Extrapolation using 2 end points on Octave/Matlab interp1

Question

I am trying to interpolate the points within a dataset but to extrapolate any x values outside of the dataset range by assuming the 2 end points continue with the same y value. I am using interp1 to carry out the interpolation. However, I see that Octave or Matlab only allows extrapolation of a single scalar value. Can someone advise how I can extrapolate using 2 separate values instead?

Answer 1

In matlab if you specify 'extrap' than it will extrapolate using the same method as interpolation, if your interpolation is not "nearest" than you can set the outside values manually:

x=[0,1,2,3];
v=[0,1,-1,5];

xq=linspace(-5,5,100);

vq = interp1(x,v,xq,'linear','extrap');
vq(xq>max(x))=v(x==max(x));
vq(xq<min(x))=v(x==min(x));

plot(x,v,'*',xq,vq)