
#1
Dec1007, 09:45 PM

P: 202

I have a set of data and I need to fit a curve to it. The equation of the curve is:
y=20a*10*log(x) My problem is just getting a curve fit to this equation. What function in MATLAB should I be looking for? How do I get the value of 'a'? Is it easier to achieve this in Excel? How? 



#2
Dec1007, 11:50 PM

Emeritus
Sci Advisor
PF Gold
P: 10,424

Plot it, and then look in the menus in the plot window for a fitting tool.
All of the functions available in that menu are also available on the command line, but I find curve fitting to be much more intuitive when done in a plot window.  Warren 



#3
Dec1107, 09:24 AM

P: 202

will i be able to enter in my own custom equation? thanks




#4
Dec1307, 10:00 AM

P: 137

curve fit in MATLAB
In excel if you have all the points plotted out, it will curve fit the points to whatever order of polynomial you want.
In Matlab I am sure you can enter your custom equations. 



#5
Dec1307, 12:07 PM

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P: 2,197

Note that if you use functions from the Optimization toolbox (if you have that installed) you can quite easily fit to a nonlinear Matlab function, i.e. you can "fit" to a piece of code which generates an output that depends on an number of inputs, even if the function includes e.g. interpolation (which is often used conventient when fitting to a large set of experimental data), ifthen statements etc. This is often very useful. Also, if you are able to rewrite your equations as a matrix equation (e.g. a sum of exponentials with unknown prefactors) you can of course also solve it directly, i.e. using something like "X\b" which gives you the best result in the leastsquare sense. 



#6
Mar1408, 06:10 AM

P: 1

You can also try Ezyfit, a free curve fitting toolbox for Matlab:
www.fast.upsud.fr/ezyfit In your case just type showfit('y=20a*10*log(x)') if your data are plotted in a figure, or f = ezfit(x,y,'y=20a*10*log(x)') if your data are stored into the vectors x,y. 


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