# Gauss-Seidel code gives wrong answers

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kemgi on 17 Apr 2020
Commented: kemgi on 23 Apr 2020
Hello,
I am hoping someone can assist me with this problem. I created a Gauss-Seidel code that will allow me to solve a set of linear equations, finding x1, x2 x3 and x4. I made two matrices; A=[4 -21 -7 1; -4 0 -3 11 ; 4 -1 10 -1; 151/8 5 8 -3] and b =[11; 15; 19; -12;]. Through using the equatio Ax=b i would be able to find the unknows, which worked using the backslash built in solver. But my code below give 4 values for x but they do not match the reuslts of the built in solver and do not equate when substitued back into a line of the linear equations.
A major aspect of the code is that it is meant to make your matrix diagonally dominant to solve.
My code is as follows:
function gauss-seidel
A=input('write matrix a')
b=input('write matrix b')
x=linspace(0,0,length(A))';
n=size(x,1);
normVal=Inf;
nmax=1000; %number of maximum iterations which can be reached%
tol=0.000001; % Tolerence for method%
iter=0;
while normVal>tol && iter<nmax
x_old=x;
[V,A]=eig(A);
TF=isdiag(A);
if TF==0
fprintf('the matrix is not diagonally dominant')
else
for i=1:n
guess=0;
for j=1:i-1
guess=guess+A(i,j)*x(j);
end
for j=i+1:n
guess=guess+A(i,j)*x_old(j);
end
x(i)=(b(i)+guess)/(A(i,i));
end
iter=iter+1;
normVal=norm(x_old-x);
end
end
fprintf('Solution of the system is : ')
for i=1:length(x)
fprintf(' %1.4f ',x(i));
end
fprintf('in %d iterations ',iter)

Prabhan Purwar on 22 Apr 2020
Hi,
Seems duplicate questions are posted by mistake, kindly refer to ans using following link:
Thanks.

#### 1 Comment

kemgi on 23 Apr 2020
Thank you ?