# How can I find X for given Y value in a fitted curve?

147 views (last 30 days)
Edited: Christy Zheng on 17 May 2020
Hi everyone.
I would like to know how to find the X value of a fitted curve given an Y value.
I had some data and fitted them using Gaussian fit. Now they look like this in my workspace:
An example of these functions as seen in the Command Window is:
f50 =
General model Gauss1:
f50(x) = a1*exp(-((x-b1)/c1)^2)
Coefficients (with 95% confidence bounds):
a1 = 7.81 (7.602, 8.018)
b1 = 1.497e-013 (-0.001774, 0.001774)
c1 = 0.08174 (0.07924, 0.08425)
Does someone wonder how to do it?
Pablo

Torsten on 3 Mar 2015
Use the inverse function:
X=b1+c1*(log(a1./Y))^0.5
or
X=b1-c1*(log(a1./Y))^0.5
Best wishes
Torsten.
Thank you! It is a good proposal but I have many fits and I have to find a quick way to do it.

Andrei Bobrov on 3 Mar 2015
other variant
X = arrayfun(@(y)fzero(f50(x)-y,1),Y);
Christy Zheng on 17 May 2020
Edited: Christy Zheng on 17 May 2020
I'm sorry for my lack of understanding, but to clarify X is the value you are looking for given a Y on the fitted curve. What are x and y referring to?

Ingrid on 3 Mar 2015
you can do this with the function
Y = feval(cfit,X);
which you could have easily found yourselve if you would have typed
doc cfit
thanks, but during the 2 hours before asking this question I also found that topic.
see you

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