MATLAB: What is wrong with the while loop

3dMATLABmatrixwhile

I have this big matrix, 592x528x903 single, called X. I want to see if the X(:,:,1) is not equal to any other in the matrix. I use this code:
b=2;
while b < 902
    if X(:,:,1) ~= X(:,:,b)
        b
    end
    b=b+1;
end
disp done
I get nothing on the screen, except done, so first I thought they all where equal. The problem is that if i use "equal to", i get the same response, nothing on the screen.
b=2;
while b < 902
    if X(:,:,1) == X(:,:,b)
        b
    end
    b=b+1;
end
disp done
Something is not working, they can't be both equal and not equal at the same time, or am I wrong?

Best Answer

Your problem is that the result of
X(:,:,1) == X(:,:,b)
or
X(:,:,1) ~= X(:,:,b)
will return a logical matrix the same size as X(:,:,1) which apparently is never all true or all false and thus the if statement always evaluates to be false. The reson is this performs an element-wise comparison. If you want to check equality of the entire matrix, you should use the isequal function (isequaln if you want to ignore NaN in the matrices).
X = randi([0 1],2,2,902); % X with some equal pages.
b=2;
while b < 902
    if isequal(X(:,:,1),X(:,:,b)) % or isequaln
        b
    end
    b=b+1;
end
disp done
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