interp1 using method 'previous' returns an unexpected NaN?

Question

Marc Cousoulis 2017-1-12

编辑： Stephen23 2017-1-12

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

Stephen23 2017-1-12

编辑：Stephen23 2017-1-12

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')