如何正确处理Python中的浮点运算？

Question

How to correctly add or subtract using floats? For example how to perform:

2.4e-07 - 1e-8

so that it returns 2.3e-7 instead of 2.2999999999999997e-07 .

Converting to int first yields unexpected results, the below returns 2.2e-07 :

int(2.4e-07 * 1e8 - 1) * 1e-8

Similarly,

(2.4e-07 * 1e8 - 1) * 1e-8

returns 2.2999999999999997e-07 .

How to perform subtraction and addition of numbers with 8 decimal point precision?

2.2999999999999997e-07 is not sufficient as the number is used as a lookup in a dictionary, and the key is 2.3e-7 . This means that any value other than 2.3e-7 results in an incorrect lookup.

Answer 1

It's really just a way of skirting around the issue of floating point arithmetic, but I suggest using the decimal package from the standard library. It lets you do exact floating point math.

Using your example,

$ from decimal import Decimal
$ x = Decimal('2.4e-7')
$ y = Decimal('1e-8')
$ x-y
Decimal('2.3E-7')

It's worth noting that Decimal objects are different than the float built-in, but they are mostly interchangeable.

Answer 2

I suggest using the decimal data type (it is present in the stardard installation of Python), because it uses fixed precision to avoid just the differences you are talking about.

>>> from decimal import Decimal
>>> x = Decimal('2.4e-7')
>>> x
Decimal('2.4E-7')
>>> y = Decimal('1e-8')
>>> y
Decimal('1E-8')
>>> x - y
Decimal('2.3E-7')

Answer 3

I do not know if it is what you are looking for but you can try that kind of thing:

a = 0.555555555
a = float("{0:.2f}".format(a))
>>> 0.56

I hope it will help you!

Adrien