Mercurial > hg > octave-nkf
annotate scripts/optimization/fzero.m @ 12632:2dbac27e0e40 stable
doc: miscellaneous touch-ups to documentation strings
* debug.txi: Correct line number for debug example.
* func.txi: Correct directory structure of functions shipped with Octave distribution.
* edit.m: Eliminate reference to editor used by bug_report.
* fzero.m: Add inline function to list of possible inputs.
* fplot.m: Add comma to break list of possible inputs.
* qqplot.m: Correct example so that it runs under current Octave distributions.
* assert.m: Add seealso reference to fail.
* fail.m: Add seealso reference to assert. Add additional calling forms of function.
* load-path.cc (addpath): Don't use @samp which created double double quotes around option.
* ov-fcn-inline.cc (vectorize): Add example usage.
* variables.cc (exist): Add seealso link to file_in_loadpath.
| author | Rik <octave@nomad.inbox5.com> |
|---|---|
| date | Mon, 25 Apr 2011 09:24:33 -0700 |
| parents | c792872f8942 |
| children | b9a89ca0fb75 |
| rev | line source |
|---|---|
| 11523 | 1 ## Copyright (C) 2008-2011 VZLU Prague, a.s. |
| 8305 | 2 ## |
| 3 ## This file is part of Octave. | |
| 4 ## | |
| 5 ## Octave is free software; you can redistribute it and/or modify it | |
| 6 ## under the terms of the GNU General Public License as published by | |
| 7 ## the Free Software Foundation; either version 3 of the License, or (at | |
| 8 ## your option) any later version. | |
| 9 ## | |
| 10 ## Octave is distributed in the hope that it will be useful, but | |
| 11 ## WITHOUT ANY WARRANTY; without even the implied warranty of | |
| 12 ## MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU | |
| 13 ## General Public License for more details. | |
| 14 ## | |
| 15 ## You should have received a copy of the GNU General Public License | |
| 16 ## along with Octave; see the file COPYING. If not, see | |
| 17 ## <http://www.gnu.org/licenses/>. | |
| 18 ## | |
| 19 ## Author: Jaroslav Hajek <highegg@gmail.com> | |
| 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 ## |
| 10470 | 34 ## @example |
| 35 ## sign (@var{fun}(@var{x0}(1))) * sign (@var{fun}(@var{x0}(2))) <= 0 | |
| 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 |
| 10470 | 40 ## is not successful, the function fails. |
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41 ## @var{options} is a structure specifying additional options. |
| 10470 | 42 ## Currently, @code{fzero} |
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43 ## recognizes these options: @code{"FunValCheck"}, @code{"OutputFcn"}, |
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44 ## @code{"TolX"}, @code{"MaxIter"}, @code{"MaxFunEvals"}. |
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45 ## For a description of these options, see @ref{doc-optimset,,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 |
| 8305 | 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: __all_opts__ ("fzero"); |
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97 |
| 8305 | 98 function [x, fval, info, output] = fzero (fun, x0, options = struct ()) |
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99 |
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100 ## Get default options if requested. |
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101 if (nargin == 1 && ischar (fun) && strcmp (fun, 'defaults')) |
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103 "OutputFcn", [], "FunValCheck", "off"); |
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104 return; |
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105 endif |
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106 |
| 8305 | 107 if (nargin < 2 || nargin > 3) |
| 108 print_usage (); | |
| 109 endif | |
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110 |
| 8305 | 111 if (ischar (fun)) |
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112 fun = str2func (fun, "global"); |
| 8305 | 113 endif |
| 114 | |
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115 ## TODO |
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116 ## displev = optimget (options, "Display", "notify"); |
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117 funvalchk = strcmpi (optimget (options, "FunValCheck", "off"), "on"); |
| 8305 | 118 outfcn = optimget (options, "OutputFcn"); |
| 10296 | 119 tolx = optimget (options, "TolX", 1e-8); |
| 8305 | 120 maxiter = optimget (options, "MaxIter", Inf); |
| 121 maxfev = optimget (options, "MaxFunEvals", Inf); | |
| 122 | |
| 123 persistent mu = 0.5; | |
| 124 | |
| 125 if (funvalchk) | |
| 8507 | 126 ## Replace fun with a guarded version. |
| 8305 | 127 fun = @(x) guarded_eval (fun, x); |
| 128 endif | |
| 129 | |
| 8507 | 130 ## The default exit flag if exceeded number of iterations. |
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131 info = 0; |
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132 niter = 0; |
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133 nfev = 0; |
| 8305 | 134 |
| 135 x = fval = a = fa = b = fb = NaN; | |
| 10296 | 136 eps = eps (class (x0)); |
| 8305 | 137 |
| 8507 | 138 ## Prepare... |
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139 a = x0(1); |
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140 fa = fun (a); |
| 8305 | 141 nfev = 1; |
| 142 if (length (x0) > 1) | |
| 143 b = x0(2); | |
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144 fb = fun (b); |
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145 nfev += 1; |
| 8305 | 146 else |
| 8507 | 147 ## Try to get b. |
| 8305 | 148 if (a == 0) |
| 149 aa = 1; | |
| 150 else | |
| 151 aa = a; | |
| 152 endif | |
| 153 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] | |
| 154 fb = fun (b); nfev += 1; | |
| 155 if (sign (fa) * sign (fb) <= 0) | |
| 156 break; | |
| 157 endif | |
| 158 endfor | |
| 159 endif | |
| 160 | |
| 161 if (b < a) | |
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162 u = a; |
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163 a = b; |
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164 b = u; |
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165 |
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166 fu = fa; |
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167 fa = fb; |
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168 fb = fu; |
| 8305 | 169 endif |
| 170 | |
| 171 if (! (sign (fa) * sign (fb) <= 0)) | |
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172 error ("fzero:bracket", "fzero: not a valid initial bracketing"); |
| 8305 | 173 endif |
| 174 | |
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175 slope0 = (fb - fa) / (b - a); |
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176 |
| 10470 | 177 if (fa == 0) |
| 178 b = a; | |
| 179 fb = fa; | |
| 180 elseif (fb == 0) | |
| 181 a = b; | |
| 182 fa = fb; | |
| 183 endif | |
| 184 | |
| 8305 | 185 itype = 1; |
| 186 | |
| 187 if (abs (fa) < abs (fb)) | |
| 188 u = a; fu = fa; | |
| 189 else | |
| 190 u = b; fu = fb; | |
| 191 endif | |
| 192 | |
| 193 d = e = u; | |
| 194 fd = fe = fu; | |
| 195 mba = mu*(b - a); | |
| 196 while (niter < maxiter && nfev < maxfev) | |
| 197 switch (itype) | |
| 198 case 1 | |
| 8507 | 199 ## The initial test. |
| 8305 | 200 if (b - a <= 2*(2 * abs (u) * eps + tolx)) |
| 201 x = u; fval = fu; | |
| 202 info = 1; | |
| 203 break; | |
| 204 endif | |
| 205 if (abs (fa) <= 1e3*abs (fb) && abs (fb) <= 1e3*abs (fa)) | |
| 8507 | 206 ## Secant step. |
| 8305 | 207 c = u - (a - b) / (fa - fb) * fu; |
| 208 else | |
| 8507 | 209 ## Bisection step. |
| 8305 | 210 c = 0.5*(a + b); |
| 211 endif | |
| 212 d = u; fd = fu; | |
| 213 itype = 5; | |
| 214 case {2, 3} | |
| 215 l = length (unique ([fa, fb, fd, fe])); | |
| 216 if (l == 4) | |
| 8507 | 217 ## Inverse cubic interpolation. |
| 8305 | 218 q11 = (d - e) * fd / (fe - fd); |
| 219 q21 = (b - d) * fb / (fd - fb); | |
| 220 q31 = (a - b) * fa / (fb - fa); | |
| 221 d21 = (b - d) * fd / (fd - fb); | |
| 222 d31 = (a - b) * fb / (fb - fa); | |
| 223 q22 = (d21 - q11) * fb / (fe - fb); | |
| 224 q32 = (d31 - q21) * fa / (fd - fa); | |
| 225 d32 = (d31 - q21) * fd / (fd - fa); | |
| 226 q33 = (d32 - q22) * fa / (fe - fa); | |
| 227 c = a + q31 + q32 + q33; | |
| 228 endif | |
| 229 if (l < 4 || sign (c - a) * sign (c - b) > 0) | |
| 8507 | 230 ## Quadratic interpolation + newton. |
| 8305 | 231 a0 = fa; |
| 232 a1 = (fb - fa)/(b - a); | |
| 233 a2 = ((fd - fb)/(d - b) - a1) / (d - a); | |
| 8507 | 234 ## Modification 1: this is simpler and does not seem to be worse. |
| 8305 | 235 c = a - a0/a1; |
| 236 if (a2 != 0) | |
| 237 c = a - a0/a1; | |
| 238 for i = 1:itype | |
| 239 pc = a0 + (a1 + a2*(c - b))*(c - a); | |
| 240 pdc = a1 + a2*(2*c - a - b); | |
| 241 if (pdc == 0) | |
| 242 c = a - a0/a1; | |
| 243 break; | |
| 244 endif | |
| 245 c -= pc/pdc; | |
| 246 endfor | |
| 247 endif | |
| 248 endif | |
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249 itype += 1; |
| 8305 | 250 case 4 |
| 8507 | 251 ## Double secant step. |
| 8305 | 252 c = u - 2*(b - a)/(fb - fa)*fu; |
| 8507 | 253 ## Bisect if too far. |
| 8305 | 254 if (abs (c - u) > 0.5*(b - a)) |
| 255 c = 0.5 * (b + a); | |
| 256 endif | |
| 257 itype = 5; | |
| 258 case 5 | |
| 8507 | 259 ## Bisection step. |
| 8305 | 260 c = 0.5 * (b + a); |
| 261 itype = 2; | |
| 262 endswitch | |
| 263 | |
| 8507 | 264 ## Don't let c come too close to a or b. |
| 8305 | 265 delta = 2*0.7*(2 * abs (u) * eps + tolx); |
| 266 if ((b - a) <= 2*delta) | |
| 267 c = (a + b)/2; | |
| 268 else | |
| 269 c = max (a + delta, min (b - delta, c)); | |
| 270 endif | |
| 271 | |
| 8507 | 272 ## Calculate new point. |
| 8305 | 273 x = c; |
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274 fval = fc = fun (c); |
| 8305 | 275 niter ++; nfev ++; |
| 276 | |
| 8507 | 277 ## Modification 2: skip inverse cubic interpolation if |
| 278 ## nonmonotonicity is detected. | |
| 8305 | 279 if (sign (fc - fa) * sign (fc - fb) >= 0) |
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280 ## The new point broke monotonicity. |
| 8507 | 281 ## Disable inverse cubic. |
| 8305 | 282 fe = fc; |
| 283 else | |
| 284 e = d; fe = fd; | |
| 285 endif | |
| 286 | |
| 8507 | 287 ## Bracketing. |
| 8305 | 288 if (sign (fa) * sign (fc) < 0) |
| 289 d = b; fd = fb; | |
| 290 b = c; fb = fc; | |
| 291 elseif (sign (fb) * sign (fc) < 0) | |
| 292 d = a; fd = fa; | |
| 293 a = c; fa = fc; | |
| 294 elseif (fc == 0) | |
| 295 a = b = c; fa = fb = fc; | |
| 296 info = 1; | |
| 297 break; | |
| 298 else | |
| 8507 | 299 ## This should never happen. |
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300 error ("fzero:bracket", "fzero: zero point is not bracketed"); |
| 8305 | 301 endif |
| 302 | |
| 8507 | 303 ## If there's an output function, use it now. |
| 8305 | 304 if (outfcn) |
| 10296 | 305 optv.funccount = nfev; |
| 8305 | 306 optv.fval = fval; |
| 307 optv.iteration = niter; | |
| 308 if (outfcn (x, optv, "iter")) | |
| 309 info = -1; | |
| 310 break; | |
| 311 endif | |
| 312 endif | |
| 313 | |
| 314 if (abs (fa) < abs (fb)) | |
| 315 u = a; fu = fa; | |
| 316 else | |
| 317 u = b; fu = fb; | |
| 318 endif | |
| 319 if (b - a <= 2*(2 * abs (u) * eps + tolx)) | |
| 320 info = 1; | |
| 321 break; | |
| 322 endif | |
| 323 | |
| 8507 | 324 ## Skip bisection step if successful reduction. |
| 8305 | 325 if (itype == 5 && (b - a) <= mba) |
| 326 itype = 2; | |
| 327 endif | |
| 328 if (itype == 2) | |
| 329 mba = mu * (b - a); | |
| 330 endif | |
| 331 endwhile | |
| 332 | |
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333 ## Check solution for a singularity by examining slope |
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334 if (info == 1) |
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335 if ((b - a) != 0 && abs ((fb - fa)/(b - a) / slope0) > max (1e6, 0.5/(eps+tolx))) |
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336 info = -5; |
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337 endif |
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338 endif |
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339 |
| 8305 | 340 output.iterations = niter; |
| 10296 | 341 output.funcCount = nfev; |
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342 output.bracketx = [a, b]; |
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343 output.brackety = [fa, fb]; |
| 8305 | 344 |
| 345 endfunction | |
| 346 | |
| 8507 | 347 ## An assistant function that evaluates a function handle and checks for |
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348 ## bad results. |
| 8305 | 349 function fx = guarded_eval (fun, x) |
| 350 fx = fun (x); | |
| 351 fx = fx(1); | |
| 352 if (! isreal (fx)) | |
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353 error ("fzero:notreal", "fzero: non-real value encountered"); |
| 8305 | 354 elseif (isnan (fx)) |
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355 error ("fzero:isnan", "fzero: NaN value encountered"); |
| 8305 | 356 endif |
| 357 endfunction | |
| 358 | |
| 10296 | 359 %!shared opt0 |
| 360 %! opt0 = optimset ("tolx", 0); | |
| 361 %!assert(fzero(@cos, [0, 3], opt0), pi/2, 10*eps) | |
| 362 %!assert(fzero(@(x) x^(1/3) - 1e-8, [0,1], opt0), 1e-24, 1e-22*eps) |