Issue with lsqcurvefit and langmuir type equation

Hi,

I have used this function quite a number of time in different codes and now I am having issues with what I thought it was going to be easy. I will need to fit a langmuir type equation (k*K*C/(1+K*C))to a set of data. I was testing the function with a fake set of data and I keep getting:

Error using lsqcurvefit (line 244) Function value and YDATA sizes are not equal.

I have tested other equations and the code seem to work for them. the problem seem to be having the C repeated at the denominator. Here the code I used:

myfun = @(x,xdata) (x(1)*x(2)*xdata)/(1+x(2)*xdata);

ydata=[0.25 0.3 0.333333333 0.357142857 0.375 0.388888889 0.4 0.409090909 0.416666667 0.428571429];

xdata=[20 30 40 50 60 70 80 90 100 120];

x0=[0.15; 0.03];

lb=[1e-5; 1e-5];

ub=[1e5; 1e5];

options=optimset('Algorithm','levenberg-marquardt');

[x,resnorm,residual,exitflag,output,lambda,jacobian] = lsqcurvefit(myfun,x0,xdata,ydata,lb,ub, options);

I have tested the same data and equation in the CFTOOL and it works fine. I am trying to use a code just because the equation will get more complicated in the future. Thanks in advance for any suggestion.

Regards

Massi

Answer 1

[bym]

see comments in revised code below

myfun = @(x,xdata) (x(1)*x(2).*xdata)./(1+x(2).*xdata); %added elementwise operators ./ .*

ydata=[0.25 0.3 0.333333333 0.357142857 0.375 0.388888889 0.4 0.409090909 0.416666667 0.428571429];

xdata=[20 30 40 50 60 70 80 90 100 120];

x0=[0.15; 0.03];

lb=[1e-5; 1e-5];

ub=[1e5; 1e5];

%options=optimset('Algorithm','levenberg-marquardt'); this algorithm does
%not support bounds

[x,resnorm,residual,exitflag,output,lambda,jacobian] = lsqcurvefit(myfun,x0,xdata,ydata,lb,ub);