Problem with eval in python

Question

As shown in the figure, when Python 2.7 uses eval to calculate floating point expressions, {code...} This makes people particularly confused. I can't find the answer on the Internet. Please give guidance to experts! Update: I have another question, a=0.3-0.1, print(a) outputs 0.19999999999999998; b=0.2, but print(...

phpcn_u1582 · Answer

Generally, it cannot be used directly in computers. == Compares whether two floating point numbers are equal.

Because floating point values have errors in computers.

python has decimal and fraction 2 modules for high-precision floating point calculations.

Example

>>> from fractions import Fraction
>>> Fraction('0.3')-Fraction('0.1') == Fraction('0.2')
True
>>> from decimal import Decimal
>>> Decimal('0.3')-Decimal('0.1') == Decimal('0.2')
True
>>>

仅有的幸福 · Answer

This has nothing to do with eval, it is mainly a matter of accuracy. This will be encountered in all programming languages. If you try 0.3-0.1 == 0.2, it will directly return False. Baidu JavaScript 0.2-0.1 problem, and then take a look at "Principles of Computer Composition" 》The initial data representation.

https://stackoverflow.com/que... This is a highly voted answer.

为情所困 · Answer

eval('0.3-0.1==0.2')  # 输出为False, 是因为0.3-0.1=0.19999999999999998
eval('%d - %d == %d'%(0.3, 0.1, 0.2))  # 输出为True, 是因为你%d传入是整数，相当于0-0=0
eval('%s - %s == %s'%(0.3, 0.1, 0.2))  # 输出为False, 参考1
eval('%s - %s == %s'%('0.3', '0.1', '0.2'))  # 输出为False, 参考1