how to find a value from a timeseries?

Question

Tony Castillo 2018-7-16

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https://ww2.mathworks.cn/matlabcentral/answers/410517-how-to-find-a-value-from-a-timeseries

编辑： Guillaume 2018-7-16

soc.mat

hi Guys, I got a problem analyzing a time series, because of its length I need to find out a way to obtain the required value without need to scroll all the time series. For instance, how to find from the attached time series the value 79?, it must appear twice in the same time series?. Could you help me?.

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KSSV 2018-7-16

The values are 69.9 and 70, there is no 79 in the data.

Tony Castillo 2018-7-16

Oh, I think I have attached the wrong time series, but surely you could check the value 70, it appears twice. How do you do? Thanks in advance

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Answer 1

KSSV 2018-7-16

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https://ww2.mathworks.cn/matlabcentral/answers/410517-how-to-find-a-value-from-a-timeseries#answer_328963

在 MATLAB Online 中打开

Go for logical indexing:

S = load('soc.mat') ;
t = S.soc.Time ;
y = S.soc.Data ;
idx = abs(y-70)<10^-5 ;
[t(idx) y(idx)]

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KSSV 2018-7-16

I want to get y values which are equal to 70. So I need to check which values of y are equal to 70 an pick those indices. To compare floating point numbers it is suggested to use abs(y-val)<tolerance. 10^-5 is a tolerance factor, so I am picking those values from y on subtracting from 70, the difference should be less then tolerance.

Guillaume 2018-7-16

编辑：Guillaume 2018-7-16

在 MATLAB Online 中打开

Do a search on floating point comparison to learn all about it.

To check that two floating point numbers are equal, you check that there difference is sufficiently small that you can consider them the same. KSSV chose 10e-5 for that sufficiently small difference. It's arbitrary, the value being informed by the magnitude of the numbers and the precision that you require.

Note that you musn't use == to compare floating point numbers, due to the limited precision inheritent to storing floating point numbers on a computer. In particular, a computer will tell you that

(0.1 + 0.1 + 0.1) == 0.3

is false, as computers can't store 0.1 exactly.

Again, do a search to learn more.

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