How do I plot the relative error for each iteration vs. iteration number?

How do I plot the relative error for each iteration vs. iteration number? I used Gauss-Seidel to solve the initial problem, but the plot comes up blank. How do I fix this? Thanks!

clear
clc
A = [4 1 2; 3 8 1; 2 3 6];
B = [200 400 300]';
x = ones(1,length(B))';

[m,n] = size(A); 
c = zeros(m,n); 
d = zeros(m,1); 

ea = 100;
tol = 0.000006;
iter = 1;

while ea > tol
    for i = 1:n 
        d(i) = B(i)/A(i,i);
        for j = 1:m
            if i == j 
                c(i,j) = 0; 
            else 
                c(i,j) = A(i,j)/A(i,i);
            end 
        end
     
        x_new(i) = d(i) - c(i,:)*x(:) ;
                
        ea_vect(i) = abs((x_new(i) - x(i))/x_new(i)*100); 
         
        x(i) = x_new(i); 
    end 
    
    ea = max(ea_vect); 
    
    fprintf('%d %9.4f %9.4f %9.4f %9.4f\n', iter, x(1),x(2),x(3),ea)

    iter = iter + 1;  
end 
fprintf('=====================================================\n')

hold on
plot(ea,iter)
grid on;
axis ([0 11 0 100])
title('Error vs. Itteration #');

Answer 1

[Karim]

Hey, you need to save the value in order to plot it. I added an extra variable (ea_plot) to achieve this, see below.

A = [4 1 2; 3 8 1; 2 3 6];
B = [200 400 300]';
x = ones(1,length(B))';

[m,n] = size(A); 
c = zeros(m,n); 
d = zeros(m,1); 

ea = 100;
tol = 0.000006;
iter = 1;

ea_plot = [];

while ea > tol
    for i = 1:n 
        d(i) = B(i)/A(i,i);
        for j = 1:m
            if i == j 
                c(i,j) = 0; 
            else 
                c(i,j) = A(i,j)/A(i,i);
            end 
        end
        
        x_new(i) = d(i) - c(i,:)*x(:) ;
        
        ea_vect(i) = abs((x_new(i) - x(i))/x_new(i)*100); 
        
        x(i) = x_new(i); 
    end 
    
    ea = max(ea_vect); 
    
    % store ea for plotting
    ea_plot(end+1,1) = ea;
    
    fprintf('%d %9.4f %9.4f %9.4f %9.4f\n', iter, x(1),x(2),x(3),ea)
    
    iter = iter + 1;  
end 

1   49.2500   31.4062   17.8802   97.9695
2   33.2083   35.3118   21.2746   48.3061
3   30.5347   35.8902   21.8767    8.7560
4   30.0891   35.9820   21.9793    1.4809
5   30.0149   35.9970   21.9965    0.2474
6   30.0025   35.9995   21.9994    0.0413
7   30.0004   35.9999   21.9999    0.0069
8   30.0001   36.0000   22.0000    0.0011
9   30.0000   36.0000   22.0000    0.0002
10   30.0000   36.0000   22.0000    0.0000
11   30.0000   36.0000   22.0000    0.0000

fprintf('=====================================================\n')

=====================================================

figure
plot(ea_plot)
grid on
title('Error vs. Itteration #')