使用条件? : (三元)运算符的好处

I find it particularly helpful when doing web development if I want to set a variable to a value sent in the request if it is defined or to some default value if it is not.

I would basically recommend using it only when the resulting statement is extremely short and represents a significant increase in conciseness over the if/else equivalent without sacrificing readability.

Good example:

int result = Check() ? 1 : 0;

Bad example:

int result = FirstCheck() ? 1 : SecondCheck() ? 1 : ThirdCheck() ? 1 : 0;

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.

The advantage of the conditional operator is that it is an operator. In other words, it returns a value. Since if is a statement, it cannot return a value.

One thing to recognize when using the ternary operator that it is an expression not a statement.

In functional languages like scheme the distinction doesn't exists:

(if (> a b) a b)

Conditional ?: Operator "Doesn't seem to be as flexible as the if/else construct"

In functional languages it is.

When programming in imperative languages I apply the ternary operator in situations where I typically would use expressions (assignment, conditional statements, etc).

I'd recommend limiting the use of the ternary(?:) operator to simple single line assignment if/else logic. Something resembling this pattern:

if(<boolCondition>) {
<variable> = <value>;
}
else {
<variable> = <anotherValue>;
}

Could be easily converted to:

<variable> = <boolCondition> ? <value> : <anotherValue>;

I would avoid using the ternary operator in situations that require if/else if/else, nested if/else, or if/else branch logic that results in the evaluation of multiple lines. Applying the ternary operator in these situations would likely result in unreadable, confusing, and unmanageable code. Hope this helps.

The conditional operator is great for short conditions, like this:

varA = boolB ? valC : valD;

I use it occasionally because it takes less time to write something that way... unfortunately, this branching can sometimes be missed by another developer browsing over your code. Plus, code isn't usually that short, so I usually help readability by putting the ? and : on separate lines, like this:

doSomeStuffToSomething(shouldSomethingBeDone()
? getTheThingThatNeedsStuffDone()
: getTheOtherThingThatNeedsStuffDone());

However, the big advantage to using if/else blocks (and why I prefer them) is that it's easier to come in later and add some additional logic to the branch,

if (shouldSomethingBeDone()) {
doSomeStuffToSomething(getTheThingThatNeedsStuffDone());
doSomeAdditionalStuff();
} else {
doSomeStuffToSomething(getTheOtherThingThatNeedsStuffDone());
}

or add another condition:

if (shouldSomethingBeDone()) {
doSomeStuffToSomething(getTheThingThatNeedsStuffDone());
doSomeAdditionalStuff();
} else if (shouldThisOtherThingBeDone()){
doSomeStuffToSomething(getTheOtherThingThatNeedsStuffDone());
}

So, in the end, it's about convenience for you now (shorter to use :?) vs. convenience for you (and others) later. It's a judgment call... but like all other code-formatting issues, the only real rule is to be consistent, and be visually courteous to those who have to maintain (or grade!) your code.

(all code eye-compiled)

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.

The scenario I most find myself using it is for defaulting values and especially in returns

return someIndex < maxIndex ? someIndex : maxIndex;

Those are really the only places I find it nice, but for them I do.

Though if you're looking for a boolean this might sometimes look like an appropriate thing to do:

bool hey = whatever < whatever_else ? true : false;

Because it's so easy to read and understand, but that idea should always be tossed for the more obvious:

bool hey = (whatever < whatever_else);

If you need multiple branches on the same condition, use an if:

if (A == 6)
f(1, 2, 3);
else
f(4, 5, 6);

If you need multiple branches with different conditions, then if statement count would snowball, you'll want to use the ternary:

f( (A == 6)? 1: 4, (B == 6)? 2: 5, (C == 6)? 3: 6 );

Also, you can use the ternary operator in initialization.

const int i = (A == 6)? 1 : 4;

Doing that with if is very messy:

int i_temp;
if (A == 6)
i_temp = 1;
else
i_temp = 4;
const int i = i_temp;

You can't put the initialization inside the if/else, because it changes the scope. But references and const variables can only be bound at initialization.

I usually choose a ternary operator when I'd have a lot of duplicate code otherwise.

if (a > 0)
answer = compute(a, b, c, d, e);
else
answer = compute(-a, b, c, d, e);

With a ternary operator, this could be accomplished with the following.

answer = compute(a > 0 ? a : -a, b, c, d, e);

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.

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.

A really cool usage is:

x = foo ? 1 :
bar ? 2 :
baz ? 3 :
4;

While the above answers are valid, and I agree with readability being important, there are 2 further points to consider:

1. In C#6, you can have expression-bodied methods.

This makes it particularly concise to use the ternary:

string GetDrink(DayOfWeek day)
=> day == DayOfWeek.Friday
? "Beer" : "Tea";

2. Behaviour differs when it comes to implicit type conversion.

If you have types T1 and T2 that can both be implicitly converted to T, then the below does not work:

T GetT() => true ? new T1() : new T2();

(because the compiler tries to determine the type of the ternary expression, and there is no conversion between T1 and T2.)

On the other hand, the if/else version below does work:

T GetT()
{
if (true) return new T1();
return new T2();
}

because T1 is converted to T and so is T2

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;

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.