How do I plot the relative error for each iteration vs. iteration number?
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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 #');
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Answers (1)
Karim
on 12 Nov 2022
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
fprintf('=====================================================\n')
figure
plot(ea_plot)
grid on
title('Error vs. Itteration #')
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