Re: Compiled function changes somehow.
- To: mathgroup at smc.vnet.net
- Subject: [mg78602] Re: Compiled function changes somehow.
- From: dh <dh at metrohm.ch>
- Date: Thu, 5 Jul 2007 04:02:57 -0400 (EDT)
- References: <f6fqn6$8mo$1@smc.vnet.net>
Hi Nacho,
try:
Max[Abs[standardresult-compiledresult]]
and you will see that the differences are very small, that is in the
last few bits.
The non compiled version first makes a rational number and then converts
it to a machine number. The compiled form on the other hand, calculates
the quotient of 2 machine numbers. As machine arithmetic is not 100%
accurate you can not expect bothe results to be identical up the the
last bit.
hope this helps, Daniel
Nacho wrote:
> Hello.
>
> I have been playing with Compile to accelerate some simple code and I
> have found some differences that I cannot explain.
>
> The standard/compiled code is as follows:
>
> standard[max_] :=
> Table[n/m, {n, 1, max}, {m, 1, max}] // N // Flatten;
>
> compiled =
> Compile[{{max, _Integer}},
> Table[n/m, {n, 1, max}, {m, 1, max}] // N // Flatten];
>
> So I can do the same calculations with both codes:
>
> In[19]:= standardresult = standard[1000]; // Timing
>
> Out[19]= {2.969, Null}
>
> In[20]:= compiledresult = compiled[1000]; // Timing
>
> Out[20]= {0.422, Null}
>
>
> The second is much faster, as expected. But are the results the same?
> Apparently, yes:
>
> In[21]:= standardresult == compiledresult
>
> Out[21]= True
>
> In[22]:= standardresult === compiledresult
>
> Out[22]= True
>
>
> But when I use Union in the lists to count the number of different
> elements, I have this surprise:
>
> Length[Union[standardresult]]
> Length[Union[compiledresult]]
>
> Out[23]= 608383
> Out[24]= 634815
>
> So they are not exactly the same... I think that the correct answer
> comes from the uncompiled version. It is the same if I remove the //N
> so it can be compared symbolically.
>
> Is this behaviour expected? Am I missing something?
>
> Both seem to be Machine Precision, but obviously there are some little
> differences. This happens with V5.2 and V6.0.
>
> Any hint?
>
> Thank you.
>
>