Why does this simple integer division in Python 3 give me one less than expected answer?

Question

Python 3. I have this command ->

int(29930125722068957199361/65537)

I get 456690506463050752, but answer should be 456690506463050753

Oh and this happens only in Python 3+ , not when executing direct in Python prompt

score 1 · Accepted Answer · answered Mar 21 '19 at 02:02

1

This is probably because of floating point error. Floating point numbers have a limited size in memory and can only be so accurate for some values.

Instead, use the // (floor division) operator for an accurate result:

print(29930125722068957199361 // 65537)
# 456690506463050753

answered Mar 21 '19 at 02:02

iz_

But the numbers being divided are integers. And since this is a case of being exactly divisible, where is the floating point aspect coming from? Other than that division is always a fp result in Python 3+. And if that was it, still my cast to int or a math.floor should bring it back to the proper int value right? – Stubborn Mar 21 '19 at 02:07
1

@Stubborn You are right, division results are always floating point. For example, `type(1 / 1)` is `float`. Anyway, when the floating point aspect comes in, you are already losing precision that cannot be "regained" through conversion to `int` or `math.floor`. – iz_ Mar 21 '19 at 02:09

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