Using interp1 on a function that is not monotonic increasing

I need to use the interp1 function on my two data sets, labeled x and y.

I wrote the following code:

yDesired = interp1(x,y, 0.009);

But, I receive the following error:

Error using griddedInterpolant
The grid vectors are not strictly monotonic increasing.

Error in interp1 (line 183)
        F = griddedInterpolant(X,V,method);

Is there any way around this?

Answer 1

[Stephen23]

John D'Errico explains here why interpolation requires a monotonic independent variable:

http://www.mathworks.com/matlabcentral/answers/121532-using-interp1-monotonic-increasing-values

It seems lots of beginners misunderstand interpolation, but the answer is always the same "the independent variable must be monotonic":

http://www.mathworks.com/matlabcentral/answers/182839-interp1-error-grid-vectors-are-not-strictly-monotonic-increasing

http://www.mathworks.com/matlabcentral/answers/180429-help-with-interpol1-the-grid-vectors-are-not-strictly-monotonic-increasing

If you have scattered data, then you need to use the appropriate tool, one of these might help you:

http://www.mathworks.com/matlabcentral/answers/159322-1d-interpolation-with-non-monotonic-scattered-data

http://www.mathworks.com/matlabcentral/answers/153528-interp1-the-grid-vectors-are-not-strictly-monotonic-increasing

Answer 2

[Steven Lord]

The X input to INTERP1 must be monotonic increasing. The Y input does not need to be monotonic increasing. Call SORT with two outputs and X as input; the first output is the reordered X vector and the second output will let you reorder Y to match the reordered X.