Hi, I have a x vs y text data file. I want to find the maximum and minimum value of first derivative of the y w.r.t x and store the corresponding x and y values. Can anyone tell me the MATLAB code for that? Thanks in advance!

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Anuradha Bhattacharya 2016-5-20

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回答： Star Strider 2016-5-20

This is the code I have written:

>> a = load('xaxisfig7line1'); >> y = a(:,2); >> x = a(1,:); >> dy = diff(y)./diff(x);

Next How do I find the corresponding x and y values for a particular value of dy and store it in a file? Thanks, Anuradha

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Answer 1

Star Strider 2016-5-20

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Use the gradient function, not diff to calculate the derivative. It is the same size as the argument vector. This assumes that your ‘x’ values are evenly-spaced. See the documentation for details.

If the ‘x’ vector are not evenly-spaced (or acceptably close), consider using the polyfit, polyder, and polyval functions. The documentation for those functions will tell you how to use them. Use the lowest-order polynomial that will provide an acceptable fit to your data. Usually you can do this with at most a 7th-order polynomial.