derdata - Data Points Mth Derivative Using Specific Methods

This function finds the derivative m of a given data points (x,y) using any method you want at xx point.

% derdata
% This function finds the derivative m of a given data points (x,y) using
% any method you want at xx point.
% ----------------------------------------------
% inputs
% x x points
% y f(x)
% method forward, backward, central
% xx the point of interest
% m derivative order
% tv - (optional) true value of the derivative.
%
% x could be evenly spaced or unevenly spaced
% central means hybrid of forward and backward (more accurate)
% it does not mean the point should be in the middle.
% ---------------------------------------------------------------------
% output
% sol.value - the value of the derivative.
% sol.te - the true error (%) if true value is given
% sol.nx - number of points used to calculate the derivative
% ---------------------------------------------------------------------
% example
% x=1:9;
% y=x.^3;
% method='central';
% xx=3;
% m=2;
% sol=derdata( x,y,method,xx,m)
%
% ---------------------------------------------------------------------
% All copyrights goes to Mohammad Al-Fetyani
% University of Jordan