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Power Law, Exponential and Logarithmic Fit

version 1.5 (20.9 KB) by

Finds and plots the linear fit to some data points when plotted on a log scale.

4.76471
17 Ratings

70 Downloads

Updated

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logfit(X,Y,graphType), where X is a vector and Y is a vector or a
matrix will plot the data with the axis scaling determined
by graphType as follows: graphType-> xscale, yscale
loglog-> log, log
logx -> log, linear
logy -> linear, log
linear -> linear, linear
A line is then fit to the scaled data in a least squares
sense.
See the 'notes' section below for help choosing a method.
logfit(X,Y), will search through all the possible axis scalings and
finish with the one that incurs the least error (with error
measured as least squares on the linear-linear data.)

Notes:
A power law relationship
[slope, intercept] = logfit(x,y,'loglog');
yApprox = (10^intercept)*x.^(slope);

An exponential relationship
[slope, intercept] = logfit(x,y,'logy');
yApprox = (10^intercept)*(10^slope).^x;

A logarithmic relationship
[slope, intercept] = logfit(x,y,'logx');
yApprox = (intercept)+(slope)*log10(x);

A linear relationship
[slope, intercept] = logfit(x,y,'linear');
yApprox = (intercept)+(slope)*x;

Comments and Ratings (18)

Zhida Li

Excellent! Thank you.

Ruijia Wang

JJ_unr

JJ_unr (view profile)

jeetesh jain

Sir I want to find the value of a and b so plz suggest the code for this curve

Yetti

Yetti (view profile)

@ashley, the b is the slope output. exp(intercept) gives a.

Great stuff. Do you know of a way to input powerlaw data and to determine the b in y = a x^b ?

Jonathan C. Lansey, you sir are a legend!
Many thanks for this function.
Would the be any way to extend the fitted line to meet x-axis at all?
Its not vital, merely for display purposes.
Many thanks anyway.
Nath

Jørgen Grythe

Perfect, exactly what I needed!

Great!
Thank you for your function!!

Budi

Budi (view profile)

very usefull,,

by the way, how to make non linear curve fit using custom equation when plot on log scale?

thx

Saskia

Saskia (view profile)

Franziska

Thanks a lot! Very helpful

Shea

Shea (view profile)

Just what the Dr. ordered. Thank You.

a

a (view profile)

Awesome!

Shatrughan

Saviour !!!!!!!!!!

Matlab2010

Nice.

I use:

% y = a x^b
function [a b] = my_powerLawFit(y, x)

logx=log(x);
logy=log(y);
p=polyfit(logx,logy,1);
b=p(1);
loga=p(2);
a=exp(loga);
end

Manas Hardas

Extremely cool!

Updates

1.5

Added new color option which lets you set the 'color' of both lines and markers with one parameter. Added robustness to NaN values.

1.4

Updated to use R2 as 'best fit' criterion rather than MSE

1.3

fixed 'skipbegin' feature functionality

1.2

Updated to include Mean Squared Error

MATLAB Release
MATLAB 7.11 (R2010b)

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