When is it appropriate to use a bitwise operator in a conditional expression?

Question

First, some background: I am an IT teacher-in-training and I'm trying to introduce the boolean operators of java to my 10th grade class. My teacher-mentor looked over a worksheet I prepared and commented that I could let them use just a single & or | to denote the operators, because they "do the same thing".

I am aware of the difference between & and &&.
& is a bitwise operator intended for use between integers, to perform "bit-twiddling".
&& is a conditional operator intended for use between boolean values.

To prove the point that these operators do not always "do the same thing" I set out to find an example where using the bitwise between boolean values would lead to an error. I found this example

boolean bitwise;
boolean conditional;
int i=10, j=12;
bitwise = (i<j) | ((i=3) > 5); // value of i after oper: 3
System.out.println(bitwise+ " "+ i);
i=10; 
conditional = (i<j) || (i=3) > 5 ;  // value of i after oper: 10
System.out.println(conditional+ " "+ i);
i=10; 
bitwise = (i>j) & (i=3) > 5;   // value of i after oper: 3
System.out.println(bitwise+ " "+ i);
i=10; 
conditional = (i>j) && (i=3) > 5;  // value of i after oper: 10
System.out.println(conditional+ " "+ i);

This example shows that if a value has to be changed by the second half of the expression, this would lead to a difference between the results, since bitwise is an eager operator, while the conditional behaves as a short circuit (does not evaluate the second half, if the first half is false in the case of && and true in the case of ||).

I have an issue with this example. Why would you want to change a value at the same time as you do a comparision on it? It doesn't seem like a robust way to code. I've always been averse to doing multiple operations in a single line in my production code. It seems like something a "coding cowboy" with no conscience as to the maintainability of his code would do. I know that in some domains code needs to be as compact as possible, but surely this is a poor practice in general?

I can explain my choice of encouraging the use of && and || over & and | because this is an accepted coding convention in software engineering.

But could someone please give me a better, even real-world, example of using a bitwise operator in a conditional expression?

Answer 1

it is appropriate when you are performing a masking operation

if ((a & b) > 0) { ... }

where a and b are integers

|| and | and && and & are not interchangable

| and & will amost never appear in a conditional expression by themselves (the point of the link you included is that such things are most often errors)

EDIT: Don't argue with your mentor; even if you win, you lose. Instead, explain that you don't want to confuse the students by mixing logical operators and bitwise operators in the same lesson. You could explain that if i=3 and j=2 then i&j = 2, while i&&j is an error. However, the simpler explanation is that you're teaching boolean (logical) operators, so throwing in special-case bitwise equivalents is a distraction from the main point of the lesson. There's no need to make the mentor "wrong", and no need to produce counter-examples. The focus of the lesson is on boolean operators, not bitwise operators.

As a corollary, when you start to teach bitwise operators, there's no need to show the special cases where + and - produce the same results as & and |

Answer 2

The non-bitwise operators && and || are short-circuit operators. In other words, with &&, if the LHS is false, the RHS will never be evaluated; with || if the LHS is true, then the RHS will never be evaluated. On the other hand, the bitwise operators & and | are non-short-circuit, and will always evaluate both the LHS and the RHS. Otherwise, they're equivalent in an if statement.

The only time I can see value in using the non-short-circuit operators is if the RHS has some kind of desirable side-effect that you want to happen in all cases. I can't think of a specific example where you would want this, and I don't believe it's good practice, but that's the difference.

Answer 3

The general philosophical answer is that using bitwise operators for boolean operators is atypical and makes the code harder to read. In practice (for code that is in production), more readable code is easier to maintain and thus more desirable.

For a real-world use of the need for short-circuit operators, see cases such as:

if (args.length > 0 && args[0] == 'test') ....

if (b != NULL && b.some_function()) ...

if (b == NULL || b.some_function()) ...

These type of operations show up frequently in real-world code.

Answer 4

You will use bitwise operators if you compare Bitmask enumerations. For example, you have an enumeration of states and an object that can be in more than one of those states. In this case, you'll do a bitwise or to assign more than one state to your object.

for example state = CONNECTED | IN_PROGRESS where CONNECTED could be 0x00000001 and IN_PROGRESS 0x00000010

For more info, look up flag enums documentation.

Answer 5

a simpler example of wrongness:

int condA=1, condB=2;

if (condA!=0 && condB!=0) {
    // correct!
}
if ((condA & condB)!=0) {
    // never executed
}

here you have two conditions, both are non-zero; but the bitwise & results in zero.