關(guān)于python浮點(diǎn)mod運(yùn)算的解釋。

luffy.deng

問(wèn)題來(lái)自http://www.72891.cn/thread-1708133-1-1.html 這個(gè)帖子
但是我感覺(jué)帖子里對(duì)python浮點(diǎn) % 運(yùn)算并沒(méi)有徹底討論清楚，因此這里說(shuō)一下我個(gè)人的理解。
關(guān)于python浮點(diǎn)mod運(yùn)算的結(jié)果 <<core python>>里提到這樣一個(gè)公式，x-(math.floor(x/y)*y)
這實(shí)際上是說(shuō)python對(duì)浮點(diǎn)商是向下取整的，這個(gè)可以通過(guò)float_divmod的實(shí)現(xiàn)得到求證。
......
mod = fmod(vx, wx);
        /* fmod is typically exact, so vx-mod is *mathematically* an
           exact multiple of wx.  But this is fp arithmetic, and fp
           vx - mod is an approximation; the result is that div may
           not be an exact integral value after the division, although
           it will always be very close to one.
        */
        div = (vx - mod) / wx;  //注意這里
       
        if (mod) {
                /* ensure the remainder has the same sign as the denominator */
                if ((wx < 0) != (mod < 0)) {
                        mod += wx;
                        div -= 1.0;
                }
        }
......

2gua

其實(shí)是，關(guān)鍵要理解地板除的含義。

t6760915

我測(cè)試了下確實(shí)是，學(xué)習(xí)了

jjj137

嗯，感謝樓主分享，以前每次用之前先自己試一遍，汗……

可可熊

math.fmod(x, y)
Return fmod(x, y), as defined by the platform C library. Note that the Python expression x % y may not return the same result. The intent of the C standard is that fmod(x, y) be exactly (mathematically; to infinite precision) equal to x - n*y for some integer n such that the result has the same sign as x and magnitude less than abs(y). Python’s x % y returns a result with the sign of y instead, and may not be exactly computable for float arguments. For example, fmod(-1e-100, 1e100) is -1e-100, but the result of Python’s -1e-100 % 1e100 is 1e100-1e-100, which cannot be represented exactly as a float, and rounds to the surprising 1e100. For this reason, function fmod() is generally preferred when working with floats, while Python’s x % y is preferred when working with integers.

看下fmod的定義以及%的定義會(huì)更清楚些：
http://python.org/doc/current/ref/binary.html#l2h-411

luffy.deng

回復(fù) 5# 可可熊
從文檔上看還真是按絕對(duì)值來(lái)的，我理解的有偏差。不過(guò)實(shí)際效果還是向下取整數(shù)。

zhuzhu316

回復(fù) 6# luffy.deng


    。。我還是想多問(wèn)句，有中文手冊(cè)嗎？

t6760915

凡事google下先

jjj137

回復(fù)  luffy.deng


    。。我還是想多問(wèn)句，有中文手冊(cè)嗎？
zhuzhu316 發(fā)表于 2010-05-25 11:33

    有也都是很舊的，而且手冊(cè)這種東西，一般翻譯了也會(huì)晦澀難懂吧……往往看起來(lái)比英文難得多。

daybreakcx

其實(shí)可以這么看，我們就是要確定k令：0 <= a + k * b < b
原來(lái)a==-25.5，b==2.25
當(dāng)k==12時(shí)-25.5 + 12 * 2.25 == 1.5
這個(gè)值就是我們要求的
其他的也是一樣的…………之所以會(huì)有地板除，是在找k的時(shí)候方便