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%% Optimization of a simple (Rosenbrock) function, with no constraints
rosen = @(x) (1-x(1)).^2 + 105*(x(2)-x(1).^2).^2;
% With no constraints, operation simply passes through
% directly to fminsearch. The solution should be [1 1]
xsol = fminsearchbnd(rosen,[3 3])
%% Only lower bound constraints
xsol = fminsearchbnd(rosen,[3 3],[2 2])
%% Only upper bound constraints
xsol = fminsearchbnd(rosen,[-5 -5],[],[0 0])
%% Dual constraints
xsol = fminsearchbnd(rosen,[2.5 2.5],[2 2],[3 3])
%% Mixed constraints
xsol = fminsearchbnd(rosen,[0 0],[2 -inf],[inf 3])
%% Provide your own fminsearch options
opts = optimset('fminsearch');
opts.Display = 'iter';
opts.TolX = 1.e-12;
opts.MaxFunEvals = 100;
n = [10,5];
H = randn(n);
H=H'*H;
Quadraticfun = @(x) x*H*x';
% Global minimizer is at [0 0 0 0 0].
% Set all lower bound constraints, all of which will
% be active in this test.
LB = [.5 .5 .5 .5 .5];
xsol = fminsearchbnd(Quadraticfun,[1 2 3 4 5],LB,[],opts)
%% Exactly fix one variable, constrain some others, and set a tolerance
opts = optimset('fminsearch');
opts.TolFun = 1.e-12;
LB = [-inf 2 1 -10];
UB = [ inf inf 1 inf];
xsol = fminsearchbnd(@(x) norm(x),[1 3 1 1],LB,UB,opts)
%% All the standard outputs from fminsearch are still returned
[xsol,fval,exitflag,output] = fminsearchbnd(@(x) norm(x),[1 3 1 1],LB,UB)

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<html xmlns:mwsh="http://www.mathworks.com/namespace/mcode/v1/syntaxhighlight.dtd">
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<title>fminsearchbnd_demo</title>
<meta name="generator" content="MATLAB 7.0.1">
<meta name="date" content="2006-07-24">
<meta name="m-file" content="fminsearchbnd_demo"><style>
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<body>
<h2>Contents</h2>
<div>
<ul>
<li><a href="#1">Optimization of a simple (Rosenbrock) function, with no constraints</a></li>
<li><a href="#2">Only lower bound constraints</a></li>
<li><a href="#3">Only upper bound constraints</a></li>
<li><a href="#4">Dual constraints</a></li>
<li><a href="#5">Mixed constraints</a></li>
<li><a href="#6">Provide your own fminsearch options</a></li>
<li><a href="#7">Exactly fix one variable, constrain some others, and set a tolerance</a></li>
<li><a href="#8">All the standard outputs from fminsearch are still returned</a></li>
</ul>
</div>
<h2>Optimization of a simple (Rosenbrock) function, with no constraints<a name="1"></a></h2><pre class="codeinput">rosen = @(x) (1-x(1)).^2 + 105*(x(2)-x(1).^2).^2;
<span class="comment">% With no constraints, operation simply passes through</span>
<span class="comment">% directly to fminsearch. The solution should be [1 1]</span>
xsol = fminsearchbnd(rosen,[3 3])
</pre><pre class="codeoutput">
xsol =
0.99998 0.99995
</pre><h2>Only lower bound constraints<a name="2"></a></h2><pre class="codeinput">xsol = fminsearchbnd(rosen,[3 3],[2 2])
</pre><pre class="codeoutput">
xsol =
2 4
</pre><h2>Only upper bound constraints<a name="3"></a></h2><pre class="codeinput">xsol = fminsearchbnd(rosen,[-5 -5],[],[0 0])
</pre><pre class="codeoutput">
xsol =
-1.0447e-13 -1.4451e-08
</pre><h2>Dual constraints<a name="4"></a></h2><pre class="codeinput">xsol = fminsearchbnd(rosen,[2.5 2.5],[2 2],[3 3])
</pre><pre class="codeoutput">
xsol =
2 3
</pre><h2>Mixed constraints<a name="5"></a></h2><pre class="codeinput">xsol = fminsearchbnd(rosen,[0 0],[2 -inf],[inf 3])
</pre><pre class="codeoutput">
xsol =
2 3
</pre><h2>Provide your own fminsearch options<a name="6"></a></h2><pre class="codeinput">opts = optimset(<span class="string">'fminsearch'</span>);
opts.Display = <span class="string">'iter'</span>;
opts.TolX = 1.e-12;
opts.MaxFunEvals = 100;
n = [10,5];
H = randn(n);
H=H'*H;
Quadraticfun = @(x) x*H*x';
<span class="comment">% Global minimizer is at [0 0 0 0 0].</span>
<span class="comment">% Set all lower bound constraints, all of which will</span>
<span class="comment">% be active in this test.</span>
LB = [.5 .5 .5 .5 .5];
xsol = fminsearchbnd(Quadraticfun,[1 2 3 4 5],LB,[],opts)
</pre><pre class="codeoutput">
Iteration Func-count min f(x) Procedure
0 1 173.731
1 6 172.028 initial simplex
2 8 162.698 expand
3 9 162.698 reflect
4 11 151.902 expand
5 13 138.235 expand
6 14 138.235 reflect
7 16 126.604 expand
8 17 126.604 reflect
9 19 97.3266 expand
10 20 97.3266 reflect
11 21 97.3266 reflect
12 22 97.3266 reflect
13 24 73.7178 expand
14 25 73.7178 reflect
15 26 73.7178 reflect
16 28 50.8236 expand
17 29 50.8236 reflect
18 31 41.6294 expand
19 33 30.4252 expand
20 34 30.4252 reflect
21 36 27.782 reflect
22 37 27.782 reflect
23 39 27.782 contract inside
24 41 22.6509 reflect
25 42 22.6509 reflect
26 43 22.6509 reflect
27 44 22.6509 reflect
28 45 22.6509 reflect
29 47 21.0211 reflect
30 48 21.0211 reflect
31 49 21.0211 reflect
32 51 21.0211 contract inside
33 52 21.0211 reflect
34 54 20.7613 contract inside
35 55 20.7613 reflect
36 56 20.7613 reflect
37 57 20.7613 reflect
38 59 20.6012 contract inside
39 61 20.5324 contract inside
40 63 20.4961 contract inside
41 65 20.3886 contract inside
42 67 20.2121 reflect
43 69 20.0876 contract inside
44 71 19.9164 reflect
45 72 19.9164 reflect
46 74 19.9164 contract inside
47 76 19.9164 contract outside
48 78 19.3349 expand
49 80 19.3349 contract inside
50 81 19.3349 reflect
51 82 19.3349 reflect
52 84 18.8721 expand
53 85 18.8721 reflect
54 87 18.6427 expand
55 89 17.4548 expand
56 90 17.4548 reflect
57 92 16.0113 expand
58 93 16.0113 reflect
59 94 16.0113 reflect
60 96 14.6134 expand
61 98 12.5445 expand
62 99 12.5445 reflect
63 101 10.7311 expand
Exiting: Maximum number of function evaluations has been exceeded
- increase MaxFunEvals option.
Current function value: 10.731146
xsol =
1.7022 1.0787 1.2034 0.5006 0.64666
</pre><h2>Exactly fix one variable, constrain some others, and set a tolerance<a name="7"></a></h2><pre class="codeinput">opts = optimset(<span class="string">'fminsearch'</span>);
opts.TolFun = 1.e-12;
LB = [-inf 2 1 -10];
UB = [ inf inf 1 inf];
xsol = fminsearchbnd(@(x) norm(x),[1 3 1 1],LB,UB,opts)
</pre><pre class="codeoutput">
xsol =
-4.9034e-07 2 1 5.1394e-07
</pre><h2>All the standard outputs from fminsearch are still returned<a name="8"></a></h2><pre class="codeinput">[xsol,fval,exitflag,output] = fminsearchbnd(@(x) norm(x),[1 3 1 1],LB,UB)
</pre><pre class="codeoutput">
xsol =
3.1094e-05 2 1 -5.1706e-05
fval =
2.2361
exitflag =
1
output =
iterations: 77
funcCount: 138
algorithm: 'Nelder-Mead simplex direct search'
message: [1x194 char]
</pre><p class="footer"><br>
Published with MATLAB&reg; 7.0.1<br></p>
<!--
##### SOURCE BEGIN #####
%% Optimization of a simple (Rosenbrock) function, with no constraints
rosen = @(x) (1-x(1)).^2 + 105*(x(2)-x(1).^2).^2;
% With no constraints, operation simply passes through
% directly to fminsearch. The solution should be [1 1]
xsol = fminsearchbnd(rosen,[3 3])
%% Only lower bound constraints
xsol = fminsearchbnd(rosen,[3 3],[2 2])
%% Only upper bound constraints
xsol = fminsearchbnd(rosen,[-5 -5],[],[0 0])
%% Dual constraints
xsol = fminsearchbnd(rosen,[2.5 2.5],[2 2],[3 3])
%% Mixed constraints
xsol = fminsearchbnd(rosen,[0 0],[2 -inf],[inf 3])
%% Provide your own fminsearch options
opts = optimset('fminsearch');
opts.Display = 'iter';
opts.TolX = 1.e-12;
opts.MaxFunEvals = 100;
n = [10,5];
H = randn(n);
H=H'*H;
Quadraticfun = @(x) x*H*x';
% Global minimizer is at [0 0 0 0 0].
% Set all lower bound constraints, all of which will
% be active in this test.
LB = [.5 .5 .5 .5 .5];
xsol = fminsearchbnd(Quadraticfun,[1 2 3 4 5],LB,[],opts)
%% Exactly fix one variable, constrain some others, and set a tolerance
opts = optimset('fminsearch');
opts.TolFun = 1.e-12;
LB = [-inf 2 1 -10];
UB = [ inf inf 1 inf];
xsol = fminsearchbnd(@(x) norm(x),[1 3 1 1],LB,UB,opts)
%% All the standard outputs from fminsearch are still returned
[xsol,fval,exitflag,output] = fminsearchbnd(@(x) norm(x),[1 3 1 1],LB,UB)
##### SOURCE END #####
-->
</body>
</html>

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\f1\b Understanding fminsearchbnd\
\
John D'Errico\
woodchips@rochester.rr.com
\f0\b0 \
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\cf0 \
Fminsearchbnd is really quite simple in concept. I've implemented lower and upper bound constraints by the careful use of transformations of the variables. In turn, the optimization engine is itself fminsearch itself.\
\
Since fminsearch does not allow bound constraints, the trick is to insert a wrapper function around the user supplied objective function. I've done all the work, so all you, as the user, needs to do is supply a set of bounds. All the other arguments are identical to that which fminsearch would have expected. \
\
There are several classes of bound constraints one might consider. Simple lower bound constraints:\
\
LB(i) <= x(i)\
\
Upper bound constraints:\
\
x(i) <= UB(i)\
\
Dual constraints\
\
LB(i) <= x(i) <= UB(i)\
\
Two other classes that I allow are fully unbounded variables, and a dual constraint where the lower and upper bounds are identical. In essence, this last class of constraint fixes the variable at the given level. Of course, internally in fminsearchbnd, I just completely remove that variable from the optimization, so fminsearch never sees the variable at all.\
\
The bounded variables are transformed such that fminsearch itself sees a fully unconstrained problem. For example, in the case of a variable bounded on the lower end by LB(i), I use the transformation\
\
x(i) = LB(i) + z(i)^2\
\
The variable z(i) is fully unconstrained, but since the square of z(i) is always non-negative (for real z), then x(i) must necessarily be always greater than or equal to LB(i). Likewise, a pure upper bound constraint is implemented as\
\
x(i) = UB(i) - z(i)^2\
\
Clearly, x(i) in this case can never rise above UB(i). And finally, the dual bounded variable is handled by a trigonometric transformation,\
\
x(i) = LB(i) + (UB(i) - LB(i))*(sin(z(i))+1)/2\
\
In this last case, I do absolutely enforce the requirement that LB(i) <= x(i) <= UB(i), since the vagaries of floating point arithmetic might sometimes cause those bounds to be subtly exceeded.\
\
\
\f1\b Multiple solutions due to the transformations\
\f0\b0 \
An artifact of the transformations used is the creation of multiple solutions to a problem that at one time may well have had a unique solution. While the presence of multiple local solutions is often a problem for an optimizer, each of these introduced solutions are fully equivalent. It matters not in the least which one is found.\
\
\
\f1\b Alternative choices for the transformations\
\f0\b0 \
I have occasionally seen it suggested that one use a sin(z)^2 transformation instead of the chosen form based on sin(z). My own feeling is that either could be used to roughly equal advantage, but that the sin^2 transformation may be slightly more nonlinear, causing subtly more problems in terms of floating point arithmetic. Similarly, I've tried other one sided and two sided transformations. A two sided transformation that I found to be of interest utilized atan(z). My testing showed that it was often more slowly convergent near the bounds, taking more iterations to converge. The atan transformation was a good choice as a way to implement exclusive bounds (see below.)\
\
\
\f1\b Inclusive versus exclusive bounds\
\f0\b0 \
A feature of the bound constraints that I have chosen to implement in fminsearchbnd is that they are inclusive bounds. That is, these constraints allow the boundary value itself to be achieved. Exclusive constraints would correspond to the strict inequalities, < and >. Why is this difference between inclusive and exclusive bound constraints an issue? As I said, fminsearchbnd allows its bounds to be fully achieved. So if your objective function includes an evaluation of log(x), where x is constrained to be greater than or equal to zero, then Matlab will generate a singularity.\
\
>> log(0)\
Warning: Log of zero.\
\
ans =\
-Inf\
\
An exclusive bound at zero would have prevented such an error. I've chosen not to implement them in that form however, as the necessary transformations tend to be somewhat more intractable, converging with less rapidity in practice. Also, the interface would have been more complex had I allowed the user to specify the actual boundary type for each constraint.\
\
All of this means that if you really need exclusive bounds, then you must offset your bound limits by a small amount.\
\
\
\f1\b Starting values, infeasible starting values, tolerances, etc.\
\f0\b0 \
The transformations chosen for fminsearchbnd are all simply invertible. This allows the user supplied starting values for each variable (prior to any transformation) to be simply mapped back to a corresponding value. Infeasible starting values are simply resolved for the bound constrained problem by simply clipping to the bounded domain.\
\
A more difficult issue is the question of tolerances on the parameters, that is, TolX. The nonlinear transformations mean that fminsearch itself will see only the transformed parameters, not the parameters in their real domain. As I've implemented fminsearchbnd as an overlay to fminsearch itself, there is no simple way to provide explicit control over the variable tolerances without re-writing fminsearch. \
\
\
\f1\b Limitations of fminsearchbnd\
\f0\b0 \
What does fminsearchbnd NOT do? You cannot provide general linear/nonlinear equality or inequality constraints, as are provided by fmincon, or lsqlin. Only simple bound constraints are allowed.\
\
\
}

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function [x,fval,exitflag,output]=fminsearchbnd3(fun,x0,LB,UB,options,varargin)
% FMINSEARCHBND: FMINSEARCH, but with bound constraints by transformation
% usage: x=FMINSEARCHBND(fun,x0)
% usage: x=FMINSEARCHBND(fun,x0,LB)
% usage: x=FMINSEARCHBND(fun,x0,LB,UB)
% usage: x=FMINSEARCHBND(fun,x0,LB,UB,options)
% usage: x=FMINSEARCHBND(fun,x0,LB,UB,options,p1,p2,...)
% usage: [x,fval,exitflag,output]=FMINSEARCHBND(fun,x0,...)
%
% arguments:
% fun, x0, options - see the help for FMINSEARCH
%
% LB - lower bound vector or array, must be the same size as x0
%
% If no lower bounds exist for one of the variables, then
% supply -inf for that variable.
%
% If no lower bounds at all, then LB may be left empty.
%
% Variables may be fixed in value by setting the corresponding
% lower and upper bounds to exactly the same value.
%
% UB - upper bound vector or array, must be the same size as x0
%
% If no upper bounds exist for one of the variables, then
% supply +inf for that variable.
%
% If no upper bounds at all, then UB may be left empty.
%
% Variables may be fixed in value by setting the corresponding
% lower and upper bounds to exactly the same value.
%
% Notes:
%
% If options is supplied, then TolX will apply to the transformed
% variables. All other FMINSEARCH parameters should be unaffected.
%
% Variables which are constrained by both a lower and an upper
% bound will use a sin transformation. Those constrained by
% only a lower or an upper bound will use a quadratic
% transformation, and unconstrained variables will be left alone.
%
% Variables may be fixed by setting their respective bounds equal.
% In this case, the problem will be reduced in size for FMINSEARCH.
%
% The bounds are inclusive inequalities, which admit the
% boundary values themselves, but will not permit ANY function
% evaluations outside the bounds. These constraints are strictly
% followed.
%
% If your problem has an EXCLUSIVE (strict) constraint which will
% not admit evaluation at the bound itself, then you must provide
% a slightly offset bound. An example of this is a function which
% contains the log of one of its parameters. If you constrain the
% variable to have a lower bound of zero, then FMINSEARCHBND may
% try to evaluate the function exactly at zero.
%
%
% Example usage:
% rosen = @(x) (1-x(1)).^2 + 105*(x(2)-x(1).^2).^2;
%
% fminsearch(rosen,[3 3]) % unconstrained
% ans =
% 1.0000 1.0000
%
% fminsearchbnd(rosen,[3 3],[2 2],[]) % constrained
% ans =
% 2.0000 4.0000
%
% See test_main.m for other examples of use.
%
%
% See also: fminsearch, fminspleas
%
%
% Author: John D'Errico
% E-mail: woodchips@rochester.rr.com
% Release: 4
% Release date: 7/23/06
% size checks
xsize = size(x0);
x0 = x0(:);
n=length(x0);
if (nargin<3) || isempty(LB)
LB = repmat(-inf,n,1);
else
LB = LB(:);
end
if (nargin<4) || isempty(UB)
UB = repmat(inf,n,1);
else
UB = UB(:);
end
if (n~=length(LB)) || (n~=length(UB))
error 'x0 is incompatible in size with either LB or UB.'
end
% set default options if necessary
if (nargin<5) || isempty(options)
options = optimset('fminsearch');
end
% stuff into a struct to pass around
params.args = varargin;
params.LB = LB;
params.UB = UB;
params.fun = fun;
params.n = n;
params.OutputFcn = [];
% 0 --> unconstrained variable
% 1 --> lower bound only
% 2 --> upper bound only
% 3 --> dual finite bounds
% 4 --> fixed variable
params.BoundClass = zeros(n,1);
for i=1:n
k = isfinite(LB(i)) + 2*isfinite(UB(i));
params.BoundClass(i) = k;
if (k==3) && (LB(i)==UB(i))
params.BoundClass(i) = 4;
end
end
% transform starting values into their unconstrained
% surrogates. Check for infeasible starting guesses.
x0u = x0;
k=1;
for i = 1:n
switch params.BoundClass(i)
case 1
% lower bound only
if x0(i)<=LB(i)
% infeasible starting value. Use bound.
x0u(k) = 0;
else
x0u(k) = sqrt(x0(i) - LB(i));
end
% increment k
k=k+1;
case 2
% upper bound only
if x0(i)>=UB(i)
% infeasible starting value. use bound.
x0u(k) = 0;
else
x0u(k) = sqrt(UB(i) - x0(i));
end
% increment k
k=k+1;
case 3
% lower and upper bounds
if x0(i)<=LB(i)
% infeasible starting value
x0u(k) = -pi/2;
elseif x0(i)>=UB(i)
% infeasible starting value
x0u(k) = pi/2;
else
x0u(k) = 2*(x0(i) - LB(i))/(UB(i)-LB(i)) - 1;
% shift by 2*pi to avoid problems at zero in fminsearch
% otherwise, the initial simplex is vanishingly small
x0u(k) = 2*pi+asin(max(-1,min(1,x0u(k))));
end
% increment k
k=k+1;
case 0
% unconstrained variable. x0u(i) is set.
x0u(k) = x0(i);
% increment k
k=k+1;
case 4
% fixed variable. drop it before fminsearch sees it.
% k is not incremented for this variable.
end
end
% if any of the unknowns were fixed, then we need to shorten
% x0u now.
if k<=n
x0u(k:n) = [];
end
% were all the variables fixed?
if isempty(x0u)
% All variables were fixed. quit immediately, setting the
% appropriate parameters, then return.
% undo the variable transformations into the original space
x = xtransform(x0u,params);
% final reshape
x = reshape(x,xsize);
% stuff fval with the final value
fval = feval(params.fun,x,params.args{:});
% fminsearchbnd was not called
exitflag = 0;
output.iterations = 0;
output.funcount = 1;
output.algorithm = 'fminsearch';
output.message = 'All variables were held fixed by the applied bounds';
% return with no call at all to fminsearch
return
end
% Check for an outputfcn. If there is any, then substitute my
% own wrapper function.
if ~isempty(options.OutputFcn)
params.OutputFcn = options.OutputFcn;
options.OutputFcn = @outfun_wrapper;
end
% now we can call fminsearch, but with our own
% intra-objective function.
[xu,fval,exitflag,output] = fminsearch(@intrafun,x0u,options,params);
% undo the variable transformations into the original space
x = xtransform(xu,params);
% final reshape
x = reshape(x,xsize);
% Use a nested function as the OutputFcn wrapper
function stop = outfun_wrapper(x,varargin);
% we need to transform x first
xtrans = xtransform(x,params);
% then call the user supplied OutputFcn
stop = params.OutputFcn(xtrans,varargin{1:(end-1)});
end
end % mainline end
% ======================================
% ========= begin subfunctions =========
% ======================================
function fval = intrafun(x,params)
% transform variables, then call original function
% transform
xtrans = xtransform(x,params);
% and call fun
fval = feval(params.fun,xtrans,params.args{:});
end % sub function intrafun end
% ======================================
function xtrans = xtransform(x,params)
% converts unconstrained variables into their original domains
xtrans = zeros(1,params.n);
% k allows some variables to be fixed, thus dropped from the
% optimization.
k=1;
for i = 1:params.n
switch params.BoundClass(i)
case 1
% lower bound only
xtrans(i) = params.LB(i) + x(k).^2;
k=k+1;
case 2
% upper bound only
xtrans(i) = params.UB(i) - x(k).^2;
k=k+1;
case 3
% lower and upper bounds
xtrans(i) = (sin(x(k))+1)/2;
xtrans(i) = xtrans(i)*(params.UB(i) - params.LB(i)) + params.LB(i);
% just in case of any floating point problems
xtrans(i) = max(params.LB(i),min(params.UB(i),xtrans(i)));
k=k+1;
case 4
% fixed variable, bounds are equal, set it at either bound
xtrans(i) = params.LB(i);
case 0
% unconstrained variable.
xtrans(i) = x(k);
k=k+1;
end
end
end % sub function xtransform end

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%% Optimization of a simple (Rosenbrock) function, with no constraints
% The unconstrained solution is at [1,1]
rosen = @(x) (1-x(1)).^2 + 105*(x(2)-x(1).^2).^2;
% With no constraints, operation simply passes through
% directly to fminsearch. The solution should be [1 1]
xsol = fminsearchbnd(rosen,[3 3])
%% Full lower and upper bound constraints which will all be inactive
xsol = fminsearchbnd(rosen,[3 3],[-1 -1],[4 4])
%% Only lower bound constraints
xsol = fminsearchbnd(rosen,[3 3],[2 2])
%% Only upper bound constraints
xsol = fminsearchbnd(rosen,[-5 -5],[],[0 0])
%% Dual constraints
xsol = fminsearchbnd(rosen,[2.5 2.5],[2 2],[3 3])
%% Dual constraints, with an infeasible starting guess
xsol = fminsearchbnd(rosen,[0 0],[2 2],[3 3])
%% Mixed constraints
xsol = fminsearchbnd(rosen,[0 0],[2 -inf],[inf 3])
%% Provide your own fminsearch options
opts = optimset('fminsearch');
opts.Display = 'iter';
opts.TolX = 1.e-12;
n = [10,5];
H = randn(n);
H=H'*H;
Quadraticfun = @(x) x*H*x';
% Global minimizer is at [0 0 0 0 0].
% Set all lower bound constraints, all of which will
% be active in this test.
LB = [.5 .5 .5 .5 .5];
xsol = fminsearchbnd(Quadraticfun,[1 2 3 4 5],LB,[],opts)
%% Exactly fix one variable, constrain some others, and set a tolerance
opts = optimset('fminsearch');
opts.TolFun = 1.e-12;
LB = [-inf 2 1 -10];
UB = [ inf inf 1 inf];
xsol = fminsearchbnd(@(x) norm(x),[1 3 1 1],LB,UB,opts)
%% All the standard outputs from fminsearch are still returned
[xsol,fval,exitflag,output] = fminsearchbnd(@(x) norm(x),[1 3 1 1],LB,UB)