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changeset 19151:e09318118fd6

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doc: Add note to gamma docstring about using gammaln when input is large. * gammainc.cc (Fgammainc): Change seealso link to gammaln from lgamma. * mappers.cc (Fgamma): Add note to docstring about using gammaln to preserve precision when input to gamma is large. * mappers.cc (Flgamma): Put gammaln first in docstring.
author Rik <rik@octave.org>
date Mon, 22 Sep 2014 20:25:21 -0700
parents adb102face2d
children 0f9c5a15c8fa
files libinterp/corefcn/gammainc.cc libinterp/corefcn/mappers.cc
diffstat 2 files changed, 10 insertions(+), 4 deletions(-) [+]
line wrap: on
line diff
--- a/libinterp/corefcn/gammainc.cc	Mon Sep 22 20:24:42 2014 -0700
+++ b/libinterp/corefcn/gammainc.cc	Mon Sep 22 20:25:21 2014 -0700
@@ -73,7 +73,7 @@
 @example\n\
 gammainc (@var{x}, @var{a}) @equiv{} 1 - gammainc (@var{x}, @var{a}, \"upper\")\n\
 @end example\n\
-@seealso{gamma, lgamma}\n\
+@seealso{gamma, gammaln}\n\
 @end deftypefn")
 {
   octave_value retval;
--- a/libinterp/corefcn/mappers.cc	Mon Sep 22 20:24:42 2014 -0700
+++ b/libinterp/corefcn/mappers.cc	Mon Sep 22 20:25:21 2014 -0700
@@ -1024,7 +1024,13 @@
 @end example\n\
 \n\
 @end ifnottex\n\
-@seealso{gammainc, lgamma}\n\
+\n\
+Programming Note: The gamma function can grow quite large even for small\n\
+input values.  In many cases it may be preferable to use the natural\n\
+logarithm of the gamma function (@code{gammaln}) in calculations to minimize\n\
+loss of precision.  The final result is then\n\
+@code{exp (@var{result_using_gammaln}).}\n\
+@seealso{gammainc, gammaln, factorial}\n\
 @end deftypefn")
 {
   octave_value retval;
@@ -1574,8 +1580,8 @@
 
 DEFUN (lgamma, args, ,
        "-*- texinfo -*-\n\
-@deftypefn  {Mapping Function} {} lgamma (@var{x})\n\
-@deftypefnx {Mapping Function} {} gammaln (@var{x})\n\
+@deftypefn  {Mapping Function} {} gammaln (@var{x})\n\
+@deftypefnx {Mapping Function} {} lgamma (@var{x})\n\
 Return the natural logarithm of the gamma function of @var{x}.\n\
 @seealso{gamma, gammainc}\n\
 @end deftypefn")