interp1nan, table lookup

looks up XI in X from the table [X,Y], if found returns Y, nan otherwise.

This function is the same operation as "table lookup". Described in "table lookup" terms, the "table" is [X,Y] and INTERP1nan "looks-up" the elements of XI in X, and, based upon their location, returns values YI within the elements of Y, and nan otherwise.