How to find a value X of given Y close to 0, where the value X comes at the first place

Hi,

I used to solved my previous question to find a value X of given Y close to 0, but there is a value X comes from the very late place, which is not what I need. How can I extract the earlier value of X?

For example,

338.00 339.00 340.00 341.00 342.00 343.00 ... 964.00

1.00 -100.00 -100.00 -100.00 -100.00 -100.00 -100.00 ... -100.00

2.00 100.00 100.00 100.00 100.00 100.00 100.00 ... 100.00

3.00 0.28 0.12 -0.05 -0.21 -0.38 -0.55 ... -0.02

4.00 0.28 0.12 -0.05 -0.21 -0.38 -0.55 ... -0.02

5.00 8.21 8.24 8.26 8.28 8.30 8.32 ... 8.35

6.00 8.21 8.24 8.26 8.28 8.30 8.32 ... 8.35

Where the 340.00 is what I need, not the 964.00. How do I write the code to get 342.00?

The current code I am using is:

dr = damp_ratio(1:6 ,:);
[~,ix]=min(abs(dr),[],'all','linear');
[~,speed]=ind2sub(size(dr),ix);

Thanks in advance.

Terence

Answer 1

[Matt J]

damp_ratio=[ -100.00  -100.00  -100.00  -100.00  -100.00  -100.00   -100.00
              100.00   100.00   100.00   100.00   100.00   100.00   +100
	      0.28      0.12 	  -0.05    -0.21    -0.38    -0.55   -0.02
              0.28 	0.12 	  -0.05    -0.21    -0.38    -0.55   -0.02
	      8.21    8.24 	   8.26     8.28     8.30     8.32   8.35 
	      8.21    8.24 	   8.26     8.28     8.30     8.32   8.35]      

damp_ratio = 6×7

-100.0000 -100.0000 -100.0000 -100.0000 -100.0000 -100.0000 -100.0000 100.0000 100.0000 100.0000 100.0000 100.0000 100.0000 100.0000 0.2800 0.1200 -0.0500 -0.2100 -0.3800 -0.5500 -0.0200 0.2800 0.1200 -0.0500 -0.2100 -0.3800 -0.5500 -0.0200 8.2100 8.2400 8.2600 8.2800 8.3000 8.3200 8.3500 8.2100 8.2400 8.2600 8.2800 8.3000 8.3200 8.3500

dr = diff(  sign(damp_ratio(1:6 ,:)),1,2)~=0 ;

[val,ix]=max(dr,[],2);

speed=ix+1;

speed(val==0)=nan

speed = 6×1

NaN NaN 3 3 NaN NaN

[Terence]

Thanks, Matt. I am a bit confused, the output is not what I need,

x = 338.00 339.00 340.00 341.00 342.00 343.00 ... 964.00;

damp_ratio =[ -100.00 -100.00 -100.00 -100.00 -100.00 -100.00 ...-100.00

100.00 100.00 100.00 100.00 100.00 100.00 ... 100

0.28 0.12 -0.05 -0.21 -0.38 -0.55 ...-0.02

0.28 0.12 -0.05 -0.21 -0.38 -0.55 ...-0.02

8.21 8.24 8.26 8.28 8.30 8.32 ... 8.35

8.21 8.24 8.26 8.28 8.30 8.32 ... 8.35]

The output I need is 340.00. but my code is to get 964.00. I just wonder if there are any ways to get the x value when the damp_ratio approaching to 0 at the first place.

Thanks!

[Matt J]

The code you showed us returns a column index "speed". Presumably, you want speed=3 which corresponds to the column of damp_ratio with x=340. This is what my code gives you - the location of the first zero crossing in each row - and you can see from the above output that it is equal to 3 in rows where a zero-crosing exists.

[Terence]

Sorry Matt, I think I made some mistakes that gets you misunderstood.

For the damp_ratio,

column x = 338.00 339.00 340.00 341.00 342.00 343.00 ... 964.00;

damp_ratio =[ -100.00 -100.00 -100.00 -100.00 -100.00 -100.00 ...-100.00

100.00 100.00 100.00 100.00 100.00 100.00 ... 100

0.28 0.12 -0.05 -0.21 -0.38 -0.55 ...-0.02

0.28 0.12 -0.05 -0.21 -0.38 -0.55 ...-0.02

8.21 8.24 8.26 8.28 8.30 8.32 ... 8.35

8.21 8.24 8.26 8.28 8.30 8.32 ... 8.35]

Run my code you can get the output value = 964.00. But what I need is 340.00, which refers to the value of -0.05. Even though -0.05 is not the value that closest to 0, like I requsted before, -0.05 is the first value approching 0. I just wodner if there are any ways to extract the value 340.00 rather than 964.00.

You can see the attached .mat. , together with the code below.

dr = damp_ratio(1:10 ,:);
[~,ix]=min(abs(dr),[],'all','linear');
[~,speed]=ind2sub(size(dr),ix);

Hope it makes the question clear.

[Matt J]

Run my code you can get the output value = 964.00.

No, that is not what I get. I get an output value of speed=7. Conversely, with my code you get an output value of speed=3.

columnX=[338.00   339.00   340.00   341.00   342.00   343.00  964.00];

damp_ratio=[ -100.00  -100.00  -100.00  -100.00  -100.00  -100.00   -100.00
              100.00   100.00   100.00   100.00   100.00   100.00   +100
	      0.28      0.12 	  -0.05    -0.21    -0.38    -0.55   -0.02
              0.28 	0.12 	  -0.05    -0.21    -0.38    -0.55   -0.02
	      8.21    8.24 	   8.26     8.28     8.30     8.32   8.35 
	      8.21    8.24 	   8.26     8.28     8.30     8.32   8.35];      

dr = damp_ratio(1:6 ,:);
[~,ix]=min(abs(dr),[],'all','linear');
[~,speed]=ind2sub(size(dr),ix)

speed = 7

[Terence]

Can you import the .mat file then run the code, you will get what I mentioned.

[Matt J]

When I run your code with matlab.mat, I get speed=930 and with my code speed=303 or speed=340, depending on the row.

load(websave('matlab.mat','https://www.mathworks.com/matlabcentral/answers/uploaded_files/495718/matlab.mat'))

dr = damp_ratio(1:10 ,:);
[~,ix]=min(abs(dr),[],'all','linear');
[~,speed]=ind2sub(size(dr),ix)

speed = 930

dr = diff(  sign(damp_ratio(1:10 ,:)),1,2)~=0 ;
[val,ix]=max(dr,[],2);
speed=ix+1;
speed(val==0)=nan

speed = 10×1

303 NaN 340 340 NaN NaN NaN NaN NaN NaN

[Terence]

Yes. So if I want 340 for output, just simply extract it from row 3 or 4?

It reminds me another question, for example, if the speed is like

speed = 10×1
   303
   NaN
   NaN
   NaN
   340
   340
   NaN
   NaN
   NaN
   NaN

where the value happens in other row, is it the same method that we extract the value of 340? Or is there a general code that could work?

[Matt J]

I don't know why you would expect a scalar result. The location of the first zero-crossing is different in every row.

[Terence]

Yes, first zero-crossing value is always changing when the different initial conditions are set, but that is the value I need to extract. I need scalar value for the next step prediction. So I just wonder if there is a general way to get the first zero-crossing value in damp_ratio.

[Terence]

Sometimes the values in 2 consecutive rows may be the same. So only one of them is needed to be extracted.

[Matt J]

So I just wonder if there is a general way to get the first zero-crossing value in damp_ratio.

It is not obvious how you propose to define a unique "first zero-crossing" for a two-dimensional array. Zero-crossings and the order in which they occur are 1-dimensional concepts. You scan through a 1 dimensional list of numbers in order until the sign of the numbers changes. A two dimensional array of numbers has no ordering.

[Matt J]

Terrence's comment moved here.

Thanks for that comments. It seems that there is no way to achieve my goal in the current situation. :(