If loop to find date where values are bigger than certain threshold

Question

I am struggling with my Matlab homework.

I have a datetime array called "date" and two arrays (X and Y) of the type double containing measurements for each time step.

I have to do the following task:

Select all time steps where both X AND Y are larger than their respective Z value. Store the dates of these time steps in a new variable. Name it 'C'

Z and Y are 52584x1 double vectors and date is a vector of the format 52584x1 datetime (dd.MM.yyyy hh:mm)

I tried:

%Some Dummy Data:
Y = 1:8:80
X = 2:4:40
t1 = datetime(2013,11,1,8,0,0)
t2 = datetime(2013,11,10,8,0,0)
date = t1:t2

Z = 8;

for i = length(date) 
    if X(i) > Z && Y(i) > Z
         C=date(i)
    end
end

I guess something is wrong with C=date(i)

Thankful for any help!

ThomasIsCoding · Accepted Answer · 2019-12-01T20:20:27.893

1

I guess what you need might be something like below:

Z = 8;
for i = 1:length(date) 
    if X(i) > Z && Y(i) > Z
         C(end+1)= date(i);
    end
end

alternative: C = date((X>Z)& (Y>Z)) or C = date(min([X;Y],[],1) > Z) is a more efficient solution instead of using for loop

edited Dec 01 '19 at 20:20

answered Dec 01 '19 at 19:29

ThomasIsCoding

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Thanks for your answer, this looks much better. However, this gives me an empty array... – Axel.Schweiß Dec 01 '19 at 19:33
@Axel.Schweiß please show some of you data, otherwise I have no idea what those variables contain – ThomasIsCoding Dec 01 '19 at 19:35
I tried to specify the data. But how can I show them? Upload the .txt file? – Axel.Schweiß Dec 01 '19 at 19:40
@Axel.Schweiß you can create some fake data for toy examples of your problem, instead of using huge and real dataset – ThomasIsCoding Dec 01 '19 at 19:42
@Axel.Schweiß with you dummy data, I don't see that `C` as an empty output....I don't know what threshold `Z` you are using when trying my code – ThomasIsCoding Dec 01 '19 at 19:56
True, but for me it does only show Nov. 10th where both conditions are given. But I am looking for all dates where X and Y is greater than 8, which should be the case for all days beginning from Nov third... – Axel.Schweiß Dec 01 '19 at 20:03
@Axel.Schweiß sorry for my typo, should be `i = 1:length(date) ` in `for-loop` – ThomasIsCoding Dec 01 '19 at 20:09
This is a really bad way of growing an array. See [this answer](https://stackoverflow.com/a/48990695/7328782). This whole code would be better without a loop at all. – Cris Luengo Dec 01 '19 at 20:12
@CrisLuengo, yes, I know. A more efficient solution is to use `date((X>Z)& (Y>Z))`, while I am not sure if OP is pursuing the efficient or just want to look at the `for-loop` – ThomasIsCoding Dec 01 '19 at 20:18
@Axel.Schweiß see my updates with alternative solutions – ThomasIsCoding Dec 01 '19 at 20:23

If loop to find date where values are bigger than certain threshold

1 Answers1