Basic Python Questions - Oct. 31, 2013

[Robert Kern]

On 2013-10-31 14:05, Chris Angelico wrote:
> On Fri, Nov 1, 2013 at 12:17 AM, Alain Ketterlin
> <alain at dpt-info.u-strasbg.fr> wrote:
>> "E.D.G." <edgrsprj at ix.netcom.com> writes:
>>
>>>        The calculation speed question just involves relatively simple
>>> math such as multiplications and divisions and trig calculations such
>>> as sin and tan etc.
>>
>> These are not "simple" computations.
>>
>> Any compiled language (Fortran, C, C++, typically) will probably go much
>> faster than any interpreted/bytecode-based language (like python or
>> perl, anything that does not use a jit).
>
> Well, they may not be simple to do, but chances are you can push the
> work down to the CPU/FPU on most modern hardware - that is, if you're
> working with IEEE floating point, which I'm pretty sure CPython always
> does; not sure about other Pythons. No need to actually calculate trig
> functions unless you need arbitrary precision (and even then, I'd bet
> the GMP libraries have that all sewn up for you). So the language
> doesn't make a lot of difference.

Sure it does. Python boxes floats into a PyObject structure. Both Python and C 
will ultimately implement the arithmetic of "a + b" with an FADD instruction, 
but Python will do a bunch of pointer dereferencing, hash lookups, and function 
calls before it gets down to that. All of that overhead typically outweighs the 
floating point computations down at the bottom, even for the more expensive trig 
functions.

This is where numpy comes in. If you can arrange your computation on arrays, 
then only the arrays need to be unboxed once, then the rest of the arithmetic 
happens in C.

-- 
Robert Kern

"I have come to believe that the whole world is an enigma, a harmless enigma
  that is made terrible by our own mad attempt to interpret it as though it had
  an underlying truth."
   -- Umberto Eco