問(wèn)題描述

我在 python 的輸出中遇到負(fù)零；例如，它的創(chuàng)建如下:

I encountered negative zero in output from python; it's created for example as follows:

k = 0.0
print(-k)

輸出將是 -0.0.

但是，當(dāng)我將 -k 與 0.0 進(jìn)行比較時(shí)，結(jié)果為 True.0.0 和 -0.0 之間有什么區(qū)別嗎(我不在乎它們可能具有不同的內(nèi)部表示；我只關(guān)心它們?cè)诔绦蛑械男袨?)有什么我應(yīng)該注意的隱藏陷阱嗎?

However, when I compare the -k to 0.0 for equality, it yields True. Is there any difference between 0.0 and -0.0 (I don't care that they presumably have different internal representation; I only care about their behavior in a program.) Is there any hidden traps I should be aware of?

推薦答案

查看 -0(維基百科中的數(shù)字)

基本上 IEEE 確實(shí)定義了一個(gè)負(fù)零.

Basically IEEE does actually define a negative zero.

并且根據(jù)這個(gè)定義用于所有目的:

And by this definition for all purposes:

-0.0 == +0.0 == 0

我同意 aaronasterling -0.0 和 +0.0 是不同的對(duì)象.使它們相等(相等運(yùn)算符)可確保代碼中不會(huì)引入細(xì)微的錯(cuò)誤.
想想 a * b == c * d

I agree with aaronasterling that -0.0 and +0.0 are different objects. Making them equal (equality operator) makes sure that subtle bugs are not introduced in the code.
Think of a * b == c * d

>>> a = 3.4
>>> b =4.4
>>> c = -0.0
>>> d = +0.0
>>> a*c
-0.0
>>> b*d
0.0
>>> a*c == b*d
True
>>> 

當(dāng)我出于所有實(shí)際目的說(shuō)這個(gè)詞時(shí)，我是相當(dāng)倉(cāng)促地選擇了這個(gè)詞.我的意思是標(biāo)準(zhǔn)的平等比較.

When I said for all practical purposes, I had chosen the word rather hastily. I meant standard equality comparison.

正如參考資料所說(shuō)，IEEE 標(biāo)準(zhǔn)定義了比較，以便 +0 = -0，而不是 -0 .+0.盡管總是可以忽略零的符號(hào)，但 IEEE 標(biāo)準(zhǔn)并沒(méi)有這樣做.當(dāng)乘法或除法涉及帶符號(hào)的零時(shí)，通常的符號(hào)規(guī)則適用于計(jì)算答案的符號(hào).

>>> divmod(-0.0,100)
(-0.0, 0.0)
>>> divmod(+0.0,100)
(0.0, 0.0)

>>> math.atan2(0.0, 0.0) == math.atan2(-0.0, 0.0)
True 
>>> math.atan2(0.0, -0.0) == math.atan2(-0.0, -0.0)
False