%% HOW DO I DO THAT IN MATLAB SERIES?

% In this series, I am answering questions that students have asked

% me about MATLAB. Most of the questions relate to a mathematical

% procedure.

%% TOPIC

% How do I find the first derivative of a discrete function y(x) if the

% x values are equidistant.

%% SUMMARY

% Language : Matlab 2010a;

% Authors : Autar Kaw and Sri Garapati;

% Mfile available at

% http://nm.mathforcollege.com/blog/discrete_diff_equidistant_blog.m

% Last Revised : January 17, 2012;

% Abstract: This program shows you how to differentiate discrete data if

% the x values are equally spaced

clc

clear all

%% INTRODUCTION

disp(‘ABSTRACT’)

disp(‘ This program shows you how to differentiate discrete data if’)

disp(‘ the x values are equally spaced ‘)

disp(‘ ‘)

disp(‘AUTHOR’)

disp(‘ Autar Kaw and Sri Garapati of http://autarkaw.wordpress.com’)

disp(‘ ‘)

disp(‘MFILE SOURCE’)

disp(‘ http://nm.mathforcollege.com/blog/discrete_diff_equidistant_blog.m’)

disp(‘ ‘)

disp(‘LAST REVISED’)

disp(‘ January 17, 2012’)

disp(‘ ‘)

%% INPUTS

% Inputs assuming

% that three or more points are given

% all x values are equidistant

% x values are in ascending or descending order.

x=[2.3 3.4 4.5 5.6 6.7 7.8];

y=[4.6 7.9 13.0 12.3 3.2 1.9];

%% DISPLAYING INPUTS

disp(‘ ‘)

disp(‘INPUTS’)

% Creating a matrix to print input data as a table

data=[x;y]’;

disp(‘ X Data Y Data’)

% Printing the input data as a table

disp(data)

%% THE CODE

% n returns the number of data points

n=length(x);

% delta is the distance between consecutive x values

delta=x(2)-x(1);

% “dy” is an array which stores the value of the derivative at each x-value

% finding the derivative from the 2nd order accurate forward divided

% difference formula at the first data point

dy(1)=(-y(3)+4*y(2)-3*y(1))/(2*delta);

% finding the derivative from the 2nd order accurate central divided

% difference formula at the second to (n-1)th data point

for i=2:1:n-1

dy(i)=(y(i+1)-y(i-1))/(2*delta);

end

% finding the derivative from the 2nd order accurate backward divided

% difference formula at the first data point

dy(n)=(y(n-2)-4*y(n-1)+3*y(n))/(2*delta);

% creating a matrix with input data and calculated derivative value at

% each data point for printing as a table

xdy=[x’ y’ dy’];

%% DISPLAYING OUTPUTS

disp(‘ ‘)

disp(‘OUTPUTS’)

disp(‘ XData YData Derivative’)

% printing the input data points and calculated derivative values(Outputs)

disp(xdy)

_________________________________________________

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