英文:
Float64 Less() implementation in standard Go library
问题
我一直在研究Go标准库中的'sort'包，研究所使用的排序算法的具体实现，有一件事对我来说似乎非常奇怪 - Float64 Less()函数的定义：
func (x Float64Slice) Less(i, j int) bool { return x[i] < x[j] || (isNaN(x[i]) && !isNaN(x[j])) }
我相当确信，浮点数值（至少在IEEE 754语义中）不应该直接进行比较。为什么标准的Go库中实现的比较是这样的呢？
英文:
I've been digging in 'sort' package of standard Go library, studying concrete implementations of used sorting algorithms and one thing seems really strange to me - definition of Float64 Less() function:
func (x Float64Slice) Less(i, j int) bool { return x[i] < x[j] || (isNaN(x[i]) && !isNaN(x[j])) }
I quite confident, that floating-point values (in IEEE 754 semantics, at least) MUST NOT be compared directly. Why the comparison implemented this way is in standard Go lib?
答案1
得分: 4
我认为你误解了浮点数不能直接进行比较的含义。
在这个上下文中是正确的：如果你拿两个浮点数，对它们进行不同的计算，数学上得到相同的结果，那么结果很有可能不相等，所以依赖浮点计算来检查数学相等性是一个非常糟糕的想法。
然而，对浮点数切片进行排序是完全有意义的：它们是可比较的，它们有一个顺序。这可能是一个好主意，也可能是一个坏主意，这取决于你的用例是什么。
如果你的用例只是对输入的浮点数据进行排序，那么没有问题。
如果你的用例涉及到一个密集区间的数学问题，那么这可能是一个坏主意。
无论如何，只要记住检查浮点数的相等性通常是一个糟糕的主意，但是比较两个浮点数可能是有意义的，否则你将无法通过比较它们来做出任何决策，而你经常需要这样做。
英文:
I think you misinterpreted that floating-point values must not be compared directly.
This is true in this context: if you take two floating-point numbers, run different calculations on both which mathematically give the same results, then there are lot of chances that the resulting numbers are not equal, so relying on floating-point computation to check mathematical equality is a very, very bad idea.
However, it is perfectly sensed to sort a slice of floating-point numbers: they are comparable, they have an order. This can be either a good or a bad idea, depending on what is your usecase.
If your usecase is just to sort input floating-point data, then there is no problem.
If your usecase is related to a mathematical problem in a dense interval, this is probably a bad idea.
Anyways, just remember that it is generally a bad idea to check for floating-point equality, but comparing two floating-point numbers may have sense, otherwise you could never take any decisions by comparing them, and you often have to do that.
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