Is this a MATLAB bug? (Logical expression)

Hello,

I have just found something that I really don't understand...

Say I have a variable called x1 which has a value of 1, and a variable x2 which has a value of 2.

Now, if I logically say "x1 or x2 is greater than 1", I should get 1, as x2 is indeed greater than 1.

And if I type this:

x1 = 1;

x2 = 2;

x1|x2>1

I get a logical "1" as answer, as expected.

However it would seem to me that writing this would yield exactly the same result, as I just add some parenthesis:

x1 = 1;

x2 = 2;

(x1|x2)>1

But instead I get a logical "0" here. What am I missing? Why are the answers different from each other?

Thank you!

Answer 1

[Jan]

(x1 | x2) is TRUE, if at least one of them is not zero. If both are zero, FALSE is replied. In the next step TRUE is converted to 1 and FALSE to 0. Neither 1 nor 0 is greater than 1, so the expression (x1 | x2) > 1 replies FALSE as expected.

Your expression "x1 or x2 is greater than 1" means something else:

(x1 > 1) | (x2 > 1)

You can combine this in English, but not in Matlab or other similar programming languages.

The different between "x1 | x2 > 1" and "(x1 | x2) > 1" is the precedence order. In the first one, the > operator has a higher precedence than the | , so X2 > 1 is evaluated at first. Then you get x1 | FALSE, which is TRUE, because x1 is not zero.

See https://www.mathworks.com/help/matlab/matlab_prog/operator-precedence.html

[Steven Lord]

To put it another way, the expressions x1 | x2 > 1 and (x1 | x2) > 1 are not equivalent. The expressions x1 | x2 > 1 and x1 | (x2 > 1) are equivalent. Jan correctly wrote out the expression equivalent to the intent of your English description, (x1 > 1) | (x2 > 1).