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

[Sebastian Berg]

Hello,

On Mon, 2011-01-31 at 10:15 -0500, DParker at 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. 
> 

You can use binary indexing instead of the if:
condition = far < 0
values[condition] = np.nan # set to NaN wherever far < 0 is True.

or if you like I suppose you could put it into cython, add some typing
to avoid creating those binary arrays etc. all over to speed things up
more.

Regards,

Sebastian

> 
> 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. +
> 3.103507e-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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