python - 为什么这会返回 False

Question

这个问题已经有答案了:

How is floating point stored? When does it matter? (9 个回答)

已关闭 8 年前。

我不明白为什么这段代码返回 False:

list_of_disks = [[170, 158, 470, 0.1], [135, 176, 410, 0.2], [100, 193, 350, 0.3], [170, 458, 470,       1.1]]
def f1(pos):
    for x in range(0, len(list_of_disks)):
        if list_of_disks[x][3] == pos:
            print list_of_disks[x+1][3] - 0.1, list_of_disks[x][3]
            print list_of_disks[x+1][3] - 0.1 == list_of_disks[x][3] # Why is this False..?
            break

f1(0.2)

当我打印出这两个值时，它们似乎是相同的..？希望有人能帮助我，谢谢!

Answer 1

这是因为0.1无法被计算机存储(没有精确的二进制表示)，所以0.3-0.1对于python来说与0.2不一样:

>>> 0.3-0.1==0.2
False

参见http://www.exploringbinary.com/why-0-point-1-does-not-exist-in-floating-point/有关浮点精度的更多详细信息。

您可以通过使用 Decimal 模块(精确十进制浮点算术的实现)来避免这些陷阱。

The purpose of this module is to support arithmetic using familiar
"schoolhouse" rules and to avoid some of the tricky representation
issues associated with binary floating point.  The package is especially
useful for financial applications or for contexts where users have
expectations that are at odds with binary floating point (for instance,
in binary floating point, 1.00 % 0.1 gives 0.09999999999999995 instead
of 0.0; Decimal('1.00') % Decimal('0.1') returns the expected
Decimal('0.00')).