Error in comparing equality of two matrices

Question

N/A 2021-12-24

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1616850-error-in-comparing-equality-of-two-matrices

评论： Walter Roberson 2021-12-27

Hi, I am trying to add a conditional statement which compares whether two matrices are equal and generates a result. But it fails everytime. Please find my code below:

alpha = input("Enter value of alpha: ")
beta = input("Enter the value of beta: ")
gamma = input("Enter the value of gamma: ")
R_BA = rotz(alpha*pi/180)*roty(beta*pi/180)*rotx(gamma*pi/180)
X = R_BA(:,1)
Y = R_BA(:,2)
Z = R_BA(:,3)
mod_X = X(1).^2 + X(2).^2 + X(3).^2
mod_Y = Y(1).^2 + Y(2).^2 + Y(3).^2
mod_Z = Z(1).^2 + Z(2).^2 + Z(3).^2
if int64(mod_X)==int64(mod_Y)==int64(mod_Z)
    disp("X, Y and Z vectors are unit vectors")
else
    disp("Unitary condition not satisfied")
end
if dot(X,Y) == dot(Y,Z) == dot(Z,X)
    disp("Orthonormal condition satisfied")
else
    disp("Orthonormal condition not satisfied")
end
inv_RBA = inv(R_BA) %inverse of rotation matrix
transpose_RBA = transpose(R_BA)
if isequal(inv_RBA,transpose_RBA)
    disp("Property satisfied")
else
    disp("Fail")
end
    

The code at the end checks if inv_RBA is equal to transpose_RBA. The two matrices have the same exact elements but I do not get a positive output.

Highly appreciate your help here.

Thanks

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Answer 1

Chunru 2021-12-24

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在 MATLAB Online 中打开

Use "isequal" for matrix comparison and "&&" for logic and.

% if int64(mod_X)==int64(mod_Y)==int64(mod_Z)
if isequal(int64(mod_X), int64(mod_Y) && isequal(int64(mod_Y), int64(mod_Z))
    ...
end

The same applies to "if dot(X,Y) == dot(Y,Z) == dot(Z,X)" as well.

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Stephen23 2021-12-27

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Note that ISEQUAL accepts any number of input arguments (not just two), so this simplifies to:

isequal(int64(mod_X), int64(mod_Y, int64(mod_Z))

Note that this is unlikely to be useful as it does not take into account the binary floating point error of those calculations.

It is not clear what hypothesis converting to INT64 (which rounds the values) is expected to confirm or reject.

Walter Roberson 2021-12-27

I suspect that they were thinking of typecast() to uint64 and then comparing those underlying representations. But that does not give any more utility than comparing double precision.

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Answer 2

N/A 2021-12-27

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https://ww2.mathworks.cn/matlabcentral/answers/1616850-error-in-comparing-equality-of-two-matrices#answer_862405

在 MATLAB Online 中打开

I had a problem using the last "if" statement in my code where I was not able to compare inv_RBA and transpose_RBA. I figured out this way to compare them both:

k = 0;
for m = 1:size(inv_RBA)
    for n = 1:size(transpose_RBA)
        
        if isequal(i,j)
            continue
        else
            disp("Fail")
            k = 1
        end
    end
end
if k==0
    disp("Property satisfied")
end

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N/A 2021-12-27

Hi, it was not entirely clear to me as I'm a beginner, but thanks for your feedback. Would you be able to recommend a better way to compare each element of these matrices and generate a result which says both are equal?

Walter Roberson 2021-12-27

在 MATLAB Online 中打开

all( abs(inv_RBA - transpose_RBA) < TOLERANCE )

where TOLERANCE is some value that you feel is "close enough" to equal. For example if the values are near 2, then you might feel that 1e-13 was "close enough"

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