Question

Duplicates:
How is floating point stored? When does it matter?

Is floating point math broken?

Why does the following occur in the Python Interpreter?

>>> 0.1+0.1+0.1-0.3
5.551115123125783e-17
>>> 0.1+0.1
0.2
>>> 0.2+0.1
0.30000000000000004
>>> 0.3-0.3
0.0
>>> 0.2+0.1
0.30000000000000004
>>> 

Why doesn’t 0.2 + 0.1 = 0.3?

Answer 1

That’s because .1 cannot be represented exactly in a binary floating point representation. If you try

>>> .1

Python will respond with .1 because it only prints up to a certain precision, but there’s already a small round-off error. The same happens with .3, but when you issue

>>> .2 + .1
0.30000000000000004

then the round-off errors in .2 and .1 accumulate. Also note:

>>> .2 + .1 == .3
False