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

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Pablo Martínez Filgueira
编辑： Christy Zheng 2020-5-17
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?
I really appreciate any help you can provide.
Pablo

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采纳的回答

Torsten
Torsten 2015-3-3
Use the inverse function:
X=b1+c1*(log(a1./Y))^0.5
or
X=b1-c1*(log(a1./Y))^0.5
Best wishes
Torsten.
  1 个评论
Pablo Martínez Filgueira
Pablo Martínez Filgueira 2015-3-3
Thank you! It is a good proposal but I have many fits and I have to find a quick way to do it.
Good answer anyway ;)

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更多回答（2 个）

Andrei Bobrov
Andrei Bobrov 2015-3-3
other variant
X = arrayfun(@(y)fzero(f50(x)-y,1),Y);
  3 个评论
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Pen-Li (Ben) Yu
Pen-Li (Ben) Yu 2018-7-12
Below is my modification to Andrei's solution:
X = arrayfun(@(y)fzero(@(x)f50(x)-y,x0),Y)
x0 is the initial value for the fzero function Y can be a scalar or a vector
Christy Zheng
Christy Zheng 2020-5-17
编辑：Christy Zheng 2020-5-17
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?

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Ingrid
Ingrid 2015-3-3
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
  3 个评论
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Ingrid
Ingrid 2015-3-3
I have read your question but overlooked the fact that you were looking for the X values and not the Y values
in this case you might find this link helpful:
http://www.mathworks.com/matlabcentral/newsreader/view_thread/287031
Pablo Martínez Filgueira
Pablo Martínez Filgueira 2015-3-4
thanks, but during the 2 hours before asking this question I also found that topic.
see you

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