MATLAB: How to fix error about interp1

errorinterp1

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?

Best Answer

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.
Related Question