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ibrahim129's blog

python division precision error
By ibrahim129, history, 3 years ago, In English

I was doing some float operations using python. But when I tried to divide 9999999999999999 by 2 (9999999999999999/2) I got 5000000000000000.0 which is wrong. I already know the issue 8.6-8.4 != 0.2 (issue with binary representation) but I couldn't figure out what is the issue. Can anyone explain this?

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3 years ago, # |
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Python float doesnt have enough precision to distinguish a difference at around 1e-16, so in Python, 99...9 (as you wrote) is stored equal to 10...00 (with as many 0s as there were 9s above).

Interactive shell: 

Input: float(10000000000000000)==float(9999999999999999)
Output: True
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3 years ago, # |
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You can also use the decimal module in python for accurate floating point arithmetic : 

from decimal import Decimal
print(Decimal(9999999999999999)/Decimal(2))
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