round down to nearest number

[noydb]

How do you round down ALWAYS to nearest 100? Like, if I have number
3268, I want that rounded down to 3200. I'm doing my rounding like
>>> round(3268, -2)
But, how to round DOWN?

[Ian Kelly]

>>> 3268 // 100 * 100
3200

For more complicated cases, Decimal objects allow you to specify
alternate rounding modes.

[noydb]

hmmm, okay.

So how would you round UP always? Say the number is 3219, so you want
3300 returned.

[Chris Rebert]

http://stackoverflow.com/questions/17944/how-to-round-up-the-result-of-integer-division/96921

Thus: (3219 + 99) // 100

Slight tangent: Beware negative numbers when using // or %.

Cheers,
Chris

[Ian Kelly]

On Thu, Feb 9, 2012 at 6:43 PM, Chris Rebert <cl...@rebertia.com> wrote:
> On Thu, Feb 9, 2012 at 5:23 PM, noydb <jenn....@gmail.com> wrote:

> http://stackoverflow.com/questions/17944/how-to-round-up-the-result-of-integer-division/96921
>
> Thus: (3219 + 99) // 100
>
> Slight tangent: Beware negative numbers when using // or %.

There's no problem with negative numbers here, as long as you actually
want to round *up* or *down*, as opposed to away from zero or toward
zero.

[noydb]

That {>>> (3219 + 99) // 100} doesnt work if the number is other then
4 digits.


(for rounding up to nearest 100):
>>> (3219 + 99)//100
33
>>> (3289 + 99)//100
33
>>> (328678 + 99)//100
3287
>>> (328 + 99)//100
4

[Terry Reedy]

On 2/9/2012 8:23 PM, noydb wrote:
> So how would you round UP always? Say the number is 3219, so you want

>>> (3333//100+1)*100
3400

--
Terry Jan Reedy

[MRAB]

>>> (3219 + 99) // 100 * 100
3300
>>> (3289 + 99) // 100 * 100
3300
>>> (328678 + 99) // 100 * 100
328700
>>> (328 + 99) // 100 * 100
400

Those are all rounded up to the nearest 100 correctly.

[MRAB]

On 10/02/2012 03:29, Terry Reedy wrote:
> On 2/9/2012 8:23 PM, noydb wrote:

>> So how would you round UP always? Say the number is 3219, so you want

> >>> (3333//100+1)*100
> 3400
>
Doing it that way doesn't always work. For example:

>>> (3400 // 100 + 1) * 100
3500

However:

>>> (3400 + 99) // 100 * 100
3400

[Ian Kelly]

On Thu, Feb 9, 2012 at 8:36 PM, MRAB <pyt...@mrabarnett.plus.com> wrote:
> On 10/02/2012 02:25, noydb wrote:
>>

>>>> (3219 + 99) // 100 * 100
> 3300
>>>> (3289 + 99) // 100 * 100
> 3300
>>>> (328678 + 99) // 100 * 100
> 328700
>>>> (328 + 99) // 100 * 100
> 400
>
> Those are all rounded up to the nearest 100 correctly.

One thing to be aware of though is that while the "round down" formula
works interchangeably for ints and floats, the "round up" formula does
not.

>>> (3300.5 + 99) // 100 * 100
3300.0

A more consistent alternative is to negate the number, round down, and
then negate again.

>>> -(-(3300.5) // 100 * 100)
3400.0

Cheers,
Ian

[Arnaud Delobelle]

On 10 February 2012 06:21, Ian Kelly <ian.g...@gmail.com> wrote:
>>>>> (3219 + 99) // 100 * 100
>> 3300
>>>>> (3289 + 99) // 100 * 100
>> 3300
>>>>> (328678 + 99) // 100 * 100
>> 328700
>>>>> (328 + 99) // 100 * 100
>> 400
>>
>> Those are all rounded up to the nearest 100 correctly.
>
> One thing to be aware of though is that while the "round down" formula
> works interchangeably for ints and floats, the "round up" formula does
> not.
>
>>>> (3300.5 + 99) // 100 * 100
> 3300.0
>

I'm surprised I haven't seen:

>>> 212 - (212 % -100)
300

Here's a function that:
* rounds up and down
* works for both integers and floats
* is only two operations (as opposed to 3 in the solutions given above)

>>> def round(n, k):
... return n - n%k
...
>>> # Round down with a positive k:
... round(167, 100)
100
>>> round(-233, 100
... )
-300
>>> # Round up with a negative k:
... round(167, -100)
200
>>> round(-233, -100)
-200
>>> # Edge cases
... round(500, -100)
500
>>> round(500, 100)
500
>>> # Floats
... round(100.5, -100)
200.0
>>> round(199.5, 100)
100.0

--
Arnaud

[Alec Taylor]

o.O

Very nice

> --
> http://mail.python.org/mailman/listinfo/python-list

[noydb]

On Feb 10, 4:58 am, Arnaud Delobelle <arno...@gmail.com> wrote:

> Arnaud- Hide quoted text -
>
> - Show quoted text -

Thanks! Covers all bases, good.

[Olive]

This trick work always (even if the entry is a float):


-(-a//100)*100

>>> -(-3219//100)*100
3300

>>> -(-3200.1//100)*100

3300.0

[Hrvoje Niksic]

Terry Reedy <tjr...@udel.edu> writes:

> On 2/9/2012 8:23 PM, noydb wrote:
>> So how would you round UP always? Say the number is 3219, so you want
>>>> (3333//100+1)*100
> 3400

Note that that doesn't work for numbers that are already round:

>>> (3300//100+1)*100
3400 # 3300 would be correct

I'd go with Chris Rebert's (x + 99) // 100.