how to find value of x for a known value of y(x), using fplot?

Hi !

I am using the function fplot to plot 5 functions. and the plot is working good.

now for each function I am trying to get the value of the x axis that makes the function equal to a certain value.

my current code is:

syms z(x)
z(x)=h(x)-f(x);
zValues=[0.6250    2.5000   10.0000   15.6250   22.5000];

%plot z(x)
figure;
fplot(z,'-','LineWidth',2)
axis([-35 35 -35 35]);
grid on;
yline(0);
xline(0);

and I get this plot:

I want to use the variable zValues to get 5 x values. such that for the blue line z(x) = 0.625 then x= ...

is there a way to do that?

Edit/ Note:

if the function z(x) was only one plot ( say the blue one ) I can use this:

solve(z(x)==zValues,x)

in this case zValues would be be one constant not a vector.

[darova]

Do you have polyxpoly? Or intersections

Answer 1

[Nora Khaled]

The only way I was able to do this is to change z(x).

insted of having all the lines in one plot. I created a loop such that at each eteration z(x) is one line.

then I used

solve(z(x)==zValues,x)

it takes time but I got the results.

[Walter Roberson]

Switch to vpasolve() instead of solve(), it will go faster.

Also for convenience you can use arrayfun:

arrayfun(@(zV) double(vpasolve(z(x) == zV, x)), zValues)

[Nora Khaled]

Thank you!

one thing though, I don't understand what the array function does?

[Walter Roberson]

https://www.mathworks.com/help/matlab/ref/arrayfun.html