how to find local minima of a graph like shown in the figure?

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pacharya 2019-5-9

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评论： Star Strider 2019-5-13

Hello Everyone,

I am trying to find local minima (to get x and y values) as circled in the figure..Any suggestion please as how to find the local minima (or x and y values for the lowest valley)? I tried to use findpeaks by inverting the plot but it gave me more number of points. Thank you!

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Star Strider 2019-5-9

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If you have R2017b or later release, the islocalmin (link) function is an option.

Your are correct in using findpeaks with your inverted (negated) signal to find the minima. There are a number of name-value pair arguments (such as 'MinPeakHeight' and 'MinPeakProminence') that can help in sorting your peaks.

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Star Strider 2019-5-10

在 MATLAB Online 中打开

Try this:

[D,S] = xlsread('file.xlsx');
xv = 1:size(D,1);
[pks,locs,widths,proms] = findpeaks(-D(:,3),xv);
wplv = (proms > 0.5) & (widths(:) > 200);               % Width & Prominence Logical Vector
figure
plot(xv, D(:,3))
hold on
plot(locs(wplv), -pks(wplv), 'or')
hold off
grid
% text(locs(plv), -pks(plv), sprintfc('W = %.1f\nP = %.1f',[widths(plv); proms(plv)']'), 'FontSize',8)

I ended up using both the ‘widths’ and ‘proms’ outputs to determine the peak values to return and plot. This appears to get all the ones you are interested in:

how to find local minima of a graph like shown in the figure - 2019 05 10.png

It also gets the last one, however you can delete that by simply using (1:end-1) as the reference, for example: -pks(wplv(1:end-1)).

I commented-out the text call I used to help me determine the widths and prominences of the various peaks. I kept in the code so you can use it if you need to in order to change the ‘wplv’ logical conditions if the current ones do not work in your other data sets. (It displays the widths and prominences of each identified peak, as a valley, on the plot.)

pacharya 2019-5-13

Thank you!

Star Strider 2019-5-13

As always, my pleasure!

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