MATLAB: Bug in least squares fitting

least squares fit

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.

Best Answer

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))))
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