Question

I have two array 451×1 , I want to fit a line to a part of my data, for x=3.8 –4.1 , and I want to evaluate interception of fitted line with line y=0, Do you have any idea? (In matlab )

data

Answer 1

You can easily perform a linear regression by indexing the points of the curve you want to use and passing them to the function POLYFIT. Here’s the code to do it and a plot of the fit line:

index = (x >= 3.8) & (x <= 4.1);   %# Get the index of the line segment
p = polyfit(x(index),y(index),1);  %# Fit polynomial coefficients for line
yfit = p(2)+x.*p(1);  %# Compute the best-fit line
plot(x,y);            %# Plot the data
hold on;              %# Add to the plot
plot(x,yfit,'r');     %# Plot the best-fit line
axis([1 7 0 4e10]);   %# Adjust the axes limits

Then you can compute where this line intercepts the x-axis (i.e. y = 0) like so:

>> -p(2)/p(1)

ans =

    3.5264