When dividing a float by 100 in Python 2.7 I get the following "rounding behaviour":
>>> 3.7e-03/100
3.7000000000000005e-05
I would expect the following:
>>> 3.7e-03/100
3.7e-05
Note that:
>>> 3.7e-03/100 == 3.7e-05
False
>>> 3.7000000000000005e-05 == 3.7e-05
False
While probably of not practical difference in most applications I find this behaviour somewhat disconcerting.
Why does this happen and how can I avoid it?
I am using Python: '2.7.5 |Anaconda 1.7.0 (32-bit)| (default, Jul 1 2013, 12:41:55) [MSC v.1500 32 bit (Intel)]'
This is a well-known deficiency of floating-point numbers.
You can think of binary floating-point as fractions with power-of-two denominators. Even a simple number such as 0.1 cannot be accurately represented as binary floating-point, and every division by a power of ten is by nature inaccurate.
If you need accurate division with arbitrary denominators in Python, use Decimal (which simulates pencil-and-paper decimals) or Fraction (which simulates pencil-and-paper fractions).
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