There is a linter rule which verifies that one don't check for null equality in the overridden == operator.
The rule is here.
I understand this rule but can't see how it is realized technically.
It seems that Dart itself makes some implicit check on other != null and == returns false in this case. Is this correct?
In other languages, e.g. Java, one needs to explicitly add this check in the overridden equals.
Second question is why then it does not check automatically on the type of other as well. Why it is ok to spare me as a programmer from checking on null, but I still need to check if other is Person? Are there cases when one overrides == and checks there for some other type then the type of that class?
The linter rule implementation is simple, it just checks whether you compare the argument of operator == to null.
The reason you don't need to is that e1 == e2 in Dart is defined to first evaluate e1 and e2 to a value, then give a result early if one or both of the values is null, and only otherwise it calls the operator== method on the value of e1.
So, when that method is called, you know for certain that the argument is not null.
The reason the == operator doesn't do more checks before calling the operator== method is that there are examples where that would be wrong.
In particular, int and double can be equal to each other. Having instances of different classes potentially being equal to each other is more common that you'd think (proxies, mocks, wrappers, Cartesian point vs polar point, int vs double).
The other check you could potentially do early is to say that an object is equal to itself, so if (identical(this, other)) return true;, but there is one counter-example forced upon the language: NaN, aka. double.nan. That particular "value" is not equal to itself (which breaks the reflexivity requirement for ==, but is specified that way by the IEEE-754 standard which is what the CPUs implement natively).
If not for NaN, the language would probably have checked for identity before calling operator== too.
It seems that Dart itself makes some implicit check on other != null and == returns false in this case. Is this correct?
Yes.
Second question is why then it does not check automatically on the type of other as well. Why it is ok to spare me as a programmer from checking on null, but I still need to check if other is Person? Are there cases when one overrides == and checks there for some other type then the type of that class?
It's less common, but there can be cases where you might want to allow a different type on the right-hand-side of the equality. For example, the left-hand-side and right-hand-side might be easily convertible to each other or to a common type. Imagine that you created, say, a Complex number class and that you wanted Complex(real: 4.0, imaginary: 0.0) == 4 to be true.
From the doc:
no class can be equivalent to [null]
Meaning, when other is Person is true, then other != null is also true. This is because:
The null object is the sole instance of the built-in class Null.
(https://dart.dev/guides/language/spec)
So every instance check against any type but Null will return false for null:
class A {}
void main() {
final x = null;
final a = A();
print(x is Null); // true
print(x is A); // false
print(a is Null); // false
print(a is A); // true
}
Consider this F# code:
let isSalary employee =
let (fName,lName,Occupation,Department,SalaryType,
HoursPerWeek, AnnualSalary, HourlyWage
) = employee
SalaryType = "Salary"
if(employee.SalaryType = SalaryType) then
true
else
false
Im getting errors in here, any fixes to it?
First things first, please post error messages and a much more specific question. Thanks! But luckily, I can about deduce the error messages from this problem.
Next, if you want to mutate SalaryType after you've deconstructed your employee 8-tuple, you should write using the mutable keyword:
let mutable (fName, lName, Occupation, Department, SalaryType,
HoursPerWeek, AnnualSalary, HourlyWage) = employee
But you shouldn't. This is explained further below.
Next problem: there is no dot notation (no tuple.member) for accessing members of a tuple. It's only possible through deconstruction. So you can't employee.SalaryType.
Here's what looks to be the crux of the problem, and it's a mistake I made many times when I was learning functional programming, and it's a difficult paradigm shift to adapt to. You should not be attempting to mutate data, or in this case, variables. Variables, or values as they are called in F#, shouldn't change, as a broad rule. Functions should be pure.
What this means is that any parameters you pass into a function should not change after leaving the function. The parameter employee should be the same after you return to the calling scope.
There's a few syntactical errors you've made that make it pretty much impossible for me to deduce what you're trying to do in the first place. Please include this in the question.
Also, one last nitpick. As you know, the last expression in an F# function is it's return value. Instead of using an if statement, just return the condition you're testing, like this:
let ...
...
employee.SalaryType = SalaryType <- but remember, you can't use dot notation on tuples; this is just an example
Please read more on
https://learn.microsoft.com/en-us/dotnet/fsharp/language-reference/
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.
Should I return a optional value like:
func someFunc(#num: Int) -> Obj? {
if num < 0 {
return nil
}
...
}
Or just use assert:
func someFunc(#num: Int) -> Obj {
assert(num >= 0, "Number should greater or equal then zero")
...
}
Edit: Now the conditions are identical in two cases, the number should greater or equal then 0. Negative values are not permitted.
If you use assert and the caller passes an invalid argument it is a non-recoverable error/crash. The caller may not be aware of all the ways the assert may be caused, that is internal logic the caller is not supposed to know.
Really the only time assert is meaningful is to check the calling arguments on method entry and even in that case it must be made clear to the user exactly what is invalid and that can never be made more stringent for the life of the method.
Since this is about Swift returning an Optional seems to make the most sense and it will be clear to the caller that a possible error must be handled. Optionals are a major feature of Swift, use them.
Or always return a useful result the way atan() handles being called with ±0 and ±Inf.
It depends on what you want the precondition of the function to be: if it is an error call it with a negative value (or non-positive; your two examples are contradictory), then go with the assert and document this. Then it becomes part of the contract that the user of the function must check the value if it's uncertain. Or if it makes more sense to support these values and return nil (e.g., the function will typically be called with such values and the nil is not an issue for that typical use), do that instead… Without knowing the details it's impossible to tell which suits best, but my guess would be the former.
I'm maintaining someone else's Lua code, and Lua is not my preferred language. This is probably a complete noob question, but I can't seem to find the answer on Google or SO...
The following code
if !(v.LastHealth == v:Health()) then
local newColor = {}
newColor.r = v.orgColor.r - (v.orgColor.r - curColor.r) --This is the line the error occurs on
newColor.g = v.orgColor.g - (v.orgColor.g * clrPercent)
newColor.b = v.orgColor.b - (v.orgColor.b * clrPercent)
newColor.a = v.orgColor.a - (v.orgColor.a - curColor.a)
v:SetColor( newColor )
produces the error
attempt to perform arithmetic on field 'r' (a table value)
orgColor (maybe, not totally certain- v.orgColor may be an outdated thing) and curColor are tables that have entries (Uh. I think. bla.x is the same as bla[x] in Lua, right?) r, g, b, and a. Apparently I can't do math on things that come from tables? Should I stow all these values in local variables before working with them? That doesn't seem right.
EDIT:
Printing v.orgColor gives table: 0x40390080, which I assume means it exists and is a table. What's odd is, v.orgColor.r gives another table! That sounds like the cause.
As it turns out, v.orgColor was not, as I had presumed, set by the host program, but was set by the same script as the code sample is from. There was an API change that made a function that used to return four RGBA values instead return a table of those same values; the old code set orgColor.r to the table containing those values, causing the error.
Moral of the story, I suppose, is that you should always make sure you know what's setting the variables you're working with.