How to get the first value in a set of array that is bigger than the first few values?

Hi everyone,

I have two arrays:

y = [1 1 1 1 1 1.5 1.7 1.9 2 2.7]

x = [25:35]

How can I find the corresponding x value for the first number that breaks the chain in y (in this case 1.5)?

y values are different for each loop but the x is almost stable.

Furthermore, how can I store the x y values of the first value that breaks the chain in a loop. Thank you so much.

Answer 1

[Star Strider]

Try this:

y = [1 1 1 1 1 1.5 1.7 1.9 2 2.7];
x = [25:35];

TF = ischange(y,'variance');
Idx = find(TF,1,'first')
Out = x(Idx)

producing:

Out =
    30

I’m not certain what you want, or how robust this would be to your other data, so you may need to experiment with it.

[Wolfgang McCormack]

@Star Strider this is exactly what I was looking for just two more things, it returns the x value of the first value that has changed. How can I make it return the x index of the last stable chain (the index of the last 1 ebfore it turns into 1.5)

Two how can I store this data in a for loop for 100 files? should I use the command store? should it be out of the loop or inside?

Thank you so much!

[Star Strider]

As always, my pleasure!

‘How can I make it return the x index of the last stable chain (the index of the last 1 ebfore it turns into 1.5’

The easiest way is simply to subtract 1 from ‘Idx’. I experimented, and cannot find a method (of the three available) that would return the last index before a change point.

‘Two how can I store this data in a for loop for 100 files? should I use the command store? should it be out of the loop or inside?’

If I understand correctly what you want to do, I would put those inside the loop. To store the values within the loop, subscript them.

So, if ‘k’ is the loop index, then:

for k = ...
        ... READ FILES ... 
        ... SPECIFY 'x' AND 'y' ...
    TF = ischange(y,'variance');    
    Idx(k) = find(TF,1,'first')-1;
    Out(k) = x(Idx(k));
end

Then after the loop, write ‘Idx’ and ‘Out’ to a file using save, writematrix, or another function, however you want to store them.

[Wolfgang McCormack]

@Star Strider Thank you so much!, I have one more question, Now, how can I calculate the slope of the line between the first and last? and how can I store the original files of them in two different folders when the slope is greater than A?

I really appreciate it! sorry for asking too many questions :D

[Star Strider]

As always, my pleasure!

‘Now, how can I calculate the slope of the line between the first and last?’

Please define ‘first and last’.

Also, for ths slope calculation, do you want to consider only those two points, or all the points in between as well? The first involves a simple calculation of only those two points, and the second involves a least-squares linear fit. (Both of these are straightforward in MATLAB, so that is not the issue. I just need to know which of these you want.) What data do you want the slope to be calculated with respect to? I assume this is essentially with ‘x’ and ‘y’ defined as previously.

‘... and how can I store the original files of them in two different folders when the slope is greater than A?’

Do you want to store one set in a folder when the slope is less than or equal to ‘A’ and in another folder when it is greater than ‘A’, or something else? Do the folders already exist or will it be necessary to create them?

Answer 2

[the cyclist]

Because x and y are not the same length, I'm not sure how to make the "correspondence" between them. However, this code snippet will give the index of the first element that is different from the preceding ones:

idx = find(diff(y)~=0,1) + 1;

Maybe you can figure out the rest from there, or explain more about what you mean.

[Wolfgang McCormack]

@the cyclist thank you so much for the code, does it only work for 1?

[the cyclist]

I'm not sure what you are asking. If your input vector is

y = [7 7 7 3 4];

then

idx = find(diff(y)~=0,1) + 1

idx = 4

gives 4 as the result, because it is the first element that "breaks the chain" of the initial values. Is that what you mean?

[Wolfgang McCormack]

@the cyclist oh I see thank you! Can I accept two answers?