Question

在使用Decimal数据类型时，我偶然发现了量化方法的问题，它似乎给出了舍入错误：

Decimal('1.0055').quantize(Decimal('0.000')) # Should output 1.006
>> Decimal('1.006') # CORRECT output
Decimal('1.0045').quantize(Decimal('0.000')) # Should output 1.005
>> Decimal('1.004') # INCORRECT output

为什么它有时会四舍五入，有时会四舍五入？

Answer 1

tl; dr这就是所谓的bankers' rounding。

默认的舍入模式是

ROUND_HALF_EVEN (to nearest with ties going to nearest even integer)

这正是您所看到的：4与5之间以及5与6之间的关系将转为偶数（{分别为{1}}和4。

如果您想要一个不同的舍入模式，您需要明确指定它。

选择是：

文档：https://docs.python.org/2/library/decimal.html#decimal.Context

请参阅ROUND_CEILING (towards Infinity), ROUND_DOWN (towards zero), ROUND_FLOOR (towards -Infinity), ROUND_HALF_DOWN (to nearest with ties going towards zero), ROUND_HALF_EVEN (to nearest with ties going to nearest even integer), ROUND_HALF_UP (to nearest with ties going away from zero), or ROUND_UP (away from zero). ROUND_05UP (away from zero if last digit after rounding towards zero would have been 0 or 5; otherwise towards zero)的{{1}}参数：https://docs.python.org/2/library/decimal.html#decimal.Decimal.quantize

Decimal.quantize舍入误差

1 个答案: