I cannot seem to get a second line for the forward finite difference

clc
clear
close all

h = [-2 -1.75 -1.5 -1.25 -1 -.75 -.5 -.25 0 .25 .5 .75 1 1.25 1.5 1.75 2];
x = [-2 -1.75 -1.5 -1.25 -1 -.75 -.5 -.25 0 .25 .5 .75 1 1.25 1.5 1.75 2];

y = @(x) x.^3 - 2*x + 4;   %start function
yd = @(x) 3*x.^2 - 2;      %first deriv
ydd = @(x) 6*x;            %second deriv

for i = 1:2
    for j = 1:17
        xi = x(i);
        hj = h(j);
        
        d1 = yd(xi);
        d2 = ydd(xi);
        
        %forward diff
        f1 = (y(xi+hj) - y(xi))/hj;
        f2 = (y(xi+2*hj) - 2*y(xi+hj))/(hj.^2);
        
        %backward diff
        b1 = (y(xi) - y(xi-hj))/hj;
        b2 = (y(xi)-2*y(xi-hj)+y(xi-2*hj))./(h.^2);
        
        %centered diff
        c1 = (y(xi+hj) - y(xi-hj))/(2*hj);
        c2 = (y(xi+hj) - 2*y(xi)+y(xi-hj))/(hj.^2);
        
        plot(x,yd(x));
        hold on
        plot(x,d2,'r');
        hold off
        legend('Analytic','Forward','Backward','Center')
    end
end

This is the code I've gotten so far. For the second plot I have tried everything I can think of and it will not plot. Why is this?

[Walter Roberson]

You should not be plotting inside your for j loop. You should be recording the results into a vector and plotting after the loop.

        plot(x,yd(x));

You current point is xi so yd(xi) what you should be concerned with recording for later.

Answer 1

[Image Analyst]

Looks like d2 is a scalar, not a vector (a list of values for each x value).