already know function: find x value given y AND plot (x,y) on the same graph

Can someone help me with this simple question? I already know the function expression. I would like to find the x value corresponding to y=0.37 and plot the point (x,y) with dash lines connecting to x/y axis in the same graph on top of the line for y?

Here is my current code:

 x = 0:100000;
 y = exp(-x/38190.7);
 figure
 semilogy(x,y)
 grid on

Thank you!

Answer 1

[Star Strider]

This works:

x = 0:100000;
ofst = 0.37;
y = @(x) exp(-x/38190.7);                           % Create Anonymous Function
xval = fzero(@(x) y(x)-ofst, 1)                     % Solve For ‘ofst’ Value

figure(1)
plot(x, y(x))
hold on
plot([xval xval], [0 ofst], '--')
plot([0 xval], [ofst ofst], '--')
plot(xval, ofst, 'bp')
hold off
grid

[Matlab_Student]

Hi, thank you very much for the answer! When I changed plot(x, y(x)) to semilogy(x, y(x)) the dash line connecting to x-axis disappeared. Do you know how to fix this?

[Star Strider]

My pleasure!

The reason is the log of zero is -Inf, so the zero didn’t plot. The fix was to change the zero to min(ylim) in the first plot call in the hold block.

This works:

x = 0:100000;
ofst = 0.37;
y = @(x) exp(-x/38190.7);                           % Create Anonymous Function
xval = fzero(@(x) y(x)-ofst, 1)                     % Solve For ‘ofst’ Value

figure(1)
semilogy(x, y(x))
hold on
plot([xval xval], [min(ylim) ofst], '--')
plot([0 xval], [ofst ofst], '--')
plot(xval, ofst, 'bp')
hold off
grid

[Walter Roberson]

When you have

plot([0 xval], [ofst ofst], '--')

then that first one has x = 0. You have a log plot so log(0) is the left coordinate. And log(0) is -infinity . MATLAB never draws lines when the coordinate is infinite. You need to change that 0 to some positive value. Likewise in the other plot() line.

[Kalyan Dash]

The code works fine. Can you please elaborate the code

plot([xval xval], [min(ylim) ofst], '--')
plot([0 xval], [ofst ofst], '--')
plot(xval, ofst, 'bp')

[Star Strider]

MATLAB requires two values for the x and two values for y in order to plot the line. It will plot a constant line at a given x or y if both values of either the x or y coordinates are the same value.