Python中的小数模块不准确

发布于 2025-01-17 17:13:20 字数 314 浏览 2 评论 0原文

from decimal import *
Pi=Decimal(3.141592653589793238462643383279502884197169399373)
print(Pi)

实际输出：

3.141592653589793115997963468544185161590576171875

输出应该是：

3.141592653589793238462643383279502884197169399373

为什么值会改变？

原文

from decimal import *
Pi=Decimal(3.141592653589793238462643383279502884197169399373)
print(Pi)

Actual output:

3.141592653589793115997963468544185161590576171875

Output should be:

3.141592653589793238462643383279502884197169399373

Why does the value change?

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青柠芒果 2025-01-24 17:13:20

您正在访问a floating-point number ，and 浮点数本质上是不正确的（另请参阅 python手册）。

要将精确编号传递给小数构造函数，请以字符串的形式传递。

>>> from decimal import Decimal

# bad
>>> Decimal(3.141592653589793238462643383279502884197169399373)
Decimal('3.141592653589793115997963468544185161590576171875')

# good
>>> Decimal('3.141592653589793238462643383279502884197169399373')
Decimal('3.141592653589793238462643383279502884197169399373')

如果您有浮点变量，则可以先将其施加到字符串，然后将其施加到十进制中，以避免一些 floating-probation-point dormision：

>>> a = 0.1 + 0.2
0.30000000000000004
>>> Decimal(a)
Decimal('0.3000000000000000444089209850062616169452667236328125')
>>> Decimal(str(a))
Decimal('0.30000000000000004')
>>>

如果您需要完整的精度，只需一路处理小数：

>>> Decimal("0.1") + Decimal("0.2")
Decimal('0.3')

You're passing in a floating-point number to the Decimal constructor, and floating-point numbers are inherently imprecise (see also the Python manual).

To pass in a precise number to the Decimal constructor, pass it in as a string.

>>> from decimal import Decimal

# bad
>>> Decimal(3.141592653589793238462643383279502884197169399373)
Decimal('3.141592653589793115997963468544185161590576171875')

# good
>>> Decimal('3.141592653589793238462643383279502884197169399373')
Decimal('3.141592653589793238462643383279502884197169399373')

If you have a floating-point variable, you can cast it to a string first, then to a Decimal to avoid some floating-point imprecision:

>>> a = 0.1 + 0.2
0.30000000000000004
>>> Decimal(a)
Decimal('0.3000000000000000444089209850062616169452667236328125')
>>> Decimal(str(a))
Decimal('0.30000000000000004')
>>>

If you need full precision, just work with Decimals all the way:

>>> Decimal("0.1") + Decimal("0.2")
Decimal('0.3')

回复收藏 0 原文

木有鱼丸 2025-01-24 17:13:20

您应该将字符串传递给 Decimal()，而不是浮点数，浮点数一开始就是不精确的。另外，请注意 Python 文档中的以下内容

与基于硬件的二进制浮点不同，十进制模块具有
用户可更改精度（默认为 28 位），可以为
根据给定问题的需要而大

from decimal import *
getcontext().prec = 100 #precision

pi = Decimal("3.141592653589793238462643383279502884197169399373")
print(pi)    #3.141592653589793238462643383279502884197169399373

You should pass a string to Decimal(), not a float, floats are imprecise to begin with. Also, note the following from the Python docs

Unlike hardware based binary floating point, the decimal module has a
user alterable precision (defaulting to 28 places) which can be as
large as needed for a given problem

from decimal import *
getcontext().prec = 100 #precision

pi = Decimal("3.141592653589793238462643383279502884197169399373")
print(pi)    #3.141592653589793238462643383279502884197169399373

回复收藏 0 原文

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