What are the benefits and drawbacks of the ?: operator as opposed to the standard if-else statement. The obvious ones being:
Conditional ?: Operator
Shorter and more concise when dealing with direct value comparisons and assignments
Doesn't seem to be as flexible as the if/else construct
Standard If/Else
Can be applied to more situations (such as function calls)
Often are unnecessarily long
Readability seems to vary for each depending on the statement. For a little while after first being exposed to the ?: operator, it took me some time to digest exactly how it worked. Would you recommend using it wherever possible, or sticking to if/else given that I work with many non-programmers?
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;
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.
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);
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.
A really cool usage is:
x = foo ? 1 :
bar ? 2 :
baz ? 3 :
4;
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;
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)
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).
While the above answers are valid, and I agree with readability being important, there are 2 further points to consider:
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";
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
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.
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.
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.
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.
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.
After updating to Xcode 7, my iOS App stopped compiling (due to several "changes"), however I've come up with something I don't know how to make any simpler. I need to sort an Array depending on the three following conditions:
appDelegate.all_breeds!.sortInPlace { (a, b) -> Bool in
(a.breedNameES == nil) ||
(b.breedNameES == nil) ||
(a.breedNameES! < b.breedNameES!)
}
Basically I need to see if either the breed A in spanish is nil, OR if the breed B in spanish is nil, otherwise I need to compare both breeds, and sort them in alphabetical order.
This had worked without problems until I updated to xCode 7, now the error I get is
Expression was too complex to be solved in reasonable time;
consider breaking up the expression into distinct subexpressions.
The problem is I can't break it into subexpressions, it's as basic as I can think of.
Any ideas how to solve this issue?
Appearently part of the problem was that Breed was a struct in my AppDelegate, which forced the compiler to iterate through all the structs to find it. This made that the expression took more time to compile.
The error message
Expression was too complex to be solved in reasonable time
can have various reasons. One is that the compiler could not infer the
types of expressions automatically from the context. In that case it
helps to add explicit type annotations. In your case
appDelegate.all_breeds!.sortInPlace { (a : AppDelegate.Breed, b : AppDelegate.Breed) -> Bool in
(a.breedNameES == nil) ||
(b.breedNameES == nil) ||
(a.breedNameES! < b.breedNameES!)
}
So I've got 3 conditions for an if-statements, and I want to ensure the first two are true before checking the third one. Here's some pseudo code demonstrating what I mean:
if (A && B && C) {
doSomething
}
Is that equivalent to:
if (A && B) {
if (C) {
doSomething
}
}
Or can we not rely on the ordering of the conditions in a short circuit evaluation?
Thanks in advance!
Yes, short circuiting respects the order and will only evaluate until it finds a failure (hence taking a short cut when it knows it can't possibly succeed any more no matter how any future check might evaluate).
If you need all 3 to be true to execute the code, then both will fit your purpose.
Literally that statement reads if A and B and C are ALL true, execute that code. If however, you want to do something where A,B are true and C is false, you'll want to use your second piece of code and add an else statement
I am looking for a more succinct F# equivalent of:
myNumber >= 2 && myNumber <= 4
I imagine something like
myNumber >=< (2, 4)
Is there some kind of operation like this?
There is no native operator, but you could define your own one.
let inline (>=<) a (b,c) = a >= b && a<= c
John's answer is exactly what you asked for, and the most practical solution. But this got me wondering if one could define operator(s) to enable a syntax closer to normal mathematical notation, i.e., a <= b <= c.
Here's one such solution:
let inline (<=.) left middle = (left <= middle, middle)
let inline (.<=) (leftResult, middle) right = leftResult && (middle <= right)
let inline (.<=.) middleLeft middleRight = (middleLeft .<= middleRight, middleRight)
1 <=. 3 .<=. 5 .<= 9 // true
1 <=. 10 .<= 5 // false
A few comments on this:
I used the . character to indicate the "middle" of the expression
. was a very deliberate choice, and is not easily changeable to some other character you like better (e.g. if you perhaps like the look of 1 <=# 3 #<= 5 better). The F# compiler changes the associativity and/or precedence of an operator based on the operator symbol's first character. We want standard left-to-right evaluation/short-circuiting, and . enables this.
A 3-number comparison is optimized away completely, but a 4+ number comparison results in CIL that allocates tuples and does various other business that isn't strictly necessary:
Is there some kind of operation like this?
Great question! The answer is "no", there isn't, but I wish there was.
Latkin's answer is nice, but it doesn't short-circuit evaluate. So if the first test fails the remaining subexpressions still get evaluated, even though their results are irrelevant.
FWIW, in Mathematica you can do 1<x<2 just like mathematics.
Does an if-statement with an && operator check for the second parameter if the first one is false / NO?
Would the following be able to crash?
NSDictionary *someRemoteData = [rawJson valueForKey:#"data"];
if( [someRemoteData isKindOfClass:[NSDictionary class]] && someRemoteData.count > 0 ){
//..do something
}
Please no simple yes or no answer, but explain why.
No, it does not evaluate the expression after learning that the answer is going to be NO. This is called short-circuiting, and it is an essential part of evaluating boolean expressions in C, C++, Objective C, and other languages with similar syntax. The conditions are evaluated left to right, making the evaluation scheme predictable.
The same rule applies to the || operator: as soon as the code knows that the value is YES, the evaluation stops.
Short-circuiting lets you guard against invalid evaluation in a single composite expression, rather than opting for an if statement. For example,
if (index >= 0 && index < Length && array[index] == 42)
would have resulted in undefined behavior if it were not for short-circuiting. But since the evaluation skips evaluation of array[index] when index is invalid, the above expression is legal.
Objective-C uses lazy evaluation, which means that only the left operand is evaluated in your case.
NO it does not. If the first statement fails the second is never checked so for example you can do this (ArrayList != null && ArrayList.size() > 0) and you will never get an error if the variable is not initialized.