CODEWITHSUNDEEP
pythonpython-3.xdecimal-point
yinka
yinka

Reputation: 134

Using "Decimal" in Python

Can someone please explain what's happening below: (I use Python 3.3)

1. >>> Decimal("0.1") + Decimal("0.1") + Decimal("0.1") - Decimal("0.3")
       Decimal('0.0')

2. >>> Decimal(0.1) + Decimal(0.1) + Decimal(0.1) - Decimal(0.3)
       Decimal('2.775557561565156540423631668E-17')

3. >>> Decimal(0.1 + 0.1 + 0.1 - 0.3)
       Decimal('5.5511151231257827021181583404541015625E-17')

I know it has to do with floating point limitation, I'd be glad if someone can explain why

Upvotes: 3

Views: 4525

Answers (1)

NPE
NPE

Reputation: 500167

In a nutshell, neither 0.1 nor 0.3 can be represented exactly as float:

In [3]: '%.20f' % 0.1
Out[3]: '0.10000000000000000555'

In [4]: '%.20f' % 0.3
Out[4]: '0.29999999999999998890'

Consequently, when you use 0.1 or 0.3 to initialize Decimal(), the resulting value is approximately 0.1 or 0.3.

Using strings ("0.1" or "0.3") does not have this problem.

Finally, your second example produces a different result to your third example because, even though both involve implicit rounding, they involve rounding to a different number of decimal places.

Upvotes: 9

Related Questions