Displaying empty array as output

Question

Mughees Asif 2019-3-19

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链接

此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/450960-displaying-empty-array-as-output

回答： Guillaume 2019-3-19

function B=IsStable(polynomial)
if AllNonZero(polynomial) == false
H = [];
B = 0;
elseif AllSameSign(polynomial) == false
H = [];
B = 0;
else
    H=HurwitzMatrix(polynomial);
        pm = length(H); 
    minor = zeros(1,pm);
        for i = 1:pm
               minor(i) = det(H(1:i,1:i)); 
        end
            if minor > 0
                B = 1;
                disp(H)
            else 
                B = 0;
                disp('[ ]')
            end
end
end 

The end if statement for the 'minor' displays the H matrix when B = 1 but does not output the disp('[ ]') when B = 0. Any suggestions?

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Adam 2019-3-19

Is the else part of that of statememnt ever get executed? I assume not, otherwise there is no reason why [ ] would not be displayed.

Adam Danz 2019-3-19

Those lines of code will only be accessed when all of the following are TRUE

AllNonZero(polynomial) == true
AllSameSign(polynomial) == true
minor <= 0

'B' can be equal to 0 under several different conditions so just because B equals zero doesn't mean that those lines of code were accessed.

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Answer 1

Guillaume 2019-3-19

0
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https://ww2.mathworks.cn/matlabcentral/answers/450960-displaying-empty-array-as-output#answer_366113

在 MATLAB Online 中打开

Hopefully, your real code doesn't use the same indentation as what you've posted above. If it does, I suggest that you let matlab reindent your code (press CTRL+I in the editor).

"The end if statement for the 'minor' displays the H matrix when B = 1"

Your sentence makes no sense. The if statement does not test the value of B. So whether B is 1 or 0 does not matter to what happens.

What the if tests is the values of minor. Note that minor is a vector and therefore minor > 0 is a logical vector. I would suspect that you do not know what happens when you pass a vector to if. What you have written is equivalent to:

if all(minor > 0)  %if is true if and only if ALL elements of minor are > 0
    B = 1;
    disp(H);  %so only displayed when ALL of minor is > 0
else   %therefore else is executed if just one element of minor is <= 0
    B = 0;
    disp('[]');
end   