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  • Machine Learning Week 3-advanced-optimization

    costfunction代码如下

    function[jVal,gradient]=costFunction(theta)
    jVal=(theta(1)-5)^2+(theta(2)-5)^2
    gradient=zeros(2,1)
    gradient(1)=2*(theta(1)-5);
    gradient(2)=2*(theta(2)-5);
    

      运行如下代码段

    options=optimset('GradObj','on','MaxIter',100)
    initialTheta=zeros(2,1)
    [optTheta,functionVal,exitFlag]=fminunc(@costFunction,initialTheta,options)
    

      结果如下

     1 jVal =
     2 
     3     50
     4 
     5 
     6 gradient =
     7 
     8      0
     9      0
    10 
    11 
    12 jVal =
    13 
    14    50.0000
    15 
    16 
    17 gradient =
    18 
    19      0
    20      0
    21 
    22 
    23 jVal =
    24 
    25    50.0000
    26 
    27 
    28 gradient =
    29 
    30      0
    31      0
    32 
    33 
    34 jVal =
    35 
    36      0
    37 
    38 
    39 gradient =
    40 
    41      0
    42      0
    43 
    44 
    45 jVal =
    46 
    47    5.5511e-15
    48 
    49 
    50 gradient =
    51 
    52      0
    53      0
    54 
    55 
    56 jVal =
    57 
    58    5.5511e-15
    59 
    60 
    61 gradient =
    62 
    63      0
    64      0
    65 
    66 
    67 Local minimum found.
    68 
    69 Optimization completed because the size of the gradient is less than
    70 the default value of the optimality tolerance.
    71 
    72 <stopping criteria details>
    73 
    74 
    75 optTheta =
    76 
    77      5
    78      5
    79 
    80 
    81 functionVal =
    82 
    83      0
    84 
    85 
    86 exitFlag =
    87 
    88      1

    exitflag帮助你了解是否收敛

    fminunc()函数

    fminunc finds a local minimum of a function of several variables.
        X = fminunc(FUN,X0) starts at X0 and attempts to find a local minimizer
        X of the function FUN. FUN accepts input X and returns a scalar
        function value F evaluated at X. X0 can be a scalar, vector or matrix. 
     
        X = fminunc(FUN,X0,OPTIONS) minimizes with the default optimization
        parameters replaced by values in OPTIONS, an argument created with the
        OPTIMOPTIONS function.  See OPTIMOPTIONS for details. Use the
        SpecifyObjectiveGradient option to specify that FUN also returns a
        second output argument G that is the partial derivatives of the
        function df/dX, at the point X. Use the HessianFcn option to specify
        that FUN also returns a third output argument H that is the 2nd partial
        derivatives of the function (the Hessian) at the point X. The Hessian
        is only used by the trust-region algorithm.
     
        X = fminunc(PROBLEM) finds the minimum for PROBLEM. PROBLEM is a
        structure with the function FUN in PROBLEM.objective, the start point
        in PROBLEM.x0, the options structure in PROBLEM.options, and solver
        name 'fminunc' in PROBLEM.solver. Use this syntax to solve at the 
        command line a problem exported from OPTIMTOOL. 
     
        [X,FVAL] = fminunc(FUN,X0,...) returns the value of the objective 
        function FUN at the solution X.
     
        [X,FVAL,EXITFLAG] = fminunc(FUN,X0,...) returns an EXITFLAG that
        describes the exit condition. Possible values of EXITFLAG and the
        corresponding exit conditions are listed below. See the documentation
        for a complete description.
     
          1  Magnitude of gradient small enough. 
          2  Change in X too small.
          3  Change in objective function too small.
          5  Cannot decrease function along search direction.
          0  Too many function evaluations or iterations.
         -1  Stopped by output/plot function.
         -3  Problem seems unbounded. 
        
        [X,FVAL,EXITFLAG,OUTPUT] = fminunc(FUN,X0,...) returns a structure 
        OUTPUT with the number of iterations taken in OUTPUT.iterations, the 
        number of function evaluations in OUTPUT.funcCount, the algorithm used 
        in OUTPUT.algorithm, the number of CG iterations (if used) in
        OUTPUT.cgiterations, the first-order optimality (if used) in
        OUTPUT.firstorderopt, and the exit message in OUTPUT.message.
     
        [X,FVAL,EXITFLAG,OUTPUT,GRAD] = fminunc(FUN,X0,...) returns the value 
        of the gradient of FUN at the solution X.
     
        [X,FVAL,EXITFLAG,OUTPUT,GRAD,HESSIAN] = fminunc(FUN,X0,...) returns the 
        value of the Hessian of the objective function FUN at the solution X.
     
        Examples
          FUN can be specified using @:
             X = fminunc(@myfun,2)
     
        where myfun is a MATLAB function such as:
     
            function F = myfun(x)
            F = sin(x) + 3;
     
          To minimize this function with the gradient provided, modify
          the function myfun so the gradient is the second output argument:
             function [f,g] = myfun(x)
              f = sin(x) + 3;
              g = cos(x);
          and indicate the gradient value is available by creating options with
          OPTIONS.SpecifyObjectiveGradient set to true (using OPTIMOPTIONS):
             options = optimoptions('fminunc','SpecifyObjectiveGradient',true);
             x = fminunc(@myfun,4,options);
     
          FUN can also be an anonymous function:
             x = fminunc(@(x) 5*x(1)^2 + x(2)^2,[5;1])
     
        If FUN is parameterized, you can use anonymous functions to capture the
        problem-dependent parameters. Suppose you want to minimize the 
        objective given in the function myfun, which is parameterized by its 
        second argument c. Here myfun is a MATLAB file function such as
     
          function [f,g] = myfun(x,c)
     
          f = c*x(1)^2 + 2*x(1)*x(2) + x(2)^2; % function
          g = [2*c*x(1) + 2*x(2)               % gradient
               2*x(1) + 2*x(2)];
     
        To optimize for a specific value of c, first assign the value to c. 
        Then create a one-argument anonymous function that captures that value 
        of c and calls myfun with two arguments. Finally, pass this anonymous 
        function to fminunc:
     
          c = 3;                              % define parameter first
          options = optimoptions('fminunc','SpecifyObjectiveGradient',true); % indicate gradient is provided 
          x = fminunc(@(x) myfun(x,c),[1;1],options)
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  • 原文地址:https://www.cnblogs.com/cswangchen/p/8528970.html
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