[Tutor] rounding up to the nearest multiple of 8

[Dave Angel]

On 10/05/2012 07:23 AM, Albert-Jan Roskam wrote:
> ----- Original Message -----
>
>> From: Asokan Pichai <pasokan at talentsprint.com>
>> To: tutor at python.org
>> Cc: 
>> Sent: Friday, October 5, 2012 11:06 AM
>> Subject: Re: [Tutor] rounding up to the nearest multiple of 8
>>
>> If you are doing so many times, it may be worth precomputing the padding for a
>> given length and adding it by a look up on the length.
>>
>> For example:
>> ------------------------
>> SPACE = ' '
>> MAX = 1000
>> TAB = 8
>> paddding = [ SPACE * (n % TAB) for n in range(MAX) ]
>>
>> .....
>> s = padding[len(s)] + s
>> .....
>> --------------------------
>> where s is string to be padded
>>
>> Asokan Pichai
>
> Good idea! I know that the string values can never exceed 32767 bytes. So when I combine all the advice I got here, the best way seems to be ver4, using Eryksun's ver3 to initialize a dictionary:
>
> from timeit import timeit
> setup = "from math import ceil; value = 1040 * '*'"
> setup += "; " + "padLookup = dict([(i, -8 * (i // -8)) for i in range(1, 32767+1)])"
> ver1 = timeit('int(ceil(len(value)/8.0)*8)', setup=setup)
> ver2 = timeit('len(value) + (-len(value) % 8)', setup=setup)
> ver3 = timeit('-8 * (len(value) // -8)', setup=setup)
> ver4 = timeit('padLookup[len(value)]', setup=setup)
>
> print ver1/ver4, ver2/ver4, ver3/ver4, ver4/ver4
>
> Thanks guys!
>

This is all fun, but what about the context?  Your original function
took an integer, not a string, and thus wasn't charged with measuring
string length, possibly multiple times.  Even so, each of these tests is
taking around a microsecond.  So are you expecting to do anything with
the result?  Just calling ljust() method more than doubled the time.  If
you actually have some code to generate the string, and/or if you're
going to take the result and write it to a file, then pretty soon this
function is negligible time.

If it were my code, i think I'd use something like   "       "[-sz%8:] 
and either prepend or append that to my string.  But if I had to do
something more complex, I'd tune it to the way the string were being used.


-- 

DaveA