Python3: why is -1 // 10 = -1?

Question

1 // 10 == 0 with integer division so how come -1 // 10 != 0 ?

MPython 3.7.0b3 (v3.7.0b3:4e7efa9c6f, Mar 29 2018, 18:42:04) [MSC v.1913 64 bit (AMD64)] on win32
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>>> -1 // 10
-1

Is this supposed to happen?

Basically, it floors the result of the plain division. This happens so that it coheres with the behavior of Python's `%` operator, which will give you a positive result when you take the modulus of a negative number. As an aside, please **always** use the generic [python] tag for all python related questions. This behavior is not specific to Python 3, and honestly Python 3 *is* Python. Python 2 is rapidly approaching it's end of life. — juanpa.arrivillaga, Aug 28 '19 at 17:32
As for what I meant by "coheres", I meant that the following holds: `a = (a // b) * b + (a % b)`, check out the linked duplicate for more details — juanpa.arrivillaga, Aug 28 '19 at 17:38

score 2 · Accepted Answer · edited Aug 28 '19 at 18:15

2

it is easier to explain like that:

operation // rounds "to the left" integer, i.e.

1//10 -> 0.1 -> 0
-1 //10 -> -0.1 -> -1 (as -1 on the X axis is to the left of -0.1)

edited Aug 28 '19 at 18:15

Evgeny

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answered Aug 28 '19 at 17:34

Sergey Chekriy

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score 0 · Answer 2 · answered Aug 28 '19 at 17:31

0

Floor division always rounds down.

1 / 10 = .1

which rounds down to 0.

-1 / 10 = -.1

which rounds down to - 1.

answered Aug 28 '19 at 17:31

Mike

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Python3: why is -1 // 10 = -1?

2 Answers2