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Best way to calculate fraction part of x?

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  Best way to calculate fraction part of x? Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2015-03-23 23:52 +1100
    Re: Best way to calculate fraction part of x? Skip Montanaro <skip.montanaro@gmail.com> - 2015-03-23 08:02 -0500
    Re: Best way to calculate fraction part of x? Mark Lawrence <breamoreboy@yahoo.co.uk> - 2015-03-23 13:11 +0000
    Re: Best way to calculate fraction part of x? wxjmfauth@gmail.com - 2015-03-23 06:17 -0700
    Re: Best way to calculate fraction part of x? Emile van Sebille <emile@fenx.com> - 2015-03-23 17:38 -0700
    Re: Best way to calculate fraction part of x? Jason Swails <jason.swails@gmail.com> - 2015-03-24 21:39 -0400
    Re: Best way to calculate fraction part of x? Emile van Sebille <emile@fenx.com> - 2015-03-26 13:48 -0700
    Re: Best way to calculate fraction part of x? Oscar Benjamin <oscar.j.benjamin@gmail.com> - 2015-03-26 21:59 +0000
    Re: Best way to calculate fraction part of x? Russell Owen <rowen@uw.edu> - 2015-03-26 15:02 -0700

#87827 — Best way to calculate fraction part of x?

I have a numeric value, possibly a float, Decimal or (improper) Fraction,
and I want the fractional part. E.g. fract(2.5) should give 0.5.

Here are two ways to do it:

py> x = 2.5
py> x % 1
0.5
py> x - int(x)
0.5

x % 1 is significantly faster, but has the disadvantage of giving the
complement of the fraction if x is negative:

py> x = -2.75
py> x % 1
0.25


Are there any other, possibly better, ways to calculate the fractional part
of a number?




-- 
Steven

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#87828

On Mon, Mar 23, 2015 at 7:52 AM, Steven D'Aprano
<steve+comp.lang.python@pearwood.info> wrote:
> x % 1 is significantly faster, but has the disadvantage of giving the
> complement of the fraction if x is negative

I suppose

abs(x) % 1

would be just as slow as

x - int(x)

? (haven't checked)

Skip

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#87829

On 23/03/2015 12:52, Steven D'Aprano wrote:
> I have a numeric value, possibly a float, Decimal or (improper) Fraction,
> and I want the fractional part. E.g. fract(2.5) should give 0.5.
>
> Here are two ways to do it:
>
> py> x = 2.5
> py> x % 1
> 0.5
> py> x - int(x)
> 0.5
>
> x % 1 is significantly faster, but has the disadvantage of giving the
> complement of the fraction if x is negative:
>
> py> x = -2.75
> py> x % 1
> 0.25
>
> Are there any other, possibly better, ways to calculate the fractional part
> of a number?
>

Any sparks here 
http://stackoverflow.com/questions/875238/fractional-part-of-the-number-question 
?  I'm assuming that the reference to "pkaeding's algorithm" refers to 
https://github.com/pkaeding but haven't confirmed that to be fact.

-- 
My fellow Pythonistas, ask not what our language can do for you, ask
what you can do for our language.

Mark Lawrence

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#87830

Le lundi 23 mars 2015 13:52:20 UTC+1, Steven D'Aprano a écrit :

>>> math.modf(2.5)
(0.5, 2.0)
>>>

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#87856

On 3/23/2015 5:52 AM, Steven D'Aprano wrote:

> Are there any other, possibly better, ways to calculate the fractional part
> of a number?

float (("%6.3f" % x)[-4:])

Emile

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#87916

[Multipart message — attachments visible in raw view] — view raw

On Mon, Mar 23, 2015 at 8:38 PM, Emile van Sebille <emile@fenx.com> wrote:

> On 3/23/2015 5:52 AM, Steven D'Aprano wrote:
>
>  Are there any other, possibly better, ways to calculate the fractional
>> part
>> of a number?
>>
>
> float (("%6.3f" % x)[-4:])


​In general you lose a lot of precision this way...​

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#88089

On 3/24/2015 6:39 PM, Jason Swails wrote:
>
>
> On Mon, Mar 23, 2015 at 8:38 PM, Emile van Sebille <emile@fenx.com

<snip>

>     float (("%6.3f" % x)[-4:])
>
>
> ​In general you lose a lot of precision this way...​
>

Even more if you use %6.1 -- but feel free to flavor to taste.   :)

Emile

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#88099

On 23 March 2015 at 12:52, Steven D'Aprano
<steve+comp.lang.python@pearwood.info> wrote:
> I have a numeric value, possibly a float, Decimal or (improper) Fraction,
> and I want the fractional part. E.g. fract(2.5) should give 0.5.
>
> Here are two ways to do it:
>
> py> x = 2.5
> py> x % 1
> 0.5
> py> x - int(x)
> 0.5
>
> x % 1 is significantly faster, but has the disadvantage of giving the
> complement of the fraction if x is negative:
>
> py> x = -2.75
> py> x % 1
> 0.25

The other version gives -0.75 in this case so I guess that's what you want.

> Are there any other, possibly better, ways to calculate the fractional part
> of a number?

What do you mean by better? Is it just faster?

To modify the % version so that it's equivalent you can do:

>>> x = -2.75
>>> (x % 1) - (x < 0)
-0.75

I'm not sure if that's faster than x - int(x) though. Obviously it
depends which numeric type you're primarily interested in.


Oscar

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#88100

On 3/24/15 6:39 PM, Jason Swails wrote:
>
>
> On Mon, Mar 23, 2015 at 8:38 PM, Emile van Sebille <emile@fenx.com
> <mailto:emile@fenx.com>> wrote:
>
>     On 3/23/2015 5:52 AM, Steven D'Aprano wrote:
>
>         Are there any other, possibly better, ways to calculate the
>         fractional part
>         of a number?
>
>
>     float (("%6.3f" % x)[-4:])
>
>
> ​In general you lose a lot of precision this way...​

I suggest modf in the math library:

math.modf(x)
Return the fractional and integer parts of x. Both results carry the 
sign of x and are floats.

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