Benefits of using the conditional ?: (ternary) operator

Question

What are the benefits and drawbacks of the ?: operator as opposed to the standard if-else statement. The obvious ones being:

Conditional ?: Operator

Answer 1

There is some performance benefit of using the the ? operator in eg. MS Visual C++, but this is a really a compiler specific thing. The compiler can actually optimize out the conditional branch in some cases.

Answer 2

This is pretty much covered by the other answers, but "it's an expression" doesn't really explain why that is so useful...

In languages like C++ and C#, you can define local readonly fields (within a method body) using them. This is not possible with a conventional if/then statement because the value of a readonly field has to be assigned within that single statement:

readonly int speed = (shiftKeyDown) ? 10 : 1;

is not the same as:

readonly int speed;  
if (shifKeyDown)  
    speed = 10;    // error - can't assign to a readonly
else  
    speed = 1;     // error  

In a similar way you can embed a tertiary expression in other code. As well as making the source code more compact (and in some cases more readable as a result) it can also make the generated machine code more compact and efficient:

MoveCar((shiftKeyDown) ? 10 : 1);

...may generate less code than having to call the same method twice:

if (shiftKeyDown)
    MoveCar(10);
else
    MoveCar(1);

Of course, it's also a more convenient and concise form (less typing, less repetition, and can reduce the chance of errors if you have to duplicate chunks of code in an if/else). In clean "common pattern" cases like this:

object thing = (reference == null) ? null : reference.Thing;

... it is simply faster to read/parse/understand (once you're used to it) than the long-winded if/else equivalent, so it can help you to 'grok' code faster.

Of course, just because it is useful does not mean it is the best thing to use in every case. I'd advise only using it for short bits of code where the meaning is clear (or made more clear) by using ?: - if you use it in more complex code, or nest ternary operators within each other it can make code horribly difficult to read.

Answer 3

The ternary operator can be included within an rvalue, whereas an if-then-else cannot; on the other hand, an if-then-else can execute loops and other statements, whereas the ternary operator can only execute (possibly void) rvalues.

On a related note, the && and || operators allow some execution patterns which are harder to implement with if-then-else. For example, if one has several functions to call and wishes to execute a piece of code if any of them fail, it can be done nicely using the && operator. Doing it without that operator will either require redundant code, a goto, or an extra flag variable.

Answer 4

With C# 7, you can use the new ref locals feature to simplify the conditional assignment of ref-compatible variables. So now, not only can you do:

int i = 0;

T b = default(T), c = default(T);

// initialization of C#7 'ref-local' variable using a conditional r-value⁽¹⁾

ref T a = ref (i == 0 ? ref b : ref c);

...but also the extremely wonderful:

// assignment of l-value⁽²⁾ conditioned by C#7 'ref-locals'

(i == 0 ? ref b : ref c) = a;

That line of code assigns the value of a to either b or c, depending on the value of i.

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Notes
1. r-value is the right-hand side of an assignment, the value that gets assigned.
2. l-value is the left-hand side of an assignment, the variable that receives the assigned value.

Answer 5

If I'm setting a value and I know it will always be one line of code to do so, I typically use the ternary (conditional) operator. If there's a chance my code and logic will change in the future, I use an if/else as it's more clear to other programmers.

Of further interest to you may be the ?? operator.

Answer 6

Sometimes it can make the assignment of a bool value easier to read at first glance:

// With
button.IsEnabled = someControl.HasError ? false : true;

// Without
button.IsEnabled = !someControl.HasError;