why double divison returns seemingly 'wrong' answer in python

Question

On basic operator / and // in Python. Why 99/9.9 returns 10.0 and 99//9.9 returns 9.0?

/ - returns result in float // - returns quotient without reminder in int

so why:

>>> 99/9.9
10.0
>>> 99//9.9
9.0
>>> 9.9*9.0
89.10000000000001

when

>>> 9.9*10.0
99.0

Answer 1

// is for integer division and returns the floor of the quotient. 5//2 = 2, whereas 5/2 = 2.5. It wouldn’t make sense to use // with floats. As explained by others here, floats are inexactly represented and attempted integer division on them yields unexpected results.

Answer 2

9.9 can't be exactly represented as a float (although it is closer than 10^-13 so just typing 9.9 doesn't show 9.9000000001 or anything like that). So a float created from the literal 9.9 is actually slightly greater than 9.9 and 99//9.9 is 9. Similarly, 99%9.9 is 9.899999999999997 and not 0 as we would expect if 9.9 could be exactly represented.

Moral of the story: "//" is mostly for integers and can produce headaches when used on floats.

Answer 3

That is common.

"Beware that due to the limitations of floating point arithmetic, rounding errors can cause unexpected results" from here.). In that reference you can see with all detail.