scala forall 并存在输出，是否有意义 [英] scala forall and exists output , does it make sense

问题描述

scala> val l = List()
l: List[Nothing] = List()

scala> l.forall(x=>false)
res0: Boolean = true

scala> l.forall(x=>true)
res1: Boolean = true

scala> l.exists(x=>false)
res2: Boolean = false

scala> l.exists(x=>true)
res3: Boolean = false

对于上面2个谓词，既然列表中不存在元素，那么forall怎么返回true呢?我很困惑.有人能帮忙解释一下吗?

For above 2 predicate, now that no element exists in the list, how come forall return true? I am confused. could you somebody help explain?

推荐答案

您可以重新表述为 forall 意味着列表中的任何元素都不会违反给定的谓词.如果有没有元素，它们都不会违反它.

You could rephrase that forall means that none of the elements of the list violate the given predicate. In case there are no elements, none of them violates it.

forall 如果集合为空，则显式返回 true:

The source code of forall explicitly returns true if a collection is empty:

def forall(p: A => Boolean): Boolean = {
    var these = this
    while (!these.isEmpty) {
       ...
    }
    true
}

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