How do I extract the exponent from a semilogy plot?

2019 2 26
1 个回答
回答已采纳
更新时间：2019 2 26
4 次查看（30 天）

0 个投票

If I use semilogy to plot some data such that the plot appears as a straight line, how can I extract the exponent of this line? Can I use polyfit to get this or is there a different way?

请先登录，再回答此问题。

 采纳的回答

Stephan
Stephan 2019-2-26
编辑：Stephan 2019-2-26

0 个投票

Hi,
i think you should use lsqcurvefit (requires Optimization Toolbox) or fit (requires Curve Fitting Toolbox) - depending on the toolboxes you have access to. I would expect an exponential function if there is a straight line in the semilogy plot. Due to this polyfit is not the correct choice. I recommend to work through this examples:
lsqcurvefit:
https://de.mathworks.com/help/optim/ug/lsqcurvefit.html#buuu2mv-1
fit:
https://de.mathworks.com/help/curvefit/fit.html#bto2vuv-9
In case of lsqcurvefit the resulting elements of solutiuon x contain the needed exponent. In case of fit you get the values with the coeffvalues function.Therefore see also Fit Postprocessing.
Best regards
Stephan

4 个评论
显示 2更早的评论 隐藏 2更早的评论

Janna Hinchliff
Janna Hinchliff 2019-2-26
Hi, thanks for your help, I'm having a go with fit but struggling to get it to work. I have a very small data set , with
x = [2.3561 2.4831 2.6233];
y = [0.2452 1.4632 1.6916];
which I'm trying to fit to the curve a*exp(-b*x). The fit isn't very good but I need to look into how good it is so I need to fit something of this form to it. I tried to use
expfit = fittype('a*exp(-b*x)','dependent',{'y'},'independent',{'x'},'coefficients',{'a','b'})
fitEa = fit(x',y',expfit)
but this gave me the error
Function 'subsindex' is not defined for values of class 'fittype'. How can I do this correctly?
Stephan
Stephan 2019-2-26
编辑：Stephan 2019-2-26
I dont get an error with your fittype - which Matlab Release do you use?
x = [2.3561 2.4831 2.6233];
y = [0.2452 1.4632 1.6916];
expfit = fittype('a*exp(-b*x)','dependent',{'y'},'independent',{'x'},...
'coefficients',{'a','b'})
f = fit(x',y',expfit);
fvals = f(x);
format long
coeffs = coeffvalues(f)
names = coeffnames(f)
format short
subplot(2,1,1)
plot(f,x,y)
legend('Location','southeast')
subplot(2,1,2)
semilogy(x,fvals)
ylim([0 2])
Janna Hinchliff
Janna Hinchliff 2019-2-26
No worries, I sorted it - I had another variable stored as fit from a different part of the code! Thanks for your help!
Stephan
Stephan 2019-2-26
No problem - if this was useful for you please accept my answer.

请先登录，再进行评论。

更多回答（0 个）

类别

在 帮助中心 和 File Exchange 中查找有关 Get Started with Curve Fitting Toolbox 的更多信息

标签

Community Treasure Hunt

Find the treasures in MATLAB Central and discover how the community can help you!

Start Hunting!

Translated by