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javaoperator-keywordternary

Java ternary operator - parameter order

I just don't get what I'm doing wrong. I'm trying to calculate the number of bytes needed for BCD. The formula is rather simple: number of digits, divided by two, rounded up.

So I tried this

int halfRoundedUp = isOddNumberOfDigits ? (numberOfDigitsInParameter/2) : (numberOfDigitsInParameter/2 + 1);

or

int halfRoundedUp = isOddNumberOfDigits ? (numberOfDigitsInParameter/2) : ((numberOfDigitsInParameter/2) + 1);

or, because it was logically more sound, originally this:

int halfRoundedUp = isOddNumberOfDigits ? (numberOfDigitsInParameter/2) : ((numberOfDigitsInParameter + 1) /2);

in several variations and bracketing. Lots of brackets. It keeps returning the wrong value when the flag is set; it should add +1 after halving, but it does not.

This workaround works just fine

int halfRoundedUp = numberOfDigitsInParameter/2;
if (isOddNumberOfDigits)
    halfRoundedUp ++;

but I'd like to understand what went wrong before. Can somebody explain why the above code snippets do not work as expected?

Solution

  • You have your arguments in the wrong order. The general for of a ternary expression is:

    condition ? return_if_true : return_if_false

    If the number has an odd number of digits, you'd want to divide by two and add one, so you need to flip the arguments around:

    int halfRoundedUp = isOddNumberOfDigits ? 
                    numberOfDigitsInParameter / 2  + 1 : 
                    numberOfDigitsInParameter / 2;

    EDIT:
    Even though the question was about the use of the ternary operator, I must say that personally, I'd just use java.util.Math.ceil(double):

    // note the floating point division
int halfRoundedUp = Math.ceil(numberOfDigitsInParameter / 2.0);