英文:
Are associate and associateBy the same?
问题
我对associateBy的两个参数版本感到困惑：
inline fun <T, K, V> Iterable<T>.associateBy(
keySelector: (T) -> K,
valueTransform: (T) -> V
): Map<K, V>
这似乎与associate做的事情完全相同，只是有不同的语法：
inline fun <T, K, V> Iterable<T>.associate(
transform: (T) -> Pair<K, V>
): Map<K, V>
等效用法：
listOf("cat", "dog", "mouse").associateBy({it.toUpperCase()}, {it.length})
listOf("cat", "dog", "mouse").associate({it.toUpperCase() to it.length})
这两个函数只是相同功能的不同语法，还是它们分别存在的特定技术原因，例如，每一个是否设计用于不同的情况？
英文:
I'm confused by the two-parameter variant of associateBy:
inline fun <T, K, V> Iterable<T>.associateBy(
keySelector: (T) -> K,
valueTransform: (T) -> V
): Map<K, V>
This seems to do exactly the same thing as associate, but with alternative syntax:
inline fun <T, K, V> Iterable<T>.associate(
transform: (T) -> Pair<K, V>
): Map<K, V>
Equivalent usages:
listOf("cat", "dog", "mouse").associateBy({it.toUpperCase()}, {it.length})
listOf("cat", "dog", "mouse").associate({it.toUpperCase() to it.length})
Are these two functions just alternative syntax for the same thing, or is there a specific technical reason they both exist separately, e.g. is each one designed to be used in a different circumstance than the other?
答案1
得分: 2
是的，它们只是执行相同操作的替代方法。不过，我可以想到一些情况下，你可能更喜欢其中一种方法。
如果键和值来自一个耗时的函数，你不想为每个元素运行两次，那么associate更适合：
list.associate {
val result = timeConsumingFunction(it) // timeConsumingFunction只在每个元素上运行一次
result.property1 to result.property2
}
如果键和值与元素的属性/方法引用很好地映射，那么对于真正喜欢属性/方法引用的人（比如我），associateBy看起来更好：
list.associateBy(Foo::property1, Foo::property2)
associatedBy还避免了创建Pair对象，而associate在大多数情况下需要，尽管这在大多数情况下是微小的优化。
英文:
Yes, they are just alternative ways of doing the same thing. I can think of some situations where you would prefer one of them though.
If the keys and values come from a time consuming function that you don't want to run twice for each element, then associate is more suitable:
list.associate {
val result = timeConsumingFunction(it) // timeConsumingFunction only gets run once per element
result.property1 to result.property2
}
If the keys and values map nicely to property/method references of the elements, associateBy looks nicer to people who really likes property/method references (e.g. me):
list.associateBy(Foo::property1, Foo::property2)
associatedBy also avoids creating the Pair object that associate would require, though this is a micro-optimisation in most cases.
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