# How to locate X value for a given Y value

3 views (last 30 days)
David Harra on 1 Feb 2023
Answered: Star Strider on 2 Feb 2023
I have Data which is Time along the x-axis and amplitude along the y -axis.
I want to know the exact time at a given value of amplitude. Its probably just one line of code but I can't seem to figure it out.
Any help would be appreciated.

Star Strider on 2 Feb 2023
In the absence of the data, getting an independent variable valuex ‘x’ from an dependent variable value ‘y’ is relatively straightforward.
Try soemthing like this —
x = linspace(0, 20, 201);
y = besselj(0, x);
yval = linspace(-0.2, 0.2, 6);
for k1 = 1:numel(yval)
zxi = find(diff(sign(y-yval(k1))));
for k2 = 1:numel(zxi)
idxrng = max(1,zxi(k2)-1) : min(numel(x), zxi(k2)+1);
xv(k1,k2) = interp1(y(idxrng), x(idxrng), yval(k1));
end
end
for k1 = 1:size(xv,1)
xv(k1,xv(k1,:)==0) = NaN;
end
figure
plot(x, y)
hold on
for k1 = 1:size(xv,1)
plot(xv(k1,:), ones(size(xv(k1,:)))*yval(k1), 's')
end
yline(yval, ':')
hold off
grid
xlabel('X')
ylabel('Y')
.

the cyclist on 1 Feb 2023
Edited: the cyclist on 1 Feb 2023
It would be helpful if you uploaded the data. You can use the paper clip icon in the INSERT section of the toolbar.
It seems like you need either
% The exact value appears in the amplitude vector, and want to find what Time it occurs
Time(amplitude == value) % be careful here if you are comparing floating point values
or
% The exact value does not appear in the amplitude vector, so you want the
% first Time where you surpass that value
Time(find(amplitude>value,1)) % this assume the values of amplitude and time are sorted appropriately
You might also want to interpolate between times, but be more exact. Then you'll need the interp1 function.
##### 2 CommentsShowHide 1 older comment
the cyclist on 2 Feb 2023
You can't upload just your Time and amplitude vectors in a MAT file?

### Categories

Find more on Surface and Mesh Plots in Help Center and File Exchange

R2022b

### Community Treasure Hunt

Find the treasures in MATLAB Central and discover how the community can help you!

Start Hunting!