Specific change points in large dataset

Question

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Hello everyone. I am new to matlab and trying to solve an assignment.

I have a large dataset with 3523873x1 size. (attached data)

I am trying to find the points (green circles).

I have tried using

z = ischange(df_10s_ufreq,'linear', 'MaxNumChanges',5);
z_index = find(z);

I could not get to the exact positions and it is also taking 2~3 minutes.

I am in search of better solutions and I will be thankful for the help.

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Answer 1

prasanth s 2019-10-17

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Here is one of the methods using 'diff' function.

use following code to get all the difference values and plot those values to findout an limit value to cut the lowermost spike.

D=diff(df_10s_ufreq);
figure,plot(D);

Let we take the limit value '-0.5e-5'.

use following code to get all indices below the limit value.

L=D<(-0.5e-5);

Again apply 'diff' and find initial change of spike. it will return desired indices.

V=diff(L);
indices=find(V>0)