Select values on x axis corresponding to a y axis value

Hi Everyone, I am trying to make a fit for the linear portion of the graph using MATLAB code, but I am stuck at this point.

I am trying to extract the value of x axis where the 10% line is made as shown in figure below. and extract x axis data where the 90% line is made. and that is defined by the variable parameter. Any help would be appreciated.

Answer 1

[Mauro Fusco]

Let's say that your curve is described by the vectors x and y (I imagine that you obtain that curve via plot(x,y)).

Then you can find the maximum of the curve as:

y_max = max(y);

You can then pick the indices of the points corresponding to 10% and 90% of the maximum as:

idx_y10 = find(y == 0.1*y_max);

idx_y90 = find(y == 0.9*y_max);

The values that you are searching on the x axis will be:

x_10 = x(idx_y10);

x_90 = x(idx_y90);

I did not try it, enjoy!

[Image Analyst]

You can't use equality like that. You'd have to use ismembertol(), or else do what I'd do

idx_y10 = find(y >= 0.1*y_max, 1, 'first');

[Darpan Verma]

Thanks for your input. I tried it but Thats not giving the y coordingates even if I see that the max value is calculated right.

[Darpan Verma]

I also tried what mauro suggested. But dont understand where I am wrong.

[Darpan Verma]

Thanks I solved it. I was just doing a small error.

Thanks to both of you

[Mauro Fusco]

Very good, but it is correct that my answer was not fully precise. Therefore, I post here a working code:

close all;

x = 0:0.01:3*pi/4;

y=sin(x);

plot(x,y);

[y_max,idx_max] = max(y);

idx_y10 = find(y > 0.1*y_max,1,'first');

idx_y90 = find(y(1:idx_max) < 0.9*y_max,1,'last');

x_10 = x(idx_y10);

x_90 = x(idx_y90);

hold on;

plot(x_10,0.1*y_max,'o red');

hold on;

plot(x_90,0.9*y_max,'o red');