interp1 fails to interpolate correctly

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I have a set of input data which the function interp1 is incorrectly interpolating.
For a statement Vq = interp1(X,V,Xq), I have
X =
-0.0124393747904463
0.0134396235257952
0.0344412390769423
0.0850313641677704
0.0850004675375172
0.126329531171773
V =
-4
-2
0
2
4
6
Clq =
0.0772456107479591
The result interp1 gives me is Vq = 3.3864734865877. This is clearly wrong, but may be related to the face that between V = 4 and V =6, X is not monotonically increasing/decreasing. Can someone help me understand why this is failing?

Accepted Answer

Matt J
Matt J on 17 Nov 2019
Edited: Matt J on 17 Nov 2019
X needs to be montonically increasing (or interp1 will pre-sort the data so that it is).

More Answers (1)

Vladimir Sovkov
Vladimir Sovkov on 18 Nov 2019
Why do you consider it is wrong? In the plot (see the attachment) this result looks quite reasonable.
  5 Comments
Walter Roberson
Walter Roberson on 19 Nov 2019
polynomial fits cannot solve double-valued functions.
Sometimes you should use a function of two variables instead.
Sometimes you can rewrite as a parameteric function.
Sometimes you just need to segment to decide which function is in control. For example hysterisis has to be handled as a pair of functions, not by trying to fit one single function.
Vladimir Sovkov
Vladimir Sovkov on 19 Nov 2019
Maybe, switching roles of X and V can help, e.g.:
VV=fzero(@(z)(interp1(V,X,z)-Clq),1)
VV=1.69220264208349

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