Integer division in Python 3 - strange result with negative number

Question

I am new to Python, and I am learning operators right now. I understood that:

The / operator is used for floating point division and
// for integer division.

Example:

7//3 = 2

And 7//-3=-3. Why is the answer -3?

I am stuck here.

See also [Why Python's Integer Division Floors](http://python-history.blogspot.com.by/2010/08/why-pythons-integer-division-floors.html) — Eugene Yarmash, Dec 11 '15 at 08:46

score 32 · Accepted Answer · edited May 23 '17 at 12:19

32

// is not integer division, but floor division:

7/-3  -> -2.33333...
7//-3 -> floor(7/-3) -> floor(-2.33333...) -> -3

PEP 238 on Changing the Division Operator:

The // operator will be available to request floor division unambiguously.

See also Why Python's Integer Division Floors (thanks to @eugene y) - Basically 7//-3 is -7//3, so you want to be able to write:

-7 = 3 * q + r

With q an integer (negative, positive or nul) and r an integer such that 0 <= r < 3. This only works if q = -3:

-7 = 3 * (-3) + 2

edited May 23 '17 at 12:19

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answered Dec 11 '15 at 07:04

Holt

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But what about `5//-3`? – Kaushik Dec 11 '19 at 10:59
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@Kaushik `5 \\ -3` does not exist in python. If you meant `5 // -3`, it's the same explanation as `7 // - 3`, because `-5 = 3 * (-2) + 1`. – Holt Dec 11 '19 at 11:02

score 8 · Answer 2 · answered Dec 11 '15 at 07:18

// is the operator for floor division.

This means that after the division is completed the "floor" function is applied (the value retrieved from the division is rounded down to the nearest integer regardless of whether the decimal part is greater or less than .5)

As for your example, be careful to note that for negative answers the floor division operator will still be rounding down (e.g. -5/2 --> -2.5 --> -3).

Integer division in Python 3 - strange result with negative number

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