Need help with central difference method..

I am very new to matlab and this is homework so I would like to just keep it simple and within the scope of what I know.. but I'm trying to plot a central difference derivative of a function as well as that function on the same figure.. Basically nothings showing up.. Here's my code:

function [void] = Task_2 clear all close all

% define numerical grid a = -3; b = 8; npt = 25; x = linspace(a,b,npt); h = abs(x(2)-x(1))^2;

% function definition f = x .* sin(x);

x_2 = linspace(a+(b-a)/npt, b-(b-a)/npt,npt-2); Dom_df = [a+(b-a)/npt,x_2] for i=2:npt-1 df(i-1) = (f(i-1)-2*f(i)+f(i+1))/h^2; end

figure(1)

set(gca,'FontSize',18) plot(x_2,df(i-1),'LineWidth',1,'MarkerSize',100) title('Random plot') legend('xsin(x)','Location','NorthWest')

I've created the x_2 vector because I know x has 25 entries while df only has 23... But yeah.. Other than the random crap that might be lying around, does anyone know why I'm not getting a plot?

Thanks!

Answer 1

[A Jenkins]

Perhaps you mean:

plot(x_2,df,'LineWidth',1,'MarkerSize',100)

instead of:

plot(x_2,df(i-1),'LineWidth',1,'MarkerSize',100)

Answer 2

[Image Analyst]

Because you put (i-1) after df so you're just plotting the same y value all the time. Plus your marker size is way way too big - it will be off the chart!

plot(x_2, df, 'bd-', 'LineWidth',2, 'MarkerSize',10)

[James]

hey, thanks for the answer but that was just a mistake i've made during trying to figure out my problem!..

[James]

even if I fix that, it still doesn't plot anything :/

[James]

after adding 'bd-', I see that it is indeed plotting a straight horizontal line... but that shouldn't be right.. I'm guessing something is wrong with my iteration.. do you know what's going on?

By the way, I checked the entries for df, the vector does not contain constant values... so why the hell is it plotting a straight horizontal line as if to say df(i) = constant for all i?