Why 2/5 print 0.400000002 ????

Question

why print(2/5) output 0.4000000002 but not 0.4?
I know is 2/5 == 0.4,but how can I get 0.4 not 0.400000000002
help

Answer 1

Binary represents 0 and positive integers. For negative integers, storage 'steals' the leftmost bit to represent 'plus' or 'minus'.

Fractions steal even more bits, causing storage to record a 'mantissa' and 'exponent'...which have to be combined to approximate your answer.

You type:
>>> x=2/5

but the computer records a binary fraction:
>>> x.as_integer_ratio()
(3602879701896397, 9007199254740992)

36.../90.... an integer over a power of 2...
= about 0.4

Finally, how to get 0.4:
>>> format(x, '.2f')
'0.40'

Just remember it's a display trick.

Answer 2

computers are not good with floating point numbers. it can not get .4 exactly. normally the closest it can get is like .3999999999999
if you want .4, find a function that will cut it to 1 decimal place for you