## how to get the x value for a given y value?

### ahmed salah (view profile)

on 22 May 2019
Latest activity Commented on by ahmed salah

on 22 May 2019

### Geoff Hayes (view profile)

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')

### Geoff Hayes (view profile)

on 22 May 2019

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.)

ahmed salah

on 22 May 2019