l1.m

fit programme for a serials of data

function X=L1(x,y,x1)
% input x is a vector that contains a list of abscissas
%       y is a vector that cntains a list of ordinates
%       x1 is the number at where the interpolation is to be obained
% output X is the linear interpolation at x1
n=length(x);
j=-1;
for i=1:n-1
    if x1<x(i+1)&&x1>x(i)
       j=i;
       break;
    end
end
if j==-1
   'false';
   return;
end
X=y(j)*(x1-x(i+1))/(x(i)-x(i+1))+y(i+1)*(x1-x(i))/(x(i+1)-x(i));