C++ Multiple statements for conditional operator

Question

I'm trying to use a conditional statement that does one thing in one condition but does two things if the other condition applies.

Consider the following:

 ( h >= 0 && h < 24 ? hour = h : hour = 0, cout << "Invalid Hour Detected\n")

If "h" is set to 25, it sets "hour" to 0 correctly. If "h" is set to 12, it correctly sets "hour" to 12.

The problem is that it outputs "Invalid Hour Detected" for both true and false conditions. I only want it to output if the conditions aren't met.

Essentially, I'm wondering if it is possible in a conditional statement to do two things for one condition.

Also tried:

( h >= 0 && h < 24 ? hour = h : hour = 0 && cout << "Invalid Hour Detected\n")

but that didn't run the cout on either case.

Answer 1

If you really want to do this, add proper parentheses and invert the order of the assignment and the output insertion (when using the comma operator, the value of the left expression is discarded):

( h >= 0 && h < 24 ) ? ( hour = h ) : (std::cout << "Invalid Hour Detected\n", hour = 0);

However, my advice is to make your code readable and abandon this kind of coding style.

Answer 2

I'm trying to use a conditional statement that does one thing in one condition but does two things if the other condition applies.

That's not a conditional statement, it's a conditional expression⁺. Conditional statement would be a lot more appropriate here from the readability perspective:

if( h >= 0 && h < 24) {
    hour = h;
} else {
    hour = 0;
    cout << "Invalid Hour Detected\n";
}

---

⁺ C++ follows C in allowing use of standalone expressions as statements. That's why you can eventually "shoehorn" your solution into the right syntax by using parentheses and switching the order of operations. The readability of that solution suffers a lot compared to that of a plain, familiar if.

Answer 3

Try

( h >= 0 && h < 24 ? hour = h : (hour = 0 || cout << "Invalid Hour Detected\n"))

Or

( h >= 0 && h < 24 ? hour = h : (hour = 0 & cout << "Invalid Hour Detected\n"))

Answer 4

The comma operator has the lowest precedence of all the operators. Consequently, your expression is evaluated like this:

(( h >= 0 && h < 24 ? hour = h : hour = 0), cout << "Invalid Hour Detected\n") 

You could express this in a conditional expression, with grouping and proper usage of the comma operator. But, if at all possible, it would be better to express this in an if/else statement. If you need to use it in an expression, consider placing it in a function.

Answer 5

Well I know that this is probably not the answer you are looking for but if you refactored a little this would be cleared up "naturally".

// Handy as a utility free function (in apt namespace)
bool isValidHour(unsigned int hour) { 
    return hour >= 0 && hour < 24;
}

isValidHour(h) ? hour = h : handleInvalidHour();
//...more

// If this is not in a class then you should pass "hour" as a reference.
void MyClass::handleInvalidHour() { 
    hour = 0;
    cout << "Invalid Hour Detected\n";
}

Of course you should be using the new std::chrono stuff if possible. It's lovely and expressive.