How to locate X value for a given Y value

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.

Answer 1

[Star Strider]

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')

.

Answer 2

[the cyclist]

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.

[David Harra]

Thanks for the reply

The type of data I have won't let me attach. My file is a .flxhst file and I use a function to read this data to obtain my time and amplitude data from a simulation.

I tried the code and it seems to give me something close to what I need but I wouild be interested in learning how to apply the interpolate function you mentioned. I have little room for error so may be more efficient

Time(find(amplitude>value,1)

[the cyclist]

You can't upload just your Time and amplitude vectors in a MAT file?