Weird behaviour inverting float

Question

With Python 3.4.3

int(1 / 1e-3)

1000

int(1 / 1e-4)

10000

int(1 / 1e-5)

99999

int(1 / 1e-6)

1000000

int(1 / 1e-7)

10000000

Bug or Feature? Any particular reason?

http://docs.oracle.com/cd/E19957-01/806-3568/ncg_goldberg.html — reptilicus, Jan 19 '16 at 17:01
Perl shows the same result: > perl -e 'print(int(1 / 1e-5), "\n")' # so it's probably inherent in the underlying C float/double format. — cdlane, Jan 19 '16 at 17:09
@cdlane Thank you, exactly the kind of answer I was looking for! — gota, Jan 19 '16 at 17:11

score 1 · Accepted Answer · answered Jan 19 '16 at 17:01

floating point numbers aren't exact. Only binary numbers are.

>>> '%.25f' % 1e-5
'0.0000100000000000000008180'
>>> '%.25f' % (1/1e-5)
'99999.9999999999854480847716331'

So 1/1e-5 is less than 100000 and int cuts off the fractal part. Converting to int, rounding is the answer:

>>> int(round(1/1e-5))
100000

1 Answers1