python - 减去 float : 128. 0 和 int : 128 results in -5. 6834188608e-14？

Question

对 python 还是新手，所以这对我来说很奇怪......

def getNormalizedStart(self):
        # center is Point: {X = 300, Y = 128}
        center = self.getTargetCenterFromStart()

        # effectiveStart is Point: {X=300.0,Y=472}
        effectiveStart = self.getFirstActiveTouchEventLocations()[0]["Point"]\
            if self.isTouchTrial() else self.getFirstMouseEventLocation()

        radians = float(MathUtils.angle(effectiveStart, center, True))

        # Point: {X=-44.0,Y=128.0}
        newStart = MathUtils.rotatePoint(effectiveStart, center, -radians)

        # newStart.X is float: -44
        # center.X is int: 300
        newStart.X -= center.X    # the result is -344. Good!

        # newStart.Y is float: 128.0
        # center.Y is int: 128
        newStart.Y -= center.Y    # the result is -5.68434188608e-14. Wut?
        return newStart

但它在控制台中有效:

>>> x = 128.0
>>> y = 128
>>> x -= y
>>> x
0.0

我是快疯了还是漏掉了一些明显的东西？

Answer 1

计算机中浮点运算的问题在于，通常您无法获得精确值。这是因为不可能用小数精确表示数字，128.0 - 128 等简单运算可能会导致非常小的错误。

像往常一样，必读的解释是What Every Computer Scientist Should Know About Floating-Point Arithmetic .