Why does
if (x) {
f();
}
call f() if x is an empty string ""?
Shouldn't empty strings in D implicitly convert to bool false like they do in Python and when empty arrays does it (in D)?
Update: I fixed the question. I had incorrectly reversed the reasoning logic. Luckily, the bright D minds understood what I meant anyway ;)
Conditions and if statements and loops are cast to bool by the compiler. So,
if(x) {...}
becomes
if(cast(bool)x) {...}
and in the case of arrays, casting to bool is equivalent to testing whether its ptr property is not null. So, it becomes
if(x.ptr !is null) {...}
In the case of arrays, this is actually a really bad test, because null arrays are considered to be the same as empty arrays. So, in most cases, you don't care whether an array is null or not. An array is essentially a struct that looks like
struct Array(T)
{
T* ptr;
size_t length;
}
The == operator will check whether all of the elements referred to by ptr are equal, but if length is 0 for both arrays, it doesn't care what the value of ptr is. That means that "" and null are equal (as are [] and null). However, the is operator explicitly checks the ptr properties for equality, so "" and null won't be the same according to the is operator, and whether a particular array which is empty has a null ptr depends on how its value was set. So, the fact that an array is empty really says nothing about whether it's null or not. You have to check with the is operator to know for sure.
The result of all this is that it's generally bad practice to put an array (or string) directly in a condition like you're doing with
if(x) {...}
Rather, you should be clear about what you're checking. Do you care whether it's empty? In that case, you should check either
if(x.empty) {...}
or
if(x.length == 0} {...}
Or do you really care that it's null? In that case, use the is operator:
if(x is null) {...}
The behavior of arrays in conditions is consistent with the rest of the language (e.g. pointer and reference types are checked to see whether they're null or not), but unfortunately, in practice, such behavior for arrays is quite bug-prone. So, I'd advise that you just don't ever put an array by itself in the condition of an if statement or loop.
the default conversion of arrays looks at the .ptr, which means only the default initialized arrays (or explicitly set to null) evaluate to false
as an added effect string literals in D are \0 terminated which means ("")[0] == '\0' and as such ("").ptr can't be null (which would lead to a segfault)
IMO it should look at the length and you can use the ptr when you need to
It does when I try it...
void main() {
import std.stdio;
string s = "";
if(s)
writeln("true"); // triggered
}
If it was "string s = null;" (which is the default initialization), it doesn't, because the null converts to false, but "" is ok on my computer. Are you sure it isn't null?
BTW, if you want to test for (non-)emptiness, the way I prefer to do it is if(x.length) and if(x.length == 0). Those work consistently for both "" and null, then if you specifically want null, do if(x is null). It is just a little more clear, especially since "" and null are interchangeable in a lot of other contexts in D.
As it currently stands, this question is not a good fit for our Q&A format. We expect answers to be supported by facts, references, or expertise, but this question will likely solicit debate, arguments, polling, or extended discussion. If you feel that this question can be improved and possibly reopened, visit the help center for guidance.
Closed 10 years ago.
I'm personally an advocate of the ternary operator: () ? :
I do realize that it has its place, but I have come across many programmers that are completely against ever using it, and some that use it too often.
What are your feelings on it? What interesting code have you seen using it?
Use it for simple expressions only:
int a = (b > 10) ? c : d;
Don't chain or nest ternary operators as it hard to read and confusing:
int a = b > 10 ? c < 20 ? 50 : 80 : e == 2 ? 4 : 8;
Moreover, when using ternary operator, consider formatting the code in a way that improves readability:
int a = (b > 10) ? some_value
: another_value;
It makes debugging slightly more difficult since you can not place breakpoints on each of the sub expressions. I use it rarely.
I love them, especially in type-safe languages.
I don't see how this:
int count = (condition) ? 1 : 0;
is any harder than this:
int count;
if (condition)
{
count = 1;
}
else
{
count = 0;
}
I'd argue that ternary operators make everything less complex and more neat than the alternative.
Chained I'm fine with - nested, not so much.
I tend to use them more in C simply because they're an if statement that has value, so it cuts down on unnecessary repetition or variables:
x = (y < 100) ? "dog" :
(y < 150) ? "cat" :
(y < 300) ? "bar" : "baz";
rather than
if (y < 100) { x = "dog"; }
else if (y < 150) { x = "cat"; }
else if (y < 300) { x = "bar"; }
else { x = "baz"; }
In assignments like this, I find it's less to refactor, and clearer.
When I'm working in ruby on the other hand, I'm more likely to use if...else...end because it's an expression too.
x = if (y < 100) then "dog"
elif (y < 150) then "cat"
elif (y < 300) then "bar"
else "baz"
end
(Although, admittedly, for something this simple, I might just use the ternary operator anyway.)
The ternary ?: operator is merely a functional equivalent of the procedural if construct. So as long as you are not using nested ?: expressions, the arguments for/against the functional representation of any operation applies here. But nesting ternary operations can result in code that is downright confusing (exercise for the reader: try writing a parser that will handle nested ternary conditionals and you will appreciate their complexity).
But there are plenty of situations where conservative use of the ?: operator can result in code that is actually easier to read than otherwise. For example:
int compareTo(Object object) {
if((isLessThan(object) && reverseOrder) || (isGreaterThan(object) && !reverseOrder)) {
return 1;
if((isLessThan(object) && !reverseOrder) || (isGreaterThan(object) && reverseOrder)) {
return -1;
else
return 0;
}
Now compare that with this:
int compareTo(Object object) {
if(isLessThan(object))
return reverseOrder ? 1 : -1;
else(isGreaterThan(object))
return reverseOrder ? -1 : 1;
else
return 0;
}
As the code is more compact, there is less syntactic noise, and by using the ternary operator judiciously (that is only in relation with the reverseOrder property) the end result isn't particularly terse.
It's a question of style, really; the subconscious rules I tend to follow are:
Only evaluate 1 expression - so foo = (bar > baz) ? true : false, but NOT foo = (bar > baz && lotto && someArray.Contains(someValue)) ? true : false
If I'm using it for display logic, e.g. <%= (foo) ? "Yes" : "No" %>
Only really use it for assignment; never flow logic (so never (foo) ? FooIsTrue(foo) : FooIsALie(foo) ) Flow logic in ternary is itself a lie, ignore that last point.
I like it because it's concise and elegant for simple assignment operations.
Like so many opinion questions, the answer is inevitably: it depends
For something like:
return x ? "Yes" : "No";
I think that is much more concise (and quicker for me to parse) than:
if (x) {
return "Yes";
} else {
return "No";
}
Now if your conditional expression is complex, then the ternary operation is not a good choice. Something like:
x && y && z >= 10 && s.Length == 0 || !foo
is not a good candidate for the ternary operator.
As an aside, if you are a C programmer, GCC actually has an extension that allows you to exclude the if-true portion of the ternary, like this:
/* 'y' is a char * */
const char *x = y ? : "Not set";
Which will set x to y assuming y is not NULL. Good stuff.
In my mind, it only makes sense to use the ternary operator in cases where an expression is needed.
In other cases, it seems like the ternary operator decreases clarity.
I use the ternary operator wherever I can, unless it makes the code extremely hard to read, but then that's usually just an indication that my code could use a little refactoring.
It always puzzles me how some people think the ternary operator is a "hidden" feature or is somewhat mysterious. It's one of the first things I learnt when I start programming in C, and I don't think it decreases readability at all. It's a natural part of the language.
By the measure of cyclomatic complexity, the use of if statements or the ternary operator are equivalent. So by that measure, the answer is no, the complexity would be exactly the same as before.
By other measures such as readability, maintainability, and DRY (don't repeat yourself), either choice may prove better than the other.
I use it quite often in places where I'm constrained to work in a constructor - for example, the new .NET 3.5 LINQ to XML constructs - to define default values when an optional parameter is null.
Contrived example:
var e = new XElement("Something",
param == null ? new XElement("Value", "Default")
: new XElement("Value", param.ToString())
);
or (thanks asterite)
var e = new XElement("Something",
new XElement("Value",
param == null ? "Default"
: param.ToString()
)
);
No matter whether you use the ternary operator or not, making sure your code is readable is the important thing. Any construct can be made unreadable.
I agree with jmulder: it shouldn't be used in place of a if, but it has its place for return expression or inside an expression:
echo "Result: " + n + " meter" + (n != 1 ? "s" : "");
return a == null ? "null" : a;
The former is just an example, and better internationalisation and localisation support of plural should be used!
If you're using the ternary operator for a simple conditional assignment I think it's fine. I've seen it (ab)used to control program flow without even making an assignment, and I think that should be avoided. Use an if statement in these cases.
(Hack of the day)
#define IF(x) x ?
#define ELSE :
Then you can do if-then-else as expression:
int b = IF(condition1) res1
ELSE IF(condition2) res2
ELSE IF(conditions3) res3
ELSE res4;
I think the ternary operator should be used when needed. It is obviously a very subjective choice, but I find that a simple expression (specially as a return expression) is much clearer than a full test. Example in C/C++:
return (a>0)?a:0;
Compared to:
if(a>0) return a;
else return 0;
You also have the case where the solution is between the ternary operator and creating a function. For example in Python:
l = [ i if i > 0 else 0 for i in lst ]
The alternative is:
def cap(value):
if value > 0:
return value
return 0
l = [ cap(i) for i in lst ]
It is needed enough that in Python (as an example), such an idiom could be seen regularly:
l = [ ((i>0 and [i]) or [0])[0] for i in lst ]
this line uses properties of the logical operators in Python: they are lazy and returns the last value computed if it is equal to the final state.
I've seen such beasts like (it was actually much worse since it was isValidDate and checked month and day as well, but I couldn't be bothered trying to remember the whole thing):
isLeapYear =
((yyyy % 400) == 0)
? 1
: ((yyyy % 100) == 0)
? 0
: ((yyyy % 4) == 0)
? 1
: 0;
where, plainly, a series of if-statements would have been better (although this one's still better than the macro version I once saw).
I don't mind it for small things like:
reportedAge = (isFemale && (Age >= 21)) ? 21 + (Age - 21) / 3 : Age;
or even slightly tricky things like:
printf ("Deleted %d file%s\n", n, (n == 1) ? "" : "s");
I like using the operator in debug code to print error values so I don't have to look them up all the time. Usually I do this for debug prints that aren't going to remain once I'm done developing.
int result = do_something();
if( result != 0 )
{
debug_printf("Error while doing something, code %x (%s)\n", result,
result == 7 ? "ERROR_YES" :
result == 8 ? "ERROR_NO" :
result == 9 ? "ERROR_FILE_NOT_FOUND" :
"Unknown");
}
I almost never use the ternary operator, because whenever I do use it, it always makes me think a lot more than I have to later when I try to maintain it.
I like to avoid verbosity, but when it makes the code a lot easier to pick up, I will go for the verbosity.
Consider:
String name = firstName;
if (middleName != null) {
name += " " + middleName;
}
name += " " + lastName;
Now, that is a bit verbose, but I find it a lot more readable than:
String name = firstName + (middleName == null ? "" : " " + middleName)
+ " " + lastName;
Or:
String name = firstName;
name += (middleName == null ? "" : " " + middleName);
name += " " + lastName;
It just seems to compress too much information into too little space, without making it clear what's going on. Every time I saw the ternary operator used, I have always found an alternative that seemed much easier to read... then again, that is an extremely subjective opinion, so if you and your colleagues find ternary very readable, go for it.
I like them. I don't know why, but I feel very cool when I use the ternary expression.
I treat ternary operators a lot like GOTO. They have their place, but they are something which you should usually avoid to make the code easier to understand.
Well, the syntax for it is horrid. I find functional ifs very useful, and they often makes code more readable.
I would suggest making a macro to make it more readable, but I'm sure someone can come up with a horrible edge case (as there always is with C++).
I typically use it in things like this:
before:
if(isheader)
drawtext(x, y, WHITE, string);
else
drawtext(x, y, BLUE, string);
after:
drawtext(x, y, isheader == true ? WHITE : BLUE, string);
As others have pointed out they are nice for short simple conditions. I especially like them for defaults (kind of like the || and or usage in JavaScript and Python), e.g.
int repCount = pRepCountIn ? *pRepCountIn : defaultRepCount;
Another common use is to initialize a reference in C++. Since references have to be declared and initialized in the same statement you can't use an if statement.
SomeType& ref = pInput ? *pInput : somethingElse;
I like Groovy's special case of the ternary operator, called the Elvis operator: ?:
expr ?: default
This code evaluates to expr if it's not null, and default if it is. Technically it's not really a ternary operator, but it's definitely related to it and saves a lot of time/typing.
I recently saw a variation on ternary operators (well, sort of) that make the standard "() ? :" variant seem to be a paragon of clarity:
var Result = [CaseIfFalse, CaseIfTrue][(boolean expression)]
or, to give a more tangible example:
var Name = ['Jane', 'John'][Gender == 'm'];
Mind you, this is JavaScript, so things like that might not be possible in other languages (thankfully).
Only when:
$var = (simple > test ? simple_result_1 : simple_result_2);
KISS.
For simple if cases, I like to use it. Actually it's much easier to read/code for instance as parameters for functions or things like that. Also to avoid the new line I like to keep with all my if/else.
Nesting it would be a big no-no in my book.
So, resuming, for a single if/else I'll use the ternary operator. For other cases, a regular if/else if/else (or switch).
For simple tasks, like assigning a different value depending on a condition, they're great. I wouldn't use them when there are longer expressions depending on the condition though.
If you and your workmates understand what they do and they aren't created in massive groups I think they make the code less complex and easier to read because there is simply less code.
The only time I think ternary operators make code harder to understand is when you have more than three or foyr in one line. Most people don't remember that they are right based precedence and when you have a stack of them it makes reading the code a nightmare.
As so many answers have said, it depends. I find that if the ternary comparison is not visible in a quick scan down the code, then it should not be used.
As a side issue, I might also note that its very existence is actually a bit of an anomaly due to the fact that in C, comparison testing is a statement. In Icon, the if construct (like most of Icon) is actually an expression. So you can do things like:
x[if y > 5 then 5 else y] := "Y"
... which I find much more readable than a ternary comparison operator. :-)
There was a discussion recently about the possibility of adding the ?: operator to Icon, but several people correctly pointed out that there was absolutely no need because of the way if works.
Which means that if you could do that in C (or any of the other languages that have the ternary operator), then you wouldn't, in fact, need the ternary operator at all.