Not equal operator not working on matrix size

I have the following code:

A = [1,2,3];
B = [4,5,6,7];

if size(A) ~= size(B)
    disp("not equal 1")
else
    disp("equal 1")
end

if size(A) == size(B)
    disp("equal 2")
else
    disp("not equal 2")
end

This creates the following output:

equal 1
not equal 2

Why does == work as expected, but ~= doesn't? Did I make a mistake?

Answer 1

[Stephen23]

"Why does == work as expected, but ~= doesn't?"

They both work exactly as expected.

"Did I make a mistake?"

The IF documentation clearly states "An expression is true when its result is nonempty and contains only nonzero elements". Take a look at your comparison:

>> size(A)~=size(B)
ans =
  0  1

Question: Are both of those values non-zero? Answer: no. Therefore your IF condition will not be considered to be true, and the ELSE part will be evaluated instead.

The trivial solution is to always use all or any (depending on your logic):

>> all(size(A)==size(B))
ans = 0

or even better use isequal because this will also work without error for arrays of any size:

>> isequal(size(A),size(B))
ans = 0

TIP: this is exactly why it is recommended to read the documentation for every operator that you use, no matter how trivial you might think it is. Beginners would avoid a lot of bugs if they did this.

[Gergely Takáts]

What does

ans =
    0  1

mean? Does it mean, the first dimension is the same, but the second is not?

[Stephen23]

"Does it mean, the first dimension is the same, but the second is not?"

Yes. The logical equivalence operators are all element-wise, which means that they compare the first elements of each array, then second elements of each array, etc. and return the logical values for each of these comparisons.

The first example in the eq and ne documentation shows that quite clearly.

[Rik]

I would support it if Mathworks decided to throw a warning for a non-scalar input to if. If people want the current behavior, what is wrong with this syntax?

if ~isempty(expr) && all(expr)