为什么在减法中使用不一致的结果会减少? [英] Why inconsistent results using subtraction in reduce?

问题描述

给出以下内容:

val rdd = List(1,2,3)

我假设rdd.reduce((x,y) => (x - y))将返回-4(即(1-2)-3=-4)，但是它返回了2.

I assumed that rdd.reduce((x,y) => (x - y)) would return -4 (i.e. (1-2)-3=-4), but it returned 2.

为什么?

推荐答案

来自RDD源代码(以及

From the RDD source code (and docs):

/**
* Reduces the elements of this RDD using the specified commutative and
* associative binary operator.
*/
def reduce(f: (T, T) => T): T

reduce是一个单调约简，因此假定该函数为可交换和 associative ，这意味着不能保证将其应用于元素的顺序.

reduce is a monoidal reduction, thus it assumes the function is commutative and associative, meaning that the order of applying it to the elements is not guaranteed.

很显然，您的函数(x,y)=>(x-y)不是可交换的也不是关联的.

Obviously, your function (x,y)=>(x-y) isn't commutative nor associative.

在您的情况下，reduce可能是这样应用的:

In your case, the reduce might have been applied this way:

3 - (2 - 1) = 2

或

1 - (2 - 3) = 2

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