pkg load optim

% create some "data"
X = linspace(-5,5,50);
Y = 2*(X-1).^2 - 1 + 1*randn(size(X));

base_settings = optimset(
   "ret_dfdp",1,"ret_covp",1,"ret_corp",1,'objf_type','wls'
); 

% fit with curvefit, parameter vector
F = @(p,x) p(1) + p(2)*x + p(3)*x.^2;
PIN = [1 1 1 1](:);
settings = optimset(base_settings,
   "fixed", [0 0 0 1](:)
);
[P, FY, CVG, OUTP] = nonlin_curvefit (F, PIN, X, Y, settings);
plot(X,Y,'.',X,F(P,X),'-');
INFO = curvefit_stat (F, P, X, Y, settings);

% fit with curvefit, parameter struct
F2 = @(p,x) p.const + p.lin*x + p.quad*x.^2;
PIN2 = struct("const",1,"lin",1,"quad",1,"dummy",0);
param_config = struct();
param_config.dummy.fixed = 1;
settings2 = optimset(base_settings,
   "f_pstruct",1,
   "param_config",param_config
); 
[P2, FY2, CVG2, OUTP2] = nonlin_curvefit (F2, PIN2, X, Y, settings2);
INFO2 = curvefit_stat (F2, P2, X, Y, settings2);