What does this lsqnonlin error mean?

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I'm attempting a nonlinear parameter fit using lsqnonlin and a Simulink model. I'm following a similarly method to: http://www.mathworks.com/help/optim/ug/lsqnonlin-with-a-simulink-model.html
After the function starts, I get this weird error below. What does it mean?
First-Order Norm of
if true
% Iteration Func-count Residual optimality Lambda step
0 3 231.377 51.4 0.01
1 6 179.496 51.1 0.001 6.82145
Subscripted assignment dimension mismatch.
Error in C:\MATLAB\R2012a Student\toolbox\shared\optimlib\finitedifferences.p>finitedifferences (line 159)
Error in levenbergMarquardt (line 235)
[JAC,~,~,numEvals,evalOK] = finitedifferences(XOUT,funfcn{3},[],[],[],costFun, ...
Error in lsqncommon (line 181)
Error in lsqnonlin (line 235)
Error in runSpringDamplsq (line 34)
param = lsqnonlin(@springDamplsq, param0, [], [], options);

Accepted Answer

Steve Grikschat
Steve Grikschat on 13 Sep 2013
This error is occurring because your function is returning arrays of varying sizes. In this case, it is being caught during finite-difference estimation of the function's Jacobian.
Check your function that you pass to lsqnonlin and make sure that it returns an array with a consistent number of elements.
  1 Comment
Amir Patel
Amir Patel on 13 Sep 2013
100% correct. I was stopping the simulation when end conditions were reached. This resulted in varying size arrays.
Thank you for the help!

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