annotate scripts/sparse/spaugment.m @ 18725:54a1e95365e1

Overhaul sprand, sprandn functions. * __sprand_impl__: Rename variable "funname" to "fcnname". Add comments to Reciprocal Condition number calculation. Rename "mynnz" to "k" to match rest of code. Add input validation test that RC is scalar or vector. Use double quotes instead of single quotes per Octave guidelines. Check for special case of output vector to avoid problems. Use randperm to replace do/until loop for speed. Pre-calculate speye() value instead of doing per loop iteration. * sprand.m: Improve docstring. Match function output variable name to documentation. Add check string to %!error tests. * sprandn.m: Improve docstring. Match function output variable name to documentation. Add check string to %!error tests.
author Rik <rik@octave.org>
date Sun, 23 Mar 2014 20:35:22 -0700
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children 53af80da6781
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1 ## Copyright (C) 2008-2013 David Bateman
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2 ##
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3 ## This file is part of Octave.
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4 ##
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5 ## Octave is free software; you can redistribute it and/or modify it
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6 ## under the terms of the GNU General Public License as published by
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7 ## the Free Software Foundation; either version 3 of the License, or (at
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8 ## your option) any later version.
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9 ##
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10 ## Octave is distributed in the hope that it will be useful, but
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11 ## WITHOUT ANY WARRANTY; without even the implied warranty of
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12 ## MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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13 ## General Public License for more details.
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14 ##
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15 ## You should have received a copy of the GNU General Public License
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16 ## along with Octave; see the file COPYING. If not, see
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17 ## <http://www.gnu.org/licenses/>.
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18
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19 ## -*- texinfo -*-
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20 ## @deftypefn {Function File} {@var{s} =} spaugment (@var{A}, @var{c})
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21 ## Create the augmented matrix of @var{A}. This is given by
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22 ##
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23 ## @example
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24 ## @group
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25 ## [@var{c} * eye(@var{m}, @var{m}), @var{A};
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26 ## @var{A}', zeros(@var{n}, @var{n})]
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27 ## @end group
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28 ## @end example
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29 ##
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30 ## @noindent
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31 ## This is related to the least squares solution of
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32 ## @code{@var{A} \ @var{b}}, by
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33 ##
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34 ## @example
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35 ## @group
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36 ## @var{s} * [ @var{r} / @var{c}; x] = [ @var{b}, zeros(@var{n}, columns(@var{b})) ]
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37 ## @end group
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38 ## @end example
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39 ##
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40 ## @noindent
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41 ## where @var{r} is the residual error
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42 ##
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43 ## @example
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44 ## @var{r} = @var{b} - @var{A} * @var{x}
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45 ## @end example
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46 ##
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47 ## As the matrix @var{s} is symmetric indefinite it can be factorized
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48 ## with @code{lu}, and the minimum norm solution can therefore be found
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49 ## without the need for a @code{qr} factorization. As the residual
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50 ## error will be @code{zeros (@var{m}, @var{m})} for under determined
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51 ## problems, and example can be
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52 ##
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53 ## @example
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54 ## @group
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55 ## m = 11; n = 10; mn = max (m, n);
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56 ## A = spdiags ([ones(mn,1), 10*ones(mn,1), -ones(mn,1)],
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57 ## [-1, 0, 1], m, n);
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58 ## x0 = A \ ones (m,1);
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59 ## s = spaugment (A);
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60 ## [L, U, P, Q] = lu (s);
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61 ## x1 = Q * (U \ (L \ (P * [ones(m,1); zeros(n,1)])));
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62 ## x1 = x1(end - n + 1 : end);
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63 ## @end group
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64 ## @end example
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65 ##
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66 ## To find the solution of an overdetermined problem needs an estimate
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67 ## of the residual error @var{r} and so it is more complex to formulate
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68 ## a minimum norm solution using the @code{spaugment} function.
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69 ##
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70 ## In general the left division operator is more stable and faster than
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71 ## using the @code{spaugment} function.
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72 ## @end deftypefn
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73
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74 function s = spaugment (A, c)
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75 if (nargin < 2)
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76 if (issparse (A))
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77 c = max (max (abs (A))) / 1000;
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78 else
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79 if (ndims (A) != 2)
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80 error ("spaugment: expecting 2-dimenisional matrix");
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81 else
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82 c = max (abs (A(:))) / 1000;
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83 endif
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84 endif
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85 elseif (!isscalar (c))
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86 error ("spaugment: C must be a scalar");
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87 endif
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88
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89 [m, n] = size (A);
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90 s = [ c * speye(m, m), A; A', sparse(n, n)];
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91 endfunction
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92
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93
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94 %!testif HAVE_UMFPACK
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95 %! m = 11; n = 10; mn = max (m ,n);
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96 %! A = spdiags ([ones(mn,1), 10*ones(mn,1), -ones(mn,1)],[-1,0,1], m, n);
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97 %! x0 = A \ ones (m,1);
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98 %! s = spaugment (A);
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99 %! [L, U, P, Q] = lu (s);
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100 %! x1 = Q * (U \ (L \ (P * [ones(m,1); zeros(n,1)])));
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101 %! x1 = x1(end - n + 1 : end);
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102 %! assert (x1, x0, 1e-6);
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103