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annotate scripts/optimization/fzero.m @ 20038:9fc020886ae9

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maint: Clean up m-files to follow Octave coding conventions. Try to trim long lines to < 80 chars. Use '##' for single line comments. Use '(...)' around tests for if/elseif/switch/while. Abut cell indexing operator '{' next to variable. Abut array indexing operator '(' next to variable. Use space between negation operator '!' and following expression. Use two newlines between endfunction and start of %!test or %!demo code. Remove unnecessary parens grouping between short-circuit operators. Remove stray extra spaces (typos) between variables and assignment operators. Remove stray extra spaces from ends of lines.
author Rik <rik@octave.org>
date Mon, 23 Feb 2015 14:54:39 -0800
parents 4197fc428c7d
children f1d0f506ee78
Ignore whitespace changes - Everywhere: Within whitespace: At end of lines:
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1 ## Copyright (C) 2008-2015 VZLU Prague, a.s.
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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 ## Author: Jaroslav Hajek <highegg@gmail.com>
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20
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21 ## -*- texinfo -*-
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22 ## @deftypefn {Function File} {} fzero (@var{fun}, @var{x0})
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23 ## @deftypefnx {Function File} {} fzero (@var{fun}, @var{x0}, @var{options})
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24 ## @deftypefnx {Function File} {[@var{x}, @var{fval}, @var{info}, @var{output}] =} fzero (@dots{})
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25 ## Find a zero of a univariate function.
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26 ##
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27 ## @var{fun} is a function handle, inline function, or string
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28 ## containing the name of the function to evaluate.
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29 ## @var{x0} should be a two-element vector specifying two points which
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30 ## bracket a zero. In other words, there must be a change in sign of the
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31 ## function between @var{x0}(1) and @var{x0}(2). More mathematically, the
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32 ## following must hold
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33 ##
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34 ## @example
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35 ## sign (@var{fun}(@var{x0}(1))) * sign (@var{fun}(@var{x0}(2))) <= 0
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36 ## @end example
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37 ##
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38 ## If @var{x0} is a single scalar then several nearby and distant
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39 ## values are probed in an attempt to obtain a valid bracketing. If this
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40 ## is not successful, the function fails.
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41 ## @var{options} is a structure specifying additional options.
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42 ## Currently, @code{fzero}
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43 ## recognizes these options: @qcode{"FunValCheck"}, @qcode{"OutputFcn"},
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44 ## @qcode{"TolX"}, @qcode{"MaxIter"}, @qcode{"MaxFunEvals"}.
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45 ## For a description of these options, see @ref{XREFoptimset,,optimset}.
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46 ##
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47 ## On exit, the function returns @var{x}, the approximate zero point
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48 ## and @var{fval}, the function value thereof.
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49 ## @var{info} is an exit flag that can have these values:
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50 ##
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51 ## @itemize
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52 ## @item 1
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53 ## The algorithm converged to a solution.
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54 ##
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55 ## @item 0
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56 ## Maximum number of iterations or function evaluations has been reached.
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57 ##
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58 ## @item -1
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59 ## The algorithm has been terminated from user output function.
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60 ##
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61 ## @item -5
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62 ## The algorithm may have converged to a singular point.
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63 ## @end itemize
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64 ##
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65 ## @var{output} is a structure containing runtime information about the
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66 ## @code{fzero} algorithm. Fields in the structure are:
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67 ##
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68 ## @itemize
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69 ## @item iterations
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70 ## Number of iterations through loop.
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71 ##
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72 ## @item nfev
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73 ## Number of function evaluations.
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74 ##
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75 ## @item bracketx
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76 ## A two-element vector with the final bracketing of the zero along the x-axis.
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77 ##
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78 ## @item brackety
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79 ## A two-element vector with the final bracketing of the zero along the y-axis.
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80 ## @end itemize
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81 ## @seealso{optimset, fsolve}
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82 ## @end deftypefn
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83
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84 ## This is essentially the ACM algorithm 748: Enclosing Zeros of
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85 ## Continuous Functions due to Alefeld, Potra and Shi, ACM Transactions
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86 ## on Mathematical Software, Vol. 21, No. 3, September 1995. Although
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87 ## the workflow should be the same, the structure of the algorithm has
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88 ## been transformed non-trivially; instead of the authors' approach of
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89 ## sequentially calling building blocks subprograms we implement here a
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90 ## FSM version using one interior point determination and one bracketing
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91 ## per iteration, thus reducing the number of temporary variables and
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92 ## simplifying the algorithm structure. Further, this approach reduces
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93 ## the need for external functions and error handling. The algorithm has
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94 ## also been slightly modified.
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95
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96 ## PKG_ADD: ## Discard result to avoid polluting workspace with ans at startup.
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97 ## PKG_ADD: [~] = __all_opts__ ("fzero");
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98
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99 function [x, fval, info, output] = fzero (fun, x0, options = struct ())
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100
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101 ## Get default options if requested.
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102 if (nargin == 1 && ischar (fun) && strcmp (fun, 'defaults'))
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103 x = optimset ("MaxIter", Inf, "MaxFunEvals", Inf, "TolX", eps,
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104 "OutputFcn", [], "FunValCheck", "off");
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105 return;
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106 endif
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107
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108 if (nargin < 2 || nargin > 3)
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109 print_usage ();
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110 endif
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111
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112 if (ischar (fun))
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113 fun = str2func (fun, "global");
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114 endif
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115
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116 ## TODO
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117 ## displev = optimget (options, "Display", "notify");
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118 funvalchk = strcmpi (optimget (options, "FunValCheck", "off"), "on");
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119 outfcn = optimget (options, "OutputFcn");
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120 tolx = optimget (options, "TolX", eps);
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121 maxiter = optimget (options, "MaxIter", Inf);
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122 maxfev = optimget (options, "MaxFunEvals", Inf);
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123
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124 persistent mu = 0.5;
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125
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126 if (funvalchk)
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127 ## Replace fun with a guarded version.
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128 fun = @(x) guarded_eval (fun, x);
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129 endif
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130
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131 ## The default exit flag if exceeded number of iterations.
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132 info = 0;
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133 niter = 0;
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134 nfev = 0;
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135
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136 x = fval = a = fa = b = fb = NaN;
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137 eps = eps (class (x0));
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138
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139 ## Prepare...
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140 a = x0(1);
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141 fa = fun (a);
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142 nfev = 1;
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143 if (length (x0) > 1)
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144 b = x0(2);
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145 fb = fun (b);
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146 nfev += 1;
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147 else
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148 ## Try to get b.
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149 if (a == 0)
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150 aa = 1;
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151 else
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152 aa = a;
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153 endif
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154 for b = [0.9*aa, 1.1*aa, aa-1, aa+1, 0.5*aa 1.5*aa, -aa, 2*aa, -10*aa, 10*aa]
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155 fb = fun (b); nfev += 1;
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156 if (sign (fa) * sign (fb) <= 0)
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157 break;
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158 endif
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159 endfor
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160 endif
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161
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162 if (b < a)
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163 u = a;
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164 a = b;
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165 b = u;
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166
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167 fu = fa;
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168 fa = fb;
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169 fb = fu;
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170 endif
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171
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172 if (! (sign (fa) * sign (fb) <= 0))
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173 error ("fzero:bracket", "fzero: not a valid initial bracketing");
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174 endif
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175
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176 slope0 = (fb - fa) / (b - a);
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177
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178 if (fa == 0)
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179 b = a;
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180 fb = fa;
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181 elseif (fb == 0)
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182 a = b;
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183 fa = fb;
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184 endif
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185
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186 itype = 1;
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187
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188 if (abs (fa) < abs (fb))
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189 u = a; fu = fa;
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190 else
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191 u = b; fu = fb;
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192 endif
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193
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194 d = e = u;
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195 fd = fe = fu;
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196 mba = mu*(b - a);
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197 while (niter < maxiter && nfev < maxfev)
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198 switch (itype)
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199 case 1
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200 ## The initial test.
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201 if (b - a <= 2*(2 * abs (u) * eps + tolx))
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202 x = u; fval = fu;
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203 info = 1;
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204 break;
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205 endif
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206 if (abs (fa) <= 1e3*abs (fb) && abs (fb) <= 1e3*abs (fa))
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207 ## Secant step.
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208 c = u - (a - b) / (fa - fb) * fu;
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209 else
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210 ## Bisection step.
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211 c = 0.5*(a + b);
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212 endif
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213 d = u; fd = fu;
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214 itype = 5;
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215 case {2, 3}
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216 l = length (unique ([fa, fb, fd, fe]));
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217 if (l == 4)
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218 ## Inverse cubic interpolation.
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219 q11 = (d - e) * fd / (fe - fd);
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220 q21 = (b - d) * fb / (fd - fb);
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221 q31 = (a - b) * fa / (fb - fa);
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222 d21 = (b - d) * fd / (fd - fb);
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223 d31 = (a - b) * fb / (fb - fa);
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diff changeset
224 q22 = (d21 - q11) * fb / (fe - fb);
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225 q32 = (d31 - q21) * fa / (fd - fa);
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diff changeset
226 d32 = (d31 - q21) * fd / (fd - fa);
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diff changeset
227 q33 = (d32 - q22) * fa / (fe - fa);
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228 c = a + q31 + q32 + q33;
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229 endif
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diff changeset
230 if (l < 4 || sign (c - a) * sign (c - b) > 0)
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diff changeset
231 ## Quadratic interpolation + newton.
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232 a0 = fa;
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233 a1 = (fb - fa)/(b - a);
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234 a2 = ((fd - fb)/(d - b) - a1) / (d - a);
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diff changeset
235 ## Modification 1: this is simpler and does not seem to be worse.
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diff changeset
236 c = a - a0/a1;
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diff changeset
237 if (a2 != 0)
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238 c = a - a0/a1;
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diff changeset
239 for i = 1:itype
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diff changeset
240 pc = a0 + (a1 + a2*(c - b))*(c - a);
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diff changeset
241 pdc = a1 + a2*(2*c - a - b);
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Rik <rik@octave.org>
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diff changeset
242 if (pdc == 0)
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Rik <rik@octave.org>
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diff changeset
243 c = a - a0/a1;
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Rik <rik@octave.org>
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diff changeset
244 break;
c3c1ebfaa7dc maint: Use common indentation for switch statement.
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diff changeset
245 endif
c3c1ebfaa7dc maint: Use common indentation for switch statement.
Rik <rik@octave.org>
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diff changeset
246 c -= pc/pdc;
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diff changeset
247 endfor
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diff changeset
248 endif
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diff changeset
249 endif
c3c1ebfaa7dc maint: Use common indentation for switch statement.
Rik <rik@octave.org>
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diff changeset
250 itype += 1;
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diff changeset
251 case 4
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Rik <rik@octave.org>
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diff changeset
252 ## Double secant step.
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253 c = u - 2*(b - a)/(fb - fa)*fu;
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diff changeset
254 ## Bisect if too far.
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diff changeset
255 if (abs (c - u) > 0.5*(b - a))
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diff changeset
256 c = 0.5 * (b + a);
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diff changeset
257 endif
c3c1ebfaa7dc maint: Use common indentation for switch statement.
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diff changeset
258 itype = 5;
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diff changeset
259 case 5
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260 ## Bisection step.
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261 c = 0.5 * (b + a);
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diff changeset
262 itype = 2;
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diff changeset
263 endswitch
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264
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265 ## Don't let c come too close to a or b.
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266 delta = 2*0.7*(2 * abs (u) * eps + tolx);
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diff changeset
267 if ((b - a) <= 2*delta)
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diff changeset
268 c = (a + b)/2;
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diff changeset
269 else
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270 c = max (a + delta, min (b - delta, c));
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271 endif
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diff changeset
272
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diff changeset
273 ## Calculate new point.
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diff changeset
274 x = c;
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diff changeset
275 fval = fc = fun (c);
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276 niter ++; nfev ++;
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277
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278 ## Modification 2: skip inverse cubic interpolation if
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diff changeset
279 ## nonmonotonicity is detected.
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diff changeset
280 if (sign (fc - fa) * sign (fc - fb) >= 0)
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281 ## The new point broke monotonicity.
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282 ## Disable inverse cubic.
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diff changeset
283 fe = fc;
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parents:
diff changeset
284 else
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diff changeset
285 e = d; fe = fd;
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286 endif
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287
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288 ## Bracketing.
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289 if (sign (fa) * sign (fc) < 0)
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290 d = b; fd = fb;
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291 b = c; fb = fc;
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292 elseif (sign (fb) * sign (fc) < 0)
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293 d = a; fd = fa;
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294 a = c; fa = fc;
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295 elseif (fc == 0)
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296 a = b = c; fa = fb = fc;
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297 info = 1;
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298 break;
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299 else
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300 ## This should never happen.
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301 error ("fzero:bracket", "fzero: zero point is not bracketed");
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302 endif
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303
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304 ## If there's an output function, use it now.
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305 if (! isempty (outfcn))
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306 optv.funccount = nfev;
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307 optv.fval = fval;
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308 optv.iteration = niter;
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309 if (outfcn (x, optv, "iter"))
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310 info = -1;
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311 break;
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312 endif
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313 endif
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314
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315 if (abs (fa) < abs (fb))
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316 u = a; fu = fa;
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317 else
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318 u = b; fu = fb;
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319 endif
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320 if (b - a <= 2*(2 * abs (u) * eps + tolx))
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321 info = 1;
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322 break;
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323 endif
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324
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325 ## Skip bisection step if successful reduction.
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326 if (itype == 5 && (b - a) <= mba)
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327 itype = 2;
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328 endif
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329 if (itype == 2)
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330 mba = mu * (b - a);
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331 endif
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332 endwhile
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333
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334 ## Check solution for a singularity by examining slope
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335 if (info == 1)
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336 if ((b - a) != 0
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337 && abs ((fb - fa)/(b - a) / slope0) > max (1e6, 0.5/(eps+tolx)))
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338 info = -5;
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339 endif
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340 endif
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341
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342 output.iterations = niter;
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343 output.funcCount = nfev;
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344 output.bracketx = [a, b];
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345 output.brackety = [fa, fb];
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346
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347 endfunction
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348
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349 ## An assistant function that evaluates a function handle and checks for
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350 ## bad results.
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351 function fx = guarded_eval (fun, x)
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352 fx = fun (x);
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353 fx = fx(1);
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354 if (! isreal (fx))
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355 error ("fzero:notreal", "fzero: non-real value encountered");
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356 elseif (isnan (fx))
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357 error ("fzero:isnan", "fzero: NaN value encountered");
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358 endif
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359 endfunction
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360
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361
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362 %!shared opt0
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363 %! opt0 = optimset ("tolx", 0);
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364 %!assert (fzero (@cos, [0, 3], opt0), pi/2, 10*eps)
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365 %!assert (fzero (@(x) x^(1/3) - 1e-8, [0,1], opt0), 1e-24, 1e-22*eps)
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366