How can I use fabs from math.h in my cython code.

[Vishal]

Hello,

I have some code where I need to get the absolute value of a double
precision number. Currently, the Python builtin abs is used. Which
makes it a little slower. I would like to use the fabs() function as
defined in math.h.
To do this I tried to add the signature of this function to math.pxd
in libc folder of Cython, as th last indented line of the pxd file,
like so:

double fabs(double x)

then in my pyx file I did the following:

from libc.math cimport fabs

This line starts giving the following error when i want to see the
annotated output.

AttributeError: 'UnspecifiedType' object has no attribute
'create_from_py_utility_code'

if I remove that line, everything goes fine?

Where to go from here?


Thanks and best regards,
Vishal Sapre

[Vishal]

Hello,

I just found out that I had the following already present:

from math import log

and on removing this, everything went well.

Thanks and best regards,

Vishal Sapre

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Thanks and best regards,
Vishal Sapre

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[Stefan Behnel]

Vishal, 18.02.2011 07:11:

> Hello,
>
> I have some code where I need to get the absolute value of a double
> precision number. Currently, the Python builtin abs is used. Which
> makes it a little slower. I would like to use the fabs() function as
> defined in math.h.

Hmm, right, math.pxd is actually lacking a lot of declarations:

http://en.wikipedia.org/wiki/Math.h

> To do this I tried to add the signature of this function to math.pxd
> in libc folder of Cython, as th last indented line of the pxd file,
> like so:
>
> double fabs(double x)

Sure, should work.

> then in my pyx file I did the following:
>
> from libc.math cimport fabs
>
> This line starts giving the following error when i want to see the
> annotated output.
>
> AttributeError: 'UnspecifiedType' object has no attribute
> 'create_from_py_utility_code'

Hmm, I assume that's not all you put into your .pyx, is it? That certainly
works for me.

> if I remove that line, everything goes fine?
>
> Where to go from here?

Could you provide some example code that fails for you? Try to strip down
your code to a short example, that will usually allow you to see what goes
wrong.

Stefan

[Stefan Behnel]

Stefan Behnel, 18.02.2011 07:26:
> Vishal, 18.02.2011 07:11:

>> I have some code where I need to get the absolute value of a double
>> precision number. Currently, the Python builtin abs is used. Which
>> makes it a little slower. I would like to use the fabs() function as
>> defined in math.h.
>
> Hmm, right, math.pxd is actually lacking a lot of declarations:
>
> http://en.wikipedia.org/wiki/Math.h

BTW, I think it shouldn't make your code slower to use the Python
functions. Applying math.abs() to a typed C floating point value should
call fabs().

http://trac.cython.org/cython_trac/ticket/662

(we've discussed this idea on cython-dev ages ago, but I couldn't find a
ticket for it)

Stefan