MATLAB: Lsqcurvefit and anonymous function error

anonymous functionerrorlsqcurvefit

I'm trying to use lsqcurvefit with an anonymous function within the script to determine the best fit for a hyperbolic tangent function. However, I keep receiving the following error as outlined by the exception variable:
'Objective function is returning undefined values at initial point. lsqcurvefit cannot continue.'
Here is the code:
% Estimate hyperbolic tangent parameters using least squares curve fit.
xo = [ 15; 20; 15 ]; % initial guess of tangent curve parameters.
lb = [ 16; 0; 0 ]; ub = [ 17; 40; 40 ]; % Bounds for the parameters
d = 0:0.1:65;
hyper = @( c, rkm ) c( 1 ).*( 1 - tanh( ( rkm - c( 2 ) ./ c( 3 ) ) ) );
for n = 1:2%size( sal, 1 )
  try
    % Fit the hyperbolic tangent parameters using the least squares curve fit.
    [ param, resnorm ] = lsqcurvefit( hyper, xo, rkm, sal(n,:), lb, ub ); 
    BSo(n) = param(1);
    Bxc(n) = param(2);
    BxL(n) = param(3);
    catch exception2
        continue
    end
end

Best Answer

You don’t give values for rkm or sal. When I created values for them, I had no problems with the following code (essentially yours, with a couple lines added to create the data):
          xo = [ 15; 20; 15 ]; % initial guess of tangent curve parameters.
          lb = [ 16; 0; 0 ]; ub = [ 17; 40; 40 ]; % Bounds for the parameters
          hyper = @( c, rkm ) c( 1 ).*( 1 - tanh( ( rkm - c( 2 ) ./ c( 3 ) ) ) );
          rkm = -10:2:10;
          sal = [hyper([16.25  11  37],rkm);  hyper([16.75  19  31],rkm)];
          d = 0:0.1:65;
          hyper = @( c, rkm ) c( 1 ).*( 1 - tanh( ( rkm - c( 2 ) ./ c( 3 ) ) ) );
          for n = 1:2     %size( sal, 1 )
            try
              % Fit the hyperbolic tangent parameters using the least squares curve fit.
              [ param, resnorm ] = lsqcurvefit( hyper, xo, rkm, sal(n,:), lb, ub ); 
              BSo(n) = param(1);
              Bxc(n) = param(2);
              BxL(n) = param(3);    
            catch exception2
                  continue
              end
          end
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