Using interp1 on a function that is not monotonic increasing

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jgillis16
jgillis16 2015-9-7
回答： Steven Lord 2015-9-7
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?

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回答（2 个）

Stephen23
Stephen23 2015-9-7
编辑：Stephen23 2015-9-7
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
Steven Lord
Steven Lord 2015-9-7
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.

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