comp.lang.idl-pvwave archive - RDF feed
type: channel
description: Messages from Usenet group comp.lang.idl-pvwave, compiled by Paulo Penteado
items: IDL is not accurate enough!
items: Re: IDL is not accurate enough!

IDL is not accurate enough!
type: item
description: 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
creator: noahh.schwartz
date: 2008-08-28T16:30:14-00:00

Re: IDL is not accurate enough!
type: item
description: Noah, other in this thread have laid out the issue, nothing to add here. If you want to go brute force with extended precision, check out the cephes math library at http://www.netlib.org/cephes/. There is an extended precision version that has a gamma function. I have used that in the past for calculating hypergeometric functions and it served my needs. I used call_external, but you can certainly turn this into a dlm. Cheers, Haje <noahh.schwartz@gmail.com> wrote in message news:c7b7fc70-b4db-4c3d-8d2b-50f88dbd1d62@s50g2000hsb.google groups.com... > 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
creator: Haje Korth
date: 2008-09-02T12:20:04-00:00

Re: IDL is not accurate enough!
type: item
description: 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
creator: Kenneth P. Bowman
date: 2008-09-04T20:33:21-00:00

Re: IDL is not accurate enough!
type: item
description: On Sep 4, 2:33 pm, "Kenneth P. Bowman" <k-bow...@null.edu> wrote: > So, when you say DLMs, does that mean Windows only? It means nothing > to me as a Linux and Mac user. DLMs (Dynamically Loadable Modules) are available on all platforms. This is a way of wrapping and loading C code to make it usable from within IDL. DLLs are a Windows thing (the equivalent of a shared object on Unix). Mike -- www.michaelgalloy.com Tech-X Corporation Software Developer
creator: Michael Galloy
date: 2008-09-05T15:57:38-00:00

Re: IDL is not accurate enough!
type: item
description: On Sep 5, 9:57 am, "mgal...@gmail.com" <mgal...@gmail.com> wrote: > DLMs (Dynamically Loadable Modules) are available on all platforms. > This is a way of wrapping and loading C code and Fortran code, too! mp
creator: MP
date: 2008-09-05T16:18:27-00:00

Re: IDL is not accurate enough!
type: item
description: In article <53e9aaf0-6cd2-41c0-95ca-1559353845a4@m36g2000hse.googlegroups.com>, "mgalloy@gmail.com" <mgalloy@gmail.com> wrote: > On Sep 4, 2:33�pm, "Kenneth P. Bowman" <k-bow...@null.edu> wrote: >> So, when you say DLMs, does that mean Windows only? �It means nothing >> to me as a Linux and Mac user. > > DLMs (Dynamically Loadable Modules) are available on all platforms. > This is a way of wrapping and loading C code to make it usable from > within IDL. DLLs are a Windows thing (the equivalent of a shared > object on Unix). > > Mike > -- > www.michaelgalloy.com > Tech-X Corporation > Software Developer Thanks, Mike. I will take a look and see what I need to do to make one work. Ken
creator: Kenneth P. Bowman
date: 2008-09-05T16:53:05-00:00

Re: IDL is not accurate enough!
type: item
description: 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
creator: pgrigis
date: 2008-09-11T14:35:27-00:00

Re: IDL is not accurate enough!
type: item
description: 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
creator: pgrigis
date: 2008-09-11T14:44:44-00:00

Re: IDL is not accurate enough!
type: item
description: 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
creator: noahh.schwartz
date: 2008-09-15T09:37:01-00:00

Re: IDL is not accurate enough!
type: item
description: 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
creator: pgrigis
date: 2008-09-15T13:40:22-00:00