Python“十进制"程序包给出错误结果 [英] Python "decimal" package gives wrong results

问题描述

我尝试通过设置 getcontext().prec = 800 来计算以下内容.

I tried to compute the following by setting getcontext().prec = 800.

>>> from decimal import *
>>> getcontext().prec = 800
>>> Decimal(22.0) / Decimal ( 10.0) - Decimal ( 0.2 )
Decimal('1.999999999999999988897769753748434595763683319091796875')
>>> 

但是预期结果是 2 .我在哪里做错了?

But the expected result is 2. Where am I doing wrong?

推荐答案

从浮点数构造小数时，您会得到浮点数的确切值，该值可能与十进制值不完全匹配，因为这就是浮点数的工作原理.

When you construct a Decimal from a floating-point number, you get the exact value of the floating-point number, which may not precisely match the decimal value because that's how floating-point numbers work.

如果要执行精确的十进制算术，请从字符串而不是浮点数构造Decimal对象:

If you want to do precise decimal arithmetic, construct your Decimal objects from strings instead of floating-point numbers:

>>> Decimal('22.0') / Decimal('10.0') - Decimal('0.2')
Decimal('2.0')

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