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Bug in least squares fitting

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Catherine Royer
Catherine Royer on 16 Nov 2016
Commented: Brendan Hamm on 16 Nov 2016
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.

Accepted Answer

Brendan Hamm
Brendan Hamm on 16 Nov 2016
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))))
  2 Comments
Brendan Hamm
Brendan Hamm on 16 Nov 2016
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.

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