Why do I get an infinity number in matrix X2?

Question

Alexandra Panayiota Gregoriou 2022-3-5

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https://ww2.mathworks.cn/matlabcentral/answers/1664074-why-do-i-get-an-infinity-number-in-matrix-x2

编辑： Torsten 2022-3-6

I am using the Jacobi method in matlab but when I run the program the matrix X2 seems to have a problem. I think its something to do with the while loop?

My program in the editor is

A=[2 -1 1; 3 -3 9; 3 3 5];
B=[7; 18; 14];
X1=[1; 0; 0];
s=3;
X2=zeros(3,1);
i=1;
j=i;
X3=rand(3,1);
while X2~=X3
    for i=1:s
        sum=0;
        if X2~=X3
            for j=1:s
                if i~=j
                    sum=sum+A(i,j)*X2(j,1);         
                end
            end
            X3(i,1)=(1/A(i,i))*(B(i)-sum);
        end
    end
    X2=X3;
    X3=ones(3,1);
end
X1
X2
X3

Plsss help me I have to submit this assignment asap

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Answer 1

Jan 2022-3-5

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X3 is growing massively until it reachs Inf. Simply insert some output, e.g. by removing the semicolon from "X2=X3;" to observe this.

This happens, because this is, what the code instructs Matlab to do. I cannot recommend a fixing, because I do not know, what you want to calculate instead.

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Image Analyst 2022-3-6

@Alexandra Panayiota Gregoriou, Also, don't use "sum" as the name of your variable since it's the name of an important built-in function. Call it "theSum" or something else.

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Answer 2

Torsten 2022-3-6

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https://ww2.mathworks.cn/matlabcentral/answers/1664074-why-do-i-get-an-infinity-number-in-matrix-x2#answer_911229

编辑：Torsten 2022-3-6

在 MATLAB Online 中打开

function main
   %A = [2 -1 1; 3 -3 9; 3 3 5];
   %b = [7; 18; 14];
   %x0 = [1; 0; 0];
  A = [4 -1 -1; -2 6 1; -1 1 7];
  b = [3; 9; -6];
  x0 = [0; 0; 0];
  Eps = 1e-8;
  x = JacobiSolve(A, b, x0, Eps);
  x
  A*x-b
end
function x = JacobiSolve(A, b, x0, Eps)
  n = length(b) ;
  x = zeros(size(x0));
  Error = 1;
  while Error >= Eps 
    for i = 1 : n
      x(i) = b(i) ;
      for j = 1 : n
        if j ~= i
          x(i) = x(i) - A(i, j)*x0(j) ;
        end
      end
      x(i) = x(i) / A(i, i) ;
    end 
    Error = norm(x-x0, inf);
    x0 = x; 
  end
end