How to fix error about interp1?

Question

Ghazal Hnr 2017-3-12

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/329458-how-to-fix-error-about-interp1

评论： Star Strider 2017-3-13

采纳的回答： Star Strider

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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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Answer 1

Star Strider 2017-3-12

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https://ww2.mathworks.cn/matlabcentral/answers/329458-how-to-fix-error-about-interp1#answer_258381

编辑：Star Strider 2017-3-12

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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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Ghazal Hnr 2017-3-13

thanks my problem solved

Star Strider 2017-3-13

My pleasure.

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Answer 2

John D'Errico 2017-3-12

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https://ww2.mathworks.cn/matlabcentral/answers/329458-how-to-fix-error-about-interp1#answer_258380

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.