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Ceil division with /// operator #87421
Description
| BPO | 43255 |
|---|---|
| Nosy | @tim-one, @rhettinger, @mdickinson, @vstinner, @serhiy-storchaka, @PedanticHacker, @nathanielmanistaatgoogle |
Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.
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GitHub fields:
assignee = None
closed_at = <Date 2021-02-18.15:00:41.696>
created_at = <Date 2021-02-18.14:18:04.452>
labels = ['type-feature', 'library', '3.10']
title = 'Ceil division with /// operator'
updated_at = <Date 2022-02-04.19:31:00.211>
user = 'https://github.com/PedanticHacker'bugs.python.org fields:
activity = <Date 2022-02-04.19:31:00.211>
actor = 'Nathaniel Manista'
assignee = 'none'
closed = True
closed_date = <Date 2021-02-18.15:00:41.696>
closer = 'vstinner'
components = ['Library (Lib)']
creation = <Date 2021-02-18.14:18:04.452>
creator = 'PedanticHacker'
dependencies = []
files = []
hgrepos = []
issue_num = 43255
keywords = []
message_count = 10.0
messages = ['387231', '387236', '387237', '387248', '387264', '387267', '387268', '387269', '387270', '387325']
nosy_count = 7.0
nosy_names = ['tim.peters', 'rhettinger', 'mark.dickinson', 'vstinner', 'serhiy.storchaka', 'PedanticHacker', 'Nathaniel Manista']
pr_nums = []
priority = 'normal'
resolution = 'rejected'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue43255'
versions = ['Python 3.10']
Activity
PedanticHacker commentedon Feb 18, 2021
I hope I'm not too late for a Python 3.10 feature request. I love the fact that Python 3.x does floor division with the // operator.
We, however, don't have a ceil division operator, besides importing the math module and using its math.ceil() function.
Is it possible we have /// as a ceil division operator in Python 3.10?
I just wanted to put this thought out of my head if anyone becomes entertained by this idea.
serhiy-storchaka commentedon Feb 18, 2021
Since adding new operator is a syntax-level change, it needs a PEP. It is better to start with discussing on the Python-ideas mailing list (https://mail.python.org/mailman3/lists/python-ideas.python.org/). Let's see if this idea will be accepted favorably.
BTW, the ceiling division can be expressed as -(-a // b) or -(a // -b).
vstinner commentedon Feb 18, 2021
I'm not convinced that this operation is so common that it deserves a new operator. As Serhiy wrote, it must be first on python-ideas first. I close the issue.
--
floor division is x//y
integer ceil division can be implemented with math.ceil(x/y) for small numbers.
For large integer numbers, I like to use something like:
I let you adjust it for negative numbers ;-)
mdickinson commentedon Feb 18, 2021
I can't recall where I saw this originally, but you can use the spaceship operator "--0--" to turn floor division into ceiling division:
There's also the ++0++ operator when you want to emphasize that you really *do* mean floor division:
But yes, -(-n // d) is the easy (if a little bit cryptic) way to get the ceiling of n / d.
PedanticHacker commentedon Feb 18, 2021
The spaceship operator is kinda cool! :)
Well, I was hoping to avoid importing the math module just to have ceil division capabilities.
Anyway, my need for ceiling is that I am coding a chess GUI where I have a table with 2 columns and I create a new row whenever White makes a move. So, my implementation for row creation is math.ceil(len(moves) // 2) where moves is a list of chess moves.
So, the sequence of table rows after White and Black make their moves is initially 0 (since there are no moves yet) and then 1 (first row created after White made a move) and then 1 (again on row 1 after Black made a move, but on column 2), then 2 2 3 3 4 4 5 5 6 6 and so on.
If we had the ceil operator in Python, I could do: len(moves) /// 2 :)
By the way -- do you happen to know any better idea to get that kind of a sequence? That is 0 1 1 2 2 3 3 4 4 5 5 6 6 and so on.
tim-one commentedon Feb 18, 2021
(len(moves) + 1) // 2
vstinner commentedon Feb 18, 2021
You may like the new int.bit_count() attribute of Python 3.10 ;-)
https://docs.python.org/dev/library/stdtypes.html#int.bit_count
PedanticHacker commentedon Feb 18, 2021
My hat off to you, Mr. Peters! Your suggestion (len(moves) + 1) // 2 works perfectly and is much more elegant than --0-- len(moves) // 2. :)
Mr. Stinner, in what way would int.bit_count() be beneficial to me?
vstinner commentedon Feb 18, 2021
I found https://www.chessprogramming.org/Population_Count when I investigated the C implementation of _Py_popcount32(), which is used by int.bit_count().
I read it when I tried to document this surprising code:
---
I reformatted it as:
---
// 32-bit SWAR (SIMD Within A Register) popcount
---
PedanticHacker commentedon Feb 19, 2021
Mr. Stinner, I really don't understand what are you hinting at.