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Re: nchoosek checks, warning, corner cases and tests
From: |
Jaroslav Hajek |
Subject: |
Re: nchoosek checks, warning, corner cases and tests |
Date: |
Sat, 13 Dec 2008 20:10:15 +0100 |
On Sat, Dec 13, 2008 at 3:17 AM, Francesco Potortì <address@hidden> wrote:
> Following the discussion about my previous changeset that disrupted some
> of Jaroslav's improvements, here is an alternative changeset that
> corrects that problem and improves the warning test for loss of
> precision.
>
> Please disregard the changeset that I had sent on December, 10th.
>
> Discussion:
> - help string now mentions some differences between this and bincoeff
> - many sanity checks are done on the arguments
> - handle k==0
> - correct k==n case, when a column vector was output
> - add error and warning tests
> - when testing against bincoeff, use a case where no precision is lost
>
> The changeset is both in the mail body and as an attachment.
>
> # HG changeset patch
> # User Francesco Potortì <address@hidden>
> # Date 1229133593 -3600
> # Node ID 1235e389bbd99059b370ddbfbc4e1c6e24268f34
> # Parent 87cca636a6c61e0cc8c3ab88a2652782203aff26
> nchoosek checks, warning, corner cases and tests
>
> diff -r 87cca636a6c6 -r 1235e389bbd9 scripts/ChangeLog
> --- a/scripts/ChangeLog Fri Dec 12 23:25:54 2008 +0100
> +++ b/scripts/ChangeLog Sat Dec 13 02:59:53 2008 +0100
> @@ -1,3 +1,8 @@ 2008-12-11 Jaroslav Hajek <address@hidden
> +2008-12-13 Francesco Potortì <address@hidden>
> +
> + * specfun/nchoosek.m: Check for input arguments, signal loss of
> + precision, correctly handle k==0 and k==n cases, add proper tests.
> +
> 2008-12-11 Jaroslav Hajek <address@hidden>
>
> * optimization/fsolve.m: Optionally allow pivoted qr factorization.
> diff -r 87cca636a6c6 -r 1235e389bbd9 scripts/specfun/nchoosek.m
> --- a/scripts/specfun/nchoosek.m Fri Dec 12 23:25:54 2008 +0100
> +++ b/scripts/specfun/nchoosek.m Sat Dec 13 02:59:53 2008 +0100
> @@ -49,31 +49,44 @@
> ## of @var{n}, taken @var{k} at a time, one row per combination. The
> ## resulting @var{c} has size @code{[nchoosek (length (@var{n}),
> ## @var{k}), @var{k}]}.
> +##
> +## @code{nchoosek} works only for nonnegative integer arguments; use
> +## @code{bincoeff} for non-integer scalar arguments and for using vector
> +## arguments to compute many coefficients at once.
> ##
> ## @seealso{bincoeff}
> ## @end deftypefn
>
> ## Author: Rolf Fabian <address@hidden>
> ## Author: Paul Kienzle <address@hidden>
> -
> -## FIXME -- This function is identical to bincoeff for scalar
> -## values, and so should probably be combined with bincoeff.
> +## Author: Jaroslav Hajek
>
> function A = nchoosek (v, k)
>
> - if (nargin != 2)
> + if (nargin != 2
> + || !isnumeric(k) || !isnumeric(v)
> + || !isscalar(k) || (!isscalar(v) && !isvector(v)))
> print_usage ();
> + endif
> + if ((isscalar(v) && v < k) || k < 0
> + || k != round(k) || any (v < 0 || v != round(v)))
> + error ("nchoosek: args are nonnegative integers with V not less than K");
> endif
>
> n = length (v);
>
> if (n == 1)
> - if (k > v/2)
> - k = v - k;
> + k = min (k, v-k); # improve precision at next step
> + A = round (prod ((v-k+1:v)./(1:k)));
> + if (A*2*k*eps >= 0.5)
> + warning ("nchoosek", "nchoosek: possible loss of precision");
> endif
> - A = round (prod ((v-k+1:v)./(1:k)));
> + elseif (k == 0)
> + A = [];
> elseif (k == 1)
> A = v(:);
> + elseif (k == n)
> + A = v(:).';
> elseif (k > n)
> A = zeros (0, k, class (v));
> else
> @@ -110,6 +123,8 @@ function A = nchoosek (v, k)
> endif
> endfunction
>
> -# nchoosek seems to be slightly more accurate (but only allowing scalar
> inputs)
> -%!assert (nchoosek(100,45), bincoeff(100,45), -1e2*eps)
> +%!warning (nchoosek(100,45));
> +%!error (nchoosek(100,45.5));
> +%!error (nchoosek(100,145));
> +%!assert (nchoosek(80,10), bincoeff(80,10))
> %!assert
> (nchoosek(1:5,3),[1:3;1,2,4;1,2,5;1,3,4;1,3,5;1,4,5;2:4;2,3,5;2,4,5;3:5])
>
> --
> Francesco Potortì (ricercatore) Voice: +39 050 315 3058 (op.2111)
> ISTI - Area della ricerca CNR Fax: +39 050 315 2040
> via G. Moruzzi 1, I-56124 Pisa Email: address@hidden
> (entrance 20, 1st floor, room C71) Web: http://fly.isti.cnr.it/
>
>
>
>
>
>
I appied it.
thanks
--
RNDr. Jaroslav Hajek
computing expert
Aeronautical Research and Test Institute (VZLU)
Prague, Czech Republic
url: www.highegg.matfyz.cz