function z=spderiv(x,y,xi)%SPDERIV Cubic Spline Derivative Interpolation.% YI=SPDERIV(X,Y,XI) uses cubic spline interpolation to fit the% data in X and Y, differentiates the spline, and returns values% of the spline derivatives evaluated at the points in XI.%% PPD=SPDERIV(PP) returns the piecewise polynomial vector PPD% describing the cubic spline derivative of the curve described by% the piecewise polynomial in PP. PP is returned by the function% SPLINE and is a data vector containing all information to% evaluate and manipulate a spline. %% YI=SPDERIV(PP,XI) differentiates the cubic spline given by% the piecewise polynomial PP, and returns the values of the% spline derivatives evaluated at the points in XI.%% See also SPLINE, PPVAL, MKPP, UNMKPP, SPINTGRL% D.C. Hanselman, University of Maine, Orono, ME 04469% 1/27/95% Copyright (c) 1996 by Prentice-Hall, Inc. if nargin==3	pp=spline(x,y);else	pp=x;end[br,co,npy,nco]=unmkpp(pp);	% take apart ppif nco==1|pp(1)~=10	error('Spline data does not have the correct PP form.')endsf=nco-1:-1:1;							% scale factors for differentiationdco=sf(ones(npy,1),:).*co(:,1:nco-1);	% derivative coefficientsppd=mkpp(br,dco); 						% build pp form for derivativeif nargin==1	z=ppd;elseif nargin==2	z=ppval(ppd,y);else	z=ppval(ppd,xi);end