0004638: Provide expm1 and log1p

(0004718)
xleroy (administrator)
2008-11-09 10:20

I agree expm1 and log1p are useful. The problem is that these functions are not in the C89 standard ("ANSI C"), only in C99. In particular, Microsoft's standard library does not implement them. So, we have a cross-platform portability issue here.

In the meantime, there exists numerically-accurate implementations in terms of regular exp() and log(), due to W. Kahan himself:
http://www.plunk.org/~hatch/rightway.php [^]
Maybe these would be good enough for your application.

(0004794)
Christophe Troestler (reporter)
2008-12-11 15:58
edited on: 2008-12-11 15:58

Thanks for your prompt reply. Since I use Linux, I did a direct binding to the C function but maybe, for windows, a binding to fdlibm http://www.netlib.org/fdlibm/ [^] can maybe envisioned?

(0004896)
xleroy (administrator)
2009-04-01 18:11

Tentative implementation in CVS trunk. Use Kahan's magic formulae if the C library doesn't provide the functions. Scheduled for inclusion in release 3.12.0.

Issue History
Date Modified	Username	Field	Change
2008-11-05 17:13	Christophe Troestler	New Issue
2008-11-09 10:20	xleroy	Note Added: 0004718
2008-11-09 10:20	xleroy	Status	new => acknowledged
2008-12-11 15:58	Christophe Troestler	Note Added: 0004794
2008-12-11 15:58	Christophe Troestler	Note Edited: 0004794
2009-03-31 13:47	xleroy	Status	acknowledged => assigned
2009-03-31 13:47	xleroy	Assigned To	=> xleroy
2009-04-01 18:11	xleroy	Note Added: 0004896
2009-04-01 18:11	xleroy	Status	assigned => resolved
2009-04-01 18:11	xleroy	Resolution	open => fixed
2009-04-01 18:11	xleroy	Fixed in Version	=> 3.12.0+dev
2011-05-29 12:14	xleroy	Status	resolved => closed

Notes
(0004718) xleroy (administrator) 2008-11-09 10:20	I agree expm1 and log1p are useful. The problem is that these functions are not in the C89 standard ("ANSI C"), only in C99. In particular, Microsoft's standard library does not implement them. So, we have a cross-platform portability issue here. In the meantime, there exists numerically-accurate implementations in terms of regular exp() and log(), due to W. Kahan himself: http://www.plunk.org/~hatch/rightway.php [^] Maybe these would be good enough for your application.

(0004794) Christophe Troestler (reporter) 2008-12-11 15:58 edited on: 2008-12-11 15:58	Thanks for your prompt reply. Since I use Linux, I did a direct binding to the C function but maybe, for windows, a binding to fdlibm http://www.netlib.org/fdlibm/ [^] can maybe envisioned?

(0004896) xleroy (administrator) 2009-04-01 18:11	Tentative implementation in CVS trunk. Use Kahan's magic formulae if the C library doesn't provide the functions. Scheduled for inclusion in release 3.12.0.

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