csplineeval.m

Interpolation routines in matlab

%
% Evaluate the cubic spline which on [ x(i) , x(i+1) ] is defined by
%
%    S(z) = a(i) + b(i)*(z-x(i)) + c(i)*(z-x(i))^2 + d(i)*(z-x(i))^3
%
% at given points z
%
% function [s] = csplineeval( x, a, b, c, d, z )
%
% input:
%        x:     vector containing the 
%               interpolation points
%
%        a:     vector containing the 
%               coefficients a(i)
%
%        b:     vector containing the 
%               coefficients b(i)
%
%        c:     vector containing the 
%               coefficients c(i)
%
%        d:     vector containing the 
%               coefficients d(i)
%
%        z:     vector containing the 
%               points at which the spline has to evaluated
%
% output:
%        s:    value of the spline at the points z
%              if z(i) is not contained in the interval [ x(1), x(n) ],
%              then s(i) is set to NaN.
%
%

function [s] = csplineeval( x, a, b, c, d, z );


n = size(x(:),1);

m = size(z(:),1);


for i = 1:m
   if( z(i) >= x(1) & z(i) <= x(n) ) 
       j = 1;
       while  z(i) > x(j+1)
            j = j+1;
       end
       t    = z(i) - x(j);
       s(i) = ((d(j)*t+c(j))*t+b(j))*t+a(j);
   else
       s(i) = NaN;
   end
end