How do I apply exponential and logarithmic curve fitting
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Hi,
I have some scatterplots and I want to check the relationships between the variables which resemble exponential and logarithmic functions. I have tried to use the functions
nlinfit
fittype
fit
but so far not successfully (poor fitting or code not working at all).
How can I check the curves for the above scatters which seem to have the following functions:
y= a*exp(b*x)+c and y=log_a(x)+b
in matlab?
Thanks
Iro
3 个评论
Star Strider
2014-2-19
What do you mean by ‘code not working at all’?
It’s not obvious to me what you mean here: y=log_a(x)+b. (I don’t understand ‘log_a(x)’.)
yeungor
2016-9-25
Have you tried fitting an exponential of the inverse? If x = f(y), then y = f^-1(x) and you can use 'fit' and the 'exp' for an exponential fit.
采纳的回答
Star Strider
2014-2-22
The nlinfit function requires that the first argument of the objective function is the parameter vector and the second the vector of independent variables. Using anonymous functions,
y = a*exp(b*x)+c
becomes
y = @(B,x) B(1).*exp(B(2).*x) + B(3); % B(1) = a, B(2) = b, B(3) = c
For the logarithmic fit, all logs to various bases are simply scaled by a constant. Consider:
a^y = b^x
Taking the log to base a (denoted by loga()) of both sides gives:
y = x*loga(b)
so the log to any base will work.
The anonymous function for your logarithmic regression is then:
y = @(B,x) log(x) + B; % B = b
or alternatively,
y = @(B,x) B(1).*log(x) + B(2);
Those should work with nlinfit.
更多回答（1 个）
Danilo NASCIMENTO
2014-2-19
0 个投票
You can use cf = fit(x,y,'exp1'); where x and y are your set of points.
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