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Message from discussion coding maxstar
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Neal Becker  
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 More options Nov 9 2012, 9:07 am
From: Neal Becker <ndbeck...@gmail.com>
Date: Fri, 9 Nov 2012 09:07:19 -0500
Local: Fri, Nov 9 2012 9:07 am
Subject: Re: [numexpr] coding maxstar
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Wow!  I'm impressed!  Even without MKL, I'm measuring:

c++ version:
%time test1()
CPU times: user 4.16 s, sys: 0.00 s, total: 4.16 s
Wall time: 4.15 s

numexpr version:
In [34]: %time test1()
CPU times: user 3.53 s, sys: 0.01 s, total: 3.53 s
Wall time: 1.87 s

The full function, which accepts n-ary args is:

def maxstar (*args):
    if len(args) == 1:
        return args[0]
    elif len(args) == 2:
        return maxstar2 (*args)
    else:
        return maxstar2 (
            maxstar (*args[:len(args)/2]),
            maxstar (*args[len(args)/2:])
            )

I'm guessing to just leave this function as pure python - nothing numexpr
can do to help here.

On Fri, Nov 9, 2012 at 8:52 AM, Francesc Alted <fal...@gmail.com> wrote:
> On 11/9/12 1:26 PM, Neal Becker wrote:

>> My problem is to code maxstar function.  For 2 arguments, it is:

>> def maxstar2 (a, b):
>>     return max (a, b ) + log1p (exp (-abs (a - b)))

>> What I have now, is using 'ndarray'
>> https://github.com/ndarray/**ndarray.git<https://github.com/ndarray/ndarray.git>

>> to convert the corresponding c++ code into a binary ufunc.

>> The way this would be applied is to pass 2 large vectors, and the
>> maxstar2 would be applied to the pairs of elements of these vectors.

>> Ultimately, it is intended to operate any more than 2 inputs, but I
>> already have recursive code in python based on maxstar2 which extends to
>> arbitrary # of inputs, and for large vectors the python overhead is
>> probably OK.

>> So I'm wondering if I can use numexpr here.  The above maxstar2 isn't
>> usable directly - it has to be converted to a binary ufunc first (as
>> written it only works on 2 scalars - not 2 vectors).

> If I understand you correctly, you can evaluate the above expression as:

> ne.evaluate('where(a>b, a, b) + log1p (exp (-abs (a - b)))')

> Here it is an actual example:

> In []: a = np.arange(1e7)

> In []: b = a + 1

> In []: time np.where(a>b, a, b) + np.log1p (np.exp (-np.abs (a - b)))
> CPU times: user 1.11 s, sys: 1.20 s, total: 2.31 s
> Wall time: 6.36 s
> Out[]:
> array([  1.31326169e+00,   2.31326169e+00,   3.31326169e+00, ...,
>          9.99999831e+06,   9.99999931e+06,   1.00000003e+07])

> In []: time ne.evaluate('where(a>b, a, b) + log1p (exp (-abs (a - b)))')
> CPU times: user 0.55 s, sys: 0.12 s, total: 0.68 s
> Wall time: 0.43 s
> Out[]:
> array([  1.31326169e+00,   2.31326169e+00,   3.31326169e+00, ...,
>          9.99999831e+06,   9.99999931e+06,   1.00000003e+07])

> [not using VML here]

> --
> Francesc Alted

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