MATLAB: Select values on x axis corresponding to a y axis value

extract data

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.

Screen Shot 2019-03-12 at 2.12.28 PM.png

Best Answer

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!

Related Solutions

MATLAB: Minimum point of Graph

Use the islocalmin function:

x = -5:0.1:5;
y = x.^2;
idx = islocalmin(y);
figure(1)
hold on
plot(x,y)
plot(x(idx),y(idx),'*r')
legend('Curve','Local Min')
hold off
fprintf('Min located at %0.2f\n',x(idx))

MATLAB: How to get an x coordinate from a given y

One usually uses

indexmax = find(max(y) == y);

when there can be several y values that are equal to the max, as indexmax will be a vector of indices of all these max values. e.g.

y = [3 2 1 1 2 3 2 1 1 3 2 1]
indexmax = find(max(y) == y);

returns

indexmax == [1 6 10]

However, later on, your code (the strmax = ...) assumes that indexmax is scalar. In that case, one would use the simpler syntax

[~, indexmax] = max(y);

This will always return just one index. In case of duplicate max, it's the index of the first max value.

As for your question, there's no guarantee that the exact value of half maximum will be present in your array so you can't search for that exact value. Instead you can search for the nearest. The position of that nearest value is the index of the absolute minimum of the difference between y and the half maximum, so:

[~, indexhalfmax] = min(abs(y - max(y)/2));

edit: fixed that last line of code.

Best Answer

Related Solutions

MATLAB: Minimum point of Graph

MATLAB: How to get an x coordinate from a given y

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