How do I return something only if all steps of an conditional statement is true?

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Allison Bushman 2019-9-12

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https://ww2.mathworks.cn/matlabcentral/answers/480187-how-do-i-return-something-only-if-all-steps-of-an-conditional-statement-is-true

编辑： Stephen23 2019-9-13

I am attempting to generate a random 4x4 matrix and if all the real parts of the eigenvalues are negative, then compute the inverse matrix of the original random matrix.

m=4;
G=randn(m);
H=eig(G);
J=real(H);
[rowsJ,colsJ]=size(J);
for r=1:rowsJ
    if J(r,1)<1
        K=G^-1
else
    printf("Unable to Compute")
    end
end

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Walter Roberson 2019-9-12

You should be testing J<0 rather than J<1 if you want negative.

John D'Errico 2019-9-12

Oh, then the test is just

all(real(eig) < 0)

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采纳的回答

Walter Roberson 2019-9-12

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/480187-how-do-i-return-something-only-if-all-steps-of-an-conditional-statement-is-true#answer_391647

在 MATLAB Online 中打开

if all(J(:)<1)
    K=inv(G);
else
    K = [];
    printf("Unable to Compute")
end

However... you should rarely be computing the inverse of a matrix. In most cases you should be using other approaches than inv(), such as using the \ operator.

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Allison Bushman 2019-9-12

Okay. Thank you

Stephen23 2019-9-13

编辑：Stephen23 2019-9-13

"but why should I not be computing the inverse of a matrix?"

inv is slower and less accurate than the recommended methods of solving systems of linear equations.

The inv documentation explains: "It is seldom necessary to form the explicit inverse of a matrix. A frequent misuse of inv arises when solving the system of linear equations Ax = b. One way to solve the equation is with x = inv(A)*b. A better way, from the standpoint of both execution time and numerical accuracy, is to use the matrix backslash operator x = A\b."

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