How to skip elements of matrix in a loop?

Question

brandon fries 2018-11-28

0
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https://ww2.mathworks.cn/matlabcentral/answers/432641-how-to-skip-elements-of-matrix-in-a-loop

回答： Image Analyst 2018-11-28

Hi

If I have a matrix defined as:

V = zeros(20,20);

%sides at 1,-1

V(1:21,1) = 1;

V(1:21,21) = -1;

V(1:21,2:20) = 2;

%zero potential triangle

V(14,8:13) = 0;

V(13,9:13) = 0;

V(12,10:13) = 0;

V(11,11:13) = 0;

V(10,12:13) = 0;

V(9,13:13) = 0;

(two metal plates with v=1 and v=-1 and a triangle in the middle v=0)

I am implementing the jacobi method by:

for j=2:column_size-1;

for i=2:row_size-1;

if V(i,j)~=1 & V(i,j)~=-1 & V(i,j)~=0 %update

V_new(i,j) = (V(i-1,j)+V(i+1,j)+V(i,j-1)+V(i,j+1))/4;

delta_V_new = delta_V_new+abs(V_new(i,j)-V(i,j));

else

V_new(i,j) = V(i,j); %keep initial conditions

end;

The loop is not converging properly. I want to keep the same boundary conditions..is there a way to skip the elements of the triangle in the middle without using the if statement? I am fine with using the if statementfor the 1 and -1

Thanks

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Answer 1

Image Analyst 2018-11-28

0
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https://ww2.mathworks.cn/matlabcentral/answers/432641-how-to-skip-elements-of-matrix-in-a-loop#answer_349483

在 MATLAB Online 中打开

Just make up a mask with true or false where you want it to process, or skip. Then

for j=2:column_size-1;
    for i=2:row_size-1;
        if ~mask(i, j)
            % Mask is false here, so skip rest of the loop.
            continue; 
        end
        % Else mask is true, so do the rest of the loop.
    end
end