Odd rounding issue

Question

I was just coding something when I noticed something odd:
2/3 returns 0, which is logical, because that's 0.6666666666666666, which gets rounded towards 0, because its an int.
int(2/3.0) returns the same, because int(0.6666666666666666) returns 0.

now:
-2/3.0 returns -0.6666666666666666 which is logical, but here comes the part that is confusing me,
int(-0.6666666666666666) returns 0 (because ints round towards 0).
But -2/3 returns -1...
Why is this?
Shouldn't this also return 0?
After all int(-0.6666666666666666) returns 0, so why does -2/3 return -1?

TL;DR: 2/3 == 0, but -2/3 != 0?

Best regards, Sjaak.

Now that you mention it, shouldn't 0.666666... round to 1 (rather than 0) if this is just doing normal rounding? — EJoshuaS - Stand with Ukraine, Feb 18 '18 at 16:19
The true duplicate appears to be [Why is -1/2 evaluated to 0 in C++, but -1 in Python?](https://stackoverflow.com/questions/22030342/) — Jeppe Stig Nielsen, Feb 18 '18 at 20:14

Martijn Pieters · Accepted Answer · 2018-02-18T16:15:12.037

7

/ floors, it doesn't truncate. It is not the same operation as int() applied to a floating point division. From the Binary arithmetic operations documentation:

the result is that of mathematical division with the ‘floor’ function applied to the result.

int() truncates:

If x is floating point, the conversion truncates towards zero.

Flooring always rounds down, so for negative numbers that'll round away from 0.

It should be noted that the - unary operator has a higher operator precedence than the / division operator, so the expression is parsed as (-2)/3, not -(2/3).

edited Feb 18 '18 at 16:15

answered Feb 18 '18 at 16:09

Martijn Pieters

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In many __other__ languages `/` will truncate (i.e. round towards zero) and then `(-2) / 3 == -(2 / 3) == -0 == 0`. – Jeppe Stig Nielsen Feb 18 '18 at 16:14
@JeppeStigNielsen: It's a choice each language makes. Python and Ruby floor while Java, C, C# and FORTRAN truncate. – Martijn Pieters Feb 18 '18 at 16:19
I’ve cleaned up the comments about what makes a duplicate; I’m happy to talk about such things on Meta or in chat. – Martijn Pieters Feb 18 '18 at 20:44

Odd rounding issue

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