| IDL is not accurate enough! [message #62192] |
Thu, 28 August 2008 09:30  |
noahh.schwartz
Messages: 10
Registered: February 2008
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Junior Member |
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Hi,
I've been having problems with IDL accuracy. I'm trying to perform
calculations using the gamma function. The problem is that it grows
VERY fast! Performing this calculation in double (namely gamma(x)/
gamma(y) with x and y big) yields the result: NaN...
Would it be possible to use a program like 'Mathematica' (or any
other) and to plug it in my ILD program? Some kind of CALL_EXTERNAL
that is to say. If it is possible, how can I do it and what is the
best program to use?
Thanks,
Noah
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| Re: IDL is not accurate enough! [message #62353 is a reply to message #62192] |
Thu, 04 September 2008 13:33  |
Kenneth P. Bowman
Messages: 585
Registered: May 2000
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Senior Member |
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In article
<e32f4688-b58e-4f27-b2f9-17faea24285a@i24g2000prf.googlegroups.com>,
rtk <oneelkruns@hotmail.com> wrote:
> On Aug 28, 2:50 pm, "Kenneth P. Bowman" <k-bow...@null.edu> wrote:
>> Mathematica can do arbitrary-precision arithmetic ... with hundreds of digits,
>> if necessary. Don't expect it to be fast, though, as it is all done
>> in software, not with the hardware floating-point unit with which we
>> normally do single- and double-precision arithmetic.
>
> Check on the IDL code contrib site sometime next week. I just
> uploaded DLMs that add arbitrary precision floating point as well as
> integer and rational types to IDL. These are wrappers on the MPFR and
> GMP libraries, respectively. Caveat emptor, but I'll answer emails.
> You will need to recompile if using something later than IDL 6.3.
> Examples included.
>
> Ron Kneusel
> rkneusel@ittvis.com
So, when you say DLMs, does that mean Windows only? It means nothing
to me as a Linux and Mac user.
Ken Bowman
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| Re: IDL is not accurate enough! [message #62508 is a reply to message #62192] |
Thu, 11 September 2008 07:44 |
pgrigis
Messages: 436
Registered: September 2007
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Senior Member |
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pgri...@gmail.com wrote:
> noahh.schwa...@gmail.com wrote:
>> On 28 ao�t, 18:42, F�LDY Lajos <fo...@rmki.kfki.hu> wrote:
>>> On Thu, 28 Aug 2008, noahh.schwa...@gmail.com wrote:
>>>> Hi,
>>>
>>>> I've been having problems with IDL accuracy. I'm trying to perform
>>>> calculations using the gamma function. The problem is that it grows
>>>> VERY fast! Performing this calculation in double (namely gamma(x)/
>>>> gamma(y) with x and y big) yields the result: NaN...
>>>> Would it be possible to use a program like 'Mathematica' (or any
>>>> other) and to plug it in my ILD program? Some kind of CALL_EXTERNAL
>>>> that is to say. If it is possible, how can I do it and what is the
>>>> best program to use?
>>>
>>>> Thanks,
>>>> Noah
>>>
>>> gamma(x)/gamma(y) => exp(lngamma(x)-lngamma(y))
>>>
>>> regards,
>>> lajos
>>
>>
>> lngamma works fine for my propose! Would you know if an equivalent
>> function exists for the beselk function? Something like lnbeselk?
>> beselk(x) for x>709 doesn't seen to work.
>
> Isn't 0 a good enough approximation?
If not, log(K(x,n))~ln(sqrt(!pi/(2*x)))-x for large x
Paolo
>
> Paolo
>
>
>> If not, I guess that I'll have to wait for the DLMs that add arbitrary
>> precision floating point...
>>
>> cheers,
>> Noah
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| Re: IDL is not accurate enough! [message #62509 is a reply to message #62192] |
Thu, 11 September 2008 07:35 |
pgrigis
Messages: 436
Registered: September 2007
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Senior Member |
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noahh.schwa...@gmail.com wrote:
> On 28 ao�t, 18:42, F�LDY Lajos <fo...@rmki.kfki.hu> wrote:
>> On Thu, 28 Aug 2008, noahh.schwa...@gmail.com wrote:
>>> Hi,
>>
>>> I've been having problems with IDL accuracy. I'm trying to perform
>>> calculations using the gamma function. The problem is that it grows
>>> VERY fast! Performing this calculation in double (namely gamma(x)/
>>> gamma(y) with x and y big) yields the result: NaN...
>>> Would it be possible to use a program like 'Mathematica' (or any
>>> other) and to plug it in my ILD program? Some kind of CALL_EXTERNAL
>>> that is to say. If it is possible, how can I do it and what is the
>>> best program to use?
>>
>>> Thanks,
>>> Noah
>>
>> gamma(x)/gamma(y) => exp(lngamma(x)-lngamma(y))
>>
>> regards,
>> lajos
>
>
> lngamma works fine for my propose! Would you know if an equivalent
> function exists for the beselk function? Something like lnbeselk?
> beselk(x) for x>709 doesn't seen to work.
Isn't 0 a good enough approximation?
Paolo
> If not, I guess that I'll have to wait for the DLMs that add arbitrary
> precision floating point...
>
> cheers,
> Noah
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| Re: IDL is not accurate enough! [message #62558 is a reply to message #62192] |
Mon, 15 September 2008 06:40 |
pgrigis
Messages: 436
Registered: September 2007
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Senior Member |
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noahh.schwa...@gmail.com wrote:
> On 11 sep, 16:44, pgri...@gmail.com wrote:
>> pgri...@gmail.com wrote:
>>> noahh.schwa...@gmail.com wrote:
>>>> On 28 ao t, 18:42, F LDY Lajos <fo...@rmki.kfki.hu> wrote:
>>>> > On Thu, 28 Aug 2008, noahh.schwa...@gmail.com wrote:
>>>> > > Hi,
>>
>>>> > > I've been having problems with IDL accuracy. I'm trying to perform
>>>> > > calculations using the gamma function. The problem is that it grows
>>>> > > VERY fast! Performing this calculation in double (namely gamma(x)/
>>>> > > gamma(y) with x and y big) yields the result: NaN...
>>>> > > Would it be possible to use a program like 'Mathematica' (or any
>>>> > > other) and to plug it in my ILD program? Some kind of CALL_EXTERNAL
>>>> > > that is to say. If it is possible, how can I do it and what is the
>>>> > > best program to use?
>>
>>>> > > Thanks,
>>>> > >Noah
>>
>>>> > gamma(x)/gamma(y) => exp(lngamma(x)-lngamma(y))
>>
>>>> > regards,
>>>> > lajos
>>
>>>> lngamma works fine for my propose! Would you know if an equivalent
>>>> function exists for the beselk function? Something like lnbeselk?
>>>> beselk(x) for x>709 doesn't seen to work.
>>
>>> Isn't 0 a good enough approximation?
>>
>> If not, log(K(x,n))~ln(sqrt(!pi/(2*x)))-x for large x
>>
>> Paolo
>>
>>
>>
>>> Paolo
>>
>>>> If not, I guess that I'll have to wait for the DLMs that add arbitrary
>>>> precision floating point...
>>
>>>> cheers,
>>>> Noah
>
>
>
> Hi Paolo,
> Your approximation seems to be missing a factor? This is what IDL
> gives me:
>
> IDL> x=705d & n=1.1 & print, alog10(beselk(x,n)), (alog(sqrt(!pi/
> (2*x)))-x)
> -307.50372 -708.05331
I meant the natural log (why should a bessel function
care about base 10 anyway?), so use alog instead.
Cheers,
Paolo
>
> Cheers,
> Noah
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| Re: IDL is not accurate enough! [message #62563 is a reply to message #62508] |
Mon, 15 September 2008 02:37 |
noahh.schwartz
Messages: 10
Registered: February 2008
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Junior Member |
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On 11 sep, 16:44, pgri...@gmail.com wrote:
> pgri...@gmail.com wrote:
>> noahh.schwa...@gmail.com wrote:
>>> On 28 ao t, 18:42, F LDY Lajos <fo...@rmki.kfki.hu> wrote:
>>>> On Thu, 28 Aug 2008, noahh.schwa...@gmail.com wrote:
>>>> > Hi,
>
>>>> > I've been having problems with IDL accuracy. I'm trying to perform
>>>> > calculations using the gamma function. The problem is that it grows
>>>> > VERY fast! Performing this calculation in double (namely gamma(x)/
>>>> > gamma(y) with x and y big) yields the result: NaN...
>>>> > Would it be possible to use a program like 'Mathematica' (or any
>>>> > other) and to plug it in my ILD program? Some kind of CALL_EXTERNAL
>>>> > that is to say. If it is possible, how can I do it and what is the
>>>> > best program to use?
>
>>>> > Thanks,
>>>> >Noah
>
>>>> gamma(x)/gamma(y) => exp(lngamma(x)-lngamma(y))
>
>>>> regards,
>>>> lajos
>
>>> lngamma works fine for my propose! Would you know if an equivalent
>>> function exists for the beselk function? Something like lnbeselk?
>>> beselk(x) for x>709 doesn't seen to work.
>
>> Isn't 0 a good enough approximation?
>
> If not, log(K(x,n))~ln(sqrt(!pi/(2*x)))-x for large x
>
> Paolo
>
>
>
>> Paolo
>
>>> If not, I guess that I'll have to wait for the DLMs that add arbitrary
>>> precision floating point...
>
>>> cheers,
>>> Noah
Hi Paolo,
Your approximation seems to be missing a factor? This is what IDL
gives me:
IDL> x=705d & n=1.1 & print, alog10(beselk(x,n)), (alog(sqrt(!pi/
(2*x)))-x)
-307.50372 -708.05331
Cheers,
Noah
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