Getting x for a given y value from a cfit object

I have a cfit object:

[expfit, gof] = fit(-pos,sam_data,'exp1','Exclude',excluded);

From this I can extract the fit parameters (which in the case of 'exp1' are) expfit.a, expfit.b. From here, I could define the inverse equation to and solve for x by evaluating at y.

I want to find out what the x-value is at:

y = exp(-1)*max(sam_data)

without finding the inverse equation. How can I do this?

Answer 1

[Star Strider]

[jdl3141]

I want to find out what the x-value is at:

y = exp(-1)*max(sam_data)

without finding the inverse equation. How can I do this?

[Star Strider]

Try this:

pos = linspace(-5,5);                                           % Create Data
a = 3;                                                          % Guess The Parameter Value
b = 5;                                                          % Guess The Parameter Value
sam_data = a*exp(b*pos);                                        % Create Data
y = exp(-1)*max(sam_data);
xq = interp1(sam_data, pos, exp(-1)*max(sam_data));             % Desired Result

.

Answer 2

[Steven Lord]

x = 2*rand(20, 1);
y = 1.25*sin(x+0.3+rand(size(x)));
f = fit(x, y, 'sin1')

f = 
     General model Sin1:
     f(x) =  a1*sin(b1*x+c1)
     Coefficients (with 95% confidence bounds):
       a1 =       1.222  (1.043, 1.401)
       b1 =       1.149  (0.8509, 1.446)
       c1 =      0.5366  (0.1605, 0.9127)

desiredy = 0.75;
fun = @(x) desiredy - f(x);
desiredx = fzero(fun, 0)

desiredx = 0.1081

f(desiredx) % should be closer to desiredy

ans = 0.7500