How to fix error about interp1?
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Ghazal Hnr
il 12 Mar 2017
Commentato: Star Strider
il 13 Mar 2017
Hello,
I have some data and i want to run this code:
xi = [85];
yi = interp1(y,x,xi)
but I have this error:
Error using griddedInterpolant
The grid vectors are not strictly monotonic increasing.
Error in interp1 (line 183)
F = griddedInterpolant(X,V,method);
Error in Q4 (line 19)
yi = interp1(y,x,xi)
How can I fix it?
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Star Strider
il 12 Mar 2017
Modificato: Star Strider
il 12 Mar 2017
One way (without seeing your data, so this is a guess on my part):
y85 = find(y <= 85, 1, 'last');
yi = interp1(y(y85-1:y85+1),x(y85-1:y85+1),xi, 'linear','extrap');
That is how I would do it. This assumes there is only 1 where ‘y’ is approximately 85.
Note — This is obviously UNTESTED CODE. It should work, if there is one ‘y’ near 85.
EDIT — Added the 'extrap' option to make my approach more robust.
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John D'Errico
il 12 Mar 2017
The fix is easy. Don't use interp1 to interpolate data that is not strictly monotone in the independent variable.
After all, how can interp1 know which value to predict on a function that is apparently multi-valued? For example, consider points around the perimeter of acircle. For any given x, there are TWO solutions. Interp1 requires a single valued function for any given x.
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