Bug in least squares fitting

I am trying to fit my data as follows and I get the error message below. Can anyone help? Thanks in advance. Cathy R

>> xdata = intvp(:,1)

xdata =

         195
         390
         590
         795
         985
        1195
        1300
        1410
        1510
        1600
        1710
        1885
        2065
        2270
        2480
        2695
        2900

>> ydata = intvp(:,2)

ydata =

    1.0139
    0.9816
    0.9606
    0.8848
    0.7149
    0.4705
    0.3130
    0.2017
    0.1224
    0.0517
    0.0210
    0.0132
    0.0041
    0.0020
    0.0172
   -0.0020
    0.0231

>> fun = @(x,xdata)x(1)*(1/(1 + (x(2)*exp(x(3)*xdata))))

fun =

    @(x,xdata)x(1)*(1/(1+(x(2)*exp(x(3)*xdata))))

>> x0 = [1.0,0.01,0.0045]

x0 =

    1.0000    0.0100    0.0045

>> x = lsqcurvefit(fun,x0,xdata,ydata) Error using lsqcurvefit (line 248) Function value and YDATA sizes are not equal.

Answer 1

[Brendan Hamm]

The issue is that you are trying to do an element-wise division but are not using an element-wise operator. You can see that:

fun(x0,xdata)

returns a row vector, which is not the same size as ydata.

Change the definition of fun and use the element-wise division operator ./ as opposed to /

fun = @(x,xdata) x(1)*(1./(1 + (x(2)*exp(x(3)*xdata))))

[Catherine Royer]

Thanks!

[Brendan Hamm]

in case you were wondering in A/B when the denominator is a vector or matrix, MATLAB is actually using an inverse (or pseudo inverse) so this calculation is treated by MATLAB as: A*inv(B). Similarly A\B is treated as inv(A)*B.