Help with linear fit

Given a curve (x vs y) I need to do a linear fit but only between two points x1 and x2 of that curve. Then, I need to calculate the slope of this line. Could you suggest me the fastest way to do that? Thanks.

Answer 1

[Mischa Kim]

Here is one way:

x      = 0:0.05:1;                          % define data
y      = sin(x).^2;

x1     = x(10);                             % define interval
x2     = x(20);

xrange = x(find(x == x1):find(x == x2));    % pick out x and y vals in interval
yrange = y(find(x == x1):find(x == x2));

p      = polyfit(xrange, yrange, 1);        % do linear curve fit 
yfit   = polyval(p, xrange);

figure
hold on; grid; box
plot(x,y,xrange,yfit,'-r')

The first element of p is the slope of the linear curve.

[aurc89]

It works, thank you very much!

Answer 2

[Jos (10584)]

Here's how:

tf = x >  x1 & x < x2     % true for x1 < x < x2
p = polyfit(x(tf), y(tf),1) % fit on selection

[aurc89]

Thanks Jos!