bug since it violates the mathematical expectations for a Set (but only for certain sizes of Set!).

作为 I.K.在评论中指出， == 如果您只是将 Sets 相互比较，则效果很好，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.

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.

更一般地说，平等可能会令人困惑 - 请注意:

More generally, equality can be confusing - note that:

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)

我已为Scala 练习，解释了 sameElements 问题.

I have submitted a fix for the Scala exercises that explains the sameElements problem.

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