What is wrong with my while loop?

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?

Answer 1

[Brendan Hamm]

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

[Jet Verheij]

Thank you!

Answer 2

[KL]

Comparing matrices is easier if you use isequal command.

https://www.mathworks.com/help/matlab/ref/isequal.html

You should use it like,

 if(isequal(X(:,:,1),X(:,:,b)))

What you're actually doing while using == or ~= is extracting a logical matrix as the result. Check this below example,

A = [1,2;3 4];
B = [1 0;3 4];
A==A
ans =
     1     1
     1     1
A==B
ans =
     1     0
     1     1

You see the problem?

Whereas, if you use isequal,

isequal(A,A)
ans =
     1
isequal(A,B)
ans =
     0

[Jet Verheij]

Thank you!