[Numpy-discussion] Vectorize or rewrite function to work with array inputs?

John Salvatier jsalvati@u.washington....
Tue Feb 1 13:30:29 CST 2011


Have you thought about using cython to work with the numpy C-API (
http://wiki.cython.org/tutorials/numpy#UsingtheNumpyCAPI)? This will be
fast, simple (you can mix and match Python and Cython).

As for your specific issue: you can simply cast to all the inputs to numpy
arrays (using asarray
http://docs.scipy.org/doc/numpy/reference/generated/numpy.asarray.html) to
deal with scalars. This will make sure they get broadcast correctly.

On Tue, Feb 1, 2011 at 11:22 AM, <DParker@chromalloy.com> wrote:

> Thanks for the advice.
>
> Using Sebastian's advice I was able to write a version that worked when the
> input arguments are both arrays with the same length. The code provided by
> eat works when t is an array, but not for an array of far.
>
> The numpy.vectorize version works with any combination of scalar or array
> input. I still haven't figured out how to rewrite my function to be as
> flexible as the numpy.vectorize version at accepting either scalars or array
> inputs and properly broadcasting the scalar arguments to the array
> arguments.
>
> David Parker
> Chromalloy - TDAG
>
>
>
> From:        eat <e.antero.tammi@gmail.com>
> To:        Discussion of Numerical Python <numpy-discussion@scipy.org>
> Date:        01/31/2011 11:37 AM
> Subject:        Re: [Numpy-discussion] Vectorize or rewrite function to
> work with array inputs?
> Sent by:        numpy-discussion-bounces@scipy.org
> ------------------------------
>
>
>
> Hi,
>
> On Mon, Jan 31, 2011 at 5:15 PM, <*DParker@chromalloy.com*<DParker@chromalloy.com>>
> wrote:
> I have several functions like the example below that I would like to make
> compatible with array inputs. The problem is the conditional statements give
> a *ValueError: The truth value of an array with more than one element is
> ambiguous. Use a.any() or a.all()*. I can use numpy.vectorize, but if
> possible I'd prefer to rewrite the function. Does anyone have any advice the
> best way to modify the code to accept array inputs? Thanks in advance for
> any assistance.
>
> If I understod your question correctly, then air_gamma could be coded as:
> def air_gamma_0(t, far=0.0):
>     """
>     Specific heat ratio (gamma) of Air/JP8
>     t - static temperature, Rankine
>     [far] - fuel air ratio [- defaults to 0.0 (dry air)]
>     air_gamma - specific heat ratio
>     """
>     if far< 0.:
>         return NAN
>     elif far < 0.005:
>         ag= air_gamma_1(t)
>         ag[np.logical_or(t< 379., t> 4731.)]= NAN
>         return ag
>     elif far< 0.069:
>         ag= air_gamma_2(t, far)
>         ag[np.logical_or(t< 699., t> 4731.)]= NAN
>         return ag
>     else:
>         return NAN
> Rest of the code is in the attachment.
>
>
> My two cents,
> eat
>
>
>
> NAN = float('nan')
>
> def air_gamma(t, far=0.0):
>     """
>     Specific heat ratio (gamma) of Air/JP8
>     t - static temperature, Rankine
>     [far] - fuel air ratio [- defaults to 0.0 (dry air)]
>     air_gamma - specific heat ratio
>     """
>     if far < 0.:
>         return NAN
>     elif far < 0.005:
>         if t < 379. or t > 4731.:
>             return NAN
>         else:
>             air_gamma = -3.472487e-22 * t ** 6. + 6.218811e-18 * t ** 5. -
> 4.428098e-14 * t ** 4. + 1.569889e-10 * t ** 3. - 0.0000002753524 * t ** 2.
> + 0.0001684666 * t + 1.368652
>     elif far < 0.069:
>         if t < 699. or t > 4731.:
>             return NAN
>         else:
>             a6 = 4.114808e-20 * far ** 3. - 1.644588e-20 * far ** 2. +
> 3.103507 <tel:+13103507>e-21 * far - 3.391308e-22
>             a5 = -6.819015e-16 * far ** 3. + 2.773945e-16 * far ** 2. -
> 5.469399e-17 * far + 6.058125e-18
>             a4 = 4.684637e-12 * far ** 3. - 1.887227e-12 * far ** 2. +
> 3.865306e-13 * far - 4.302534e-14
>             a3 = -0.00000001700602 * far ** 3. + 0.000000006593809 * far **
> 2. - 0.000000001392629 * far + 1.520583e-10
>             a2 = 0.00003431136 * far ** 3. - 0.00001248285 * far ** 2. +
> 0.000002688007 * far - 0.0000002651616
>             a1 = -0.03792449 * far ** 3. + 0.01261025 * far ** 2. -
> 0.002676877 * far + 0.0001580424
>             a0 = 13.65379 * far ** 3. - 3.311225 * far ** 2. + 0.3573201 *
> far + 1.372714
>             air_gamma = a6 * t ** 6. + a5 * t ** 5. + a4 * t ** 4. + a3 * t
> ** 3. + a2 * t ** 2. + a1 * t + a0
>     elif far >= 0.069:
>         return NAN
>     else:
>         return NAN
>     return air_gamma
>
> David Parker
> Chromalloy - TDAG
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>
> [attachment "air_gamma.py" deleted by Dave Parker/Chromalloy]
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