MATLAB: Interp1 using method ‘previous’ returns an unexpected NaN

Question

x = [1 2 4]; v = [17 100 17]; xq = 1:5; vq = interp1(x,v,xq,'previous');

vq = 17 100 100 17 NaN

Why is vq(5) not equal to 17?

Answer 1

This is clearly explained in the interp1 documentation, in the section "Extrapolation strategy":

• If you specify the 'pchip' or 'spline' interpolation methods, then the default behavior is 'extrap'.
• All other interpolation methods return NaN by default for query points outside the domain.

Your query point is outside of the input domain (from 1 to 4), so NaN is the correct and documented output.

As the documentation states, if you want to extrapolate, then you need to specify this:

 vq = interp1(x,v,xq,'previous','extrap')