using += and ternary operator in java clarification

Question

I was running through some practice questions for an upcoming exam and came across a question that me nor my class mates can seem to understand. It is as follows:

where all variables are int or int array.

score += (rounds[i]) ? i + START : 0

How does the ternary operator work in java with += ? This is my understanding:

so it is score += round[i] == i+start or == 0.

Is this a correct understanding?

Kind regards, James

Answer 1

As with any combination of operators, it's a question of operator precedence and (where the operators have the same precedence) associativity. In Java, the simple assignment and all the operator/assignment operators share the lowest precedence tier. The ternary operator is the sole occupant of the next higher precedence tier. Therefore, your expression is equivalent to

score += ((rounds[i]) ? (i + START) : 0)

That is, the ternary expression is evaluiated, and its result is the right-hand operand of the +=.

As others have observed, that's not valid in Java if the type of rounds[i] is int, though that would be ok in C. But the expression could be sensible in Java if rounds[i] were an array of boolean, or it could be rewritten like this ...

score += ((rounds[i] != 0) ? (i + START) : 0)

... on the assumption that a C-style boolean interpretation of integer rounds[i] is what is wanted.

Answer 2

score += (some condition which is true or false) ? value to add if true : value to add if false;

Answer 3

We can try it.

    int START = 3;

    int score = 0;
    boolean[] rounds = { true, false };
    for (int i = 0; i < rounds.length; i++) {
        score += (rounds[i]) ? i + START : 0;
        System.out.format("i is %d, score is %d%n", i, score);
    }

Output:

i is 0, score is 3
i is 1, score is 3

So the first time through the loop i is 0 and rounds[i] is true. In this case Java adds i and START to get 3 and adds this to score. Second time i is 1 and rounds[i] is false, so instead just 0 is added.

The statement you ask about adds a value to score. The value added is i + START if rounds[i] can be evaluated to true and 0 if it’s false. If i and START are both numeric, a number will be added. If score is numeric, adding 0 usually makes no difference, so you may think of the statement as adding a value only if rounds[i] is true.

so it is score += round[i] == i+start or == 0.

No, there is no implicit == comparison in the statement (as others have said, it requires that rounds[i] is a Boolean value, true or false).

Answer 4

The ternary operator ?: is used as follows:

• a = bolean_expression ? this if true : else this if false.

        int start = 5;

        // In this case, start was added to itself to obtain 10.                
        start += true ? start  : 0;
        System.out.println(start); // prints 10

        // with out the compound assignment operator (+=) the
        // expression should be enclosed in () 
        start = start + (true ? start : 0);
        System.out.println(start); // prints 20

In the above cases, if the boolean was false, the start always have the value 5 since 0 would be added each time.