MATLAB: How to get the x value for a given y value

floating pointgraphplotx value

I plot x-y graph (Gaussian function) and want to get the x-axis value (will be two points in this case) at a certain y-axis value (half of the maximum)

I tried this but it didn't work:

clc;
clear all;
Fs = 150; % Sampling frequency
t = -0.5:1/Fs:0.5; % Time vector of 1 second
x = 1/(sqrt(2*pi*0.01))*(exp(-t.^2/(2*0.01)));
figure;
plot(t,x);
xi = 0.5*max(x) ;
z=find(x==xi);
ti = x(z) ;
hold on
plot(ti,xi,'*r')

Best Answer

ahmed - your code assumes that there is an x value that is identical to xi

z=find(x==xi);

This need not be true. And comparing doubles in this manner is not generally a good idea (due to precision, see Why is 0.3 - 0.2 - 0.1 (or similar) not equal to zero?). Usually a tolerance of some kind should be used (i.e. abs(x - y) < eps). In your case, a tolerance might not work as well because you will not know what that tolerance should be. You could try different values...the following seems to work for this dataset

z=find(abs(x-xi)< 0.10);
ti = t(z) ;
hold on
plot(ti,xi,'*r')

(Note how ti is obtained from the t array instead of x.)

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MATLAB: Getting data from plot

I agree with Jan - it's ambiguous. You already have x, and y since you plotted it, so there's no need to extract anything from the axes (graph) at all. So in that case, I'd just use the x and y which are already available, and if the x is in the x array that you plotted, you can do this:

index = find(x == desiredXValue);   % May be multiple indexes, possibly
yDesired = y(index);

Now, if the desired x is not in your x array, then you can use interp1() to get the interpolated/estimated y value for that x.

yDesired = interp1(x,y, desiredXValue);

MATLAB: Plotting a surface or mesh

This sort of works (it runs and produces an interpolated surface):

x1 = [2,4,6];
x2 = [2,4,4];
x3 = [0.1,0.3,0.5];
x = x1; y = x2; z = x3;                         % Assumption
xi = [2:0.1:6];
yi = [2:0.1:4];
[XI,YI] = meshgrid(xi,yi);                      % Interpolation Grid Must Match Data Range
ZI = griddata(x,y,z,XI,YI);
mesh(XI,YI,ZI), hold;
plot3(x,y,z,'o'), hold off

The griddata function does not allow extrapolation (extrapolation is not mentioned in the documentation), so asking it to do so produces a matrix of NaN values, and NaN values do not plot. This was the reason you were not getting any surface plotted. The scatteredInterpolant class allows extrapolation, so consider using it if you want to extrapolate.

Best Answer

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