Extrapolating data outside given data

Hi,

I need to extrapolate 15 points of data with 3 given corresponding x and y values. is there any way to do it?

%x = 1:15, y = 9.85 9.82 9.84 (extrapolate to 15)

any help would be greatly appreciated !!

[Walter Roberson]

Could you confirm that you have an x vector of length 3, and a y vector of length 3?

Is the task to extrapolate y where x would be 15? Or is the task to interpolate at 15 different locations, such as linspace(min(x), max(x), 15) ?

[Taha Rizvi]

Hi Walter,

sorry for that, the current x vector is x = [1 2 3] to correspond with that y vector.

and yes i need to extrapolate data all the way up to 15, so in total i would have 15 y values for 15 x values. like you said linspace(min(x), max(x), 15)

Answer 1

[Walter Roberson]

xq = linspace(min(x), max(x), 15);
yq = interp1(x, y, xq);

This does not extrapolate to x = 15, this interpolates at 1, 8/7, 9/7, 10/7, 11/7 and so on.

If you were trying to extrapolate out to x = 15, then

xq = 1 : 15;
yq = interp1(x, y, xq, 'linear', 'extrap')

or 'spline', 'extrap'

Or you could,

xq = 1 : 15;
[P, S, MU] = polyfit(x, y, 2);
yq = polyval(P, xq, S, MU);

[Taha Rizvi]

Im a beginner in Matlab, could you explain how that code is able to extrapolate from those values please?