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I have an MPMoviePlayerController instance. I wish to check its playbackState property for one of a number of values. As such I do something like this:
if (moviePlayer.playbackState == (MPMoviePlaybackStateStopped ||
MPMoviePlaybackStatePlaying ||
MPMoviePlaybackStatePaused)) {
// ...
// Perform some logic
// ...
}
This works as expected but causes a compiler warning:
Use of logical '||' with constant operand.
The compiler's fix is to use the bitwise | operator instead. Searching on Stack Overflow you will find a couple of answers suggesting the same thing. BUT using the bitwise OR really isn't what I need here.
MPMoviePlaybackState is declared in MPMoviePlayerController.h:
enum {
MPMoviePlaybackStateStopped,
MPMoviePlaybackStatePlaying,
MPMoviePlaybackStatePaused,
MPMoviePlaybackStateInterrupted,
MPMoviePlaybackStateSeekingForward,
MPMoviePlaybackStateSeekingBackward
};
typedef NSInteger MPMoviePlaybackState;
This isn't a bitmask (and nor would it make much sense for it to be so — the enumerated values are mutually exclusive modes, not flags to be combined). I really do want to use the logical ||.
(In my particular case, with the underlying values being 0,1,2 the bitwise example might work but that's just a coincidence.)
How should I rephrase to avoid the warning or what #pragma clang diagnostic ignored ... can I use to silence the warning?
(Bonus points for pointing to a list of all such diagnostics — I cannot seem to locate one in the manual.)
Thanks in advance!
Obviously, the (enumval1 || enumval2 || ..) is wrong. You can't use the || operator like this, but only with logical expressions.
The | operator works, because it's a simple bitwise OR, which will do job for you only and only if your enum members are different powers of 2 (e.g. 1, 2, 4, 8, ...).
It's connected with bitwise representation of numbers in binary, which , if the number if the power of 2, is like this: 2->10, 4->100, 8->1000 etc. So, for 2 | 8 it will be like 0010 | 1000 = 1010, which isn't zero, and if statement will proceed.
The compiler warnings are fully right and helping at this point. Use the switch(..) or if(..) else if(..) statements, or make your enum like this:
enum yourEnum
{
enumval1 = 1 << 0;
enumval2 = 1 << 1;
enumval3 = 1 << 2;
// ...
}
why wouldn't you just do this?
if ((moviePlayer.playbackState == MPMoviePlaybackStateStopped) ||
(moviePlayer.playbackState == MPMoviePlaybackStatePlaying) ||
(moviePlayer.playbackState == MPMoviePlaybackStatePaused)) {
// ...
// Perform some logic
// ...
}
I suggest using a switch/case block with fall trough logic like that:
switch(moviePlayer.playbackState){
case MPMoviePlaybackStateStopped: /* falls through */
case MPMoviePlaybackStatePlaying: /* falls through */
case MPMoviePlaybackStatePaused: /* falls through */
// your stuff
}
This will result in the intended behaviour with as less code as possible. Enums are made for exact the switch case kind of business. And they are performance optimized more than "if" statements, becuase the CPU does not even have to test the values when reaching the code. The compiler calculates the correct ASM jump offset at that location. So its as fast as lighning :)
i would like to have one identifier responsible for several values for one time. if i had one this is what i could do:
if (myVariable == IDENTIFIER)//instead of if(myVariable == 5 || myVariable == 7) if i need A LOT of values
[myObject doSomething];
Is there a possibility to implement it somehow?
I think the closest you can come is by using bitmasks, so that you represent the set of allowable values with a mask that has all of the values set:
const int ALL_VALUES = (1 << 5) | (1 << 7);
if ((1 << myVariable) & ALL_VALUES)
[myObject doSomething];
Above, bit-wise AND is used to compute the intersection between the current value (seen as a 1-bit mask) and the mask of all allowed values. Note that this will only work if the number of values (and their actual values) is less than the number of bits in an int.
You could have a NSSet of possible values:
NSSet *possibleValues = [NSSet setWithObjects:#"Value1", #"Value2", #"Value3", nil];
if ([possibleValues containsObject:myVariable])
If you need something that works with a raw integer, let me know.
This is what methods are for:
- (BOOL)isFoo(int identifier) {
return identifier == 5 || identifier == 7;
}
Combine the answers. First use a function (variant of grahamparks):
BOOL isFoo(int identifier)
{
...
return ...;
}
For something this simple a function is probably better than a method - calling is a lot quicker and there is no need to ever override. Further if the function is only ever required in the current file declare it static BOOL isFoo... to limit the visibility of isFoo to just the file.
Now pick the body which suits the data - a couple of values, comparisons (grahamparks); more than a few values but all within 0-31 (uint32_t) or 0-63 (uint64_t) consider the bit-mask (unwind); many values all over the range consider sets (Richard J. Ross III); or roll your own. The important point which ever algorithm you choose is isolated within the function and can be changed easily if needed without affecting the rest of your code.
As existing similar examples consider isDigit() et al in the standard C library. Some implementations of these use a pre-allocated arrays of booleans (256 elements as the argument is a character) so testing for membership of the set is just an array index operation.
This question already has answers here:
Closed 12 years ago.
Possible Duplicate:
Why do most programming languages only have binary equality comparison operators?
I have had a simple question for a fairly long time--since I started learning programming languages.
I'd like to write like "if x is either 1 or 2 => TRUE (otherwise FALSE)."
But when I write it in a programming language, say in C,
( x == 1 || x == 2 )
it really works but looks awkward and hard to read. I guess it should be possible to simplify such an or operation, and so if you have any idea, please tell me. Thanks, Nathan
Python allows test for membership in a sequence:
if x in (1, 2):
An extension version in C#
step 1: create an extension method
public static class ObjectExtensions
{
public static bool Either(this object value, params object[] array)
{
return array.Any(p => Equals(value, p));
}
}
step 2: use the extension method
if (x.Either(1,2,3,4,5,6))
{
}
else
{
}
While there are a number of quite interesting answers in this thread, I would like to point out that they may have performance implications if you're doing this kind of logic inside of a loop depending on the language. A far as for the computer to understand, the if (x == 1 || x == 2) is by far the easiest to understand and optimize when it's compiled into machine code.
When I started programming it seemed weird to me as well that instead of something like:
(1 < x < 10)
I had to write:
(1 < x && x < 10)
But this is how most programming languages work, and after a while you will get used to it.
So I believe it is perfectly fine to write
( x == 1 || x == 2 )
Writing it this way also has the advantage that other programmers will understand easily what you wrote. Using a function to encapsulate it might just make things more complicated because the other programmers would need to find that function and see what it does.
Only more recent programming languages like Python, Ruby etc. allow you to write it in a simpler, nicer way. That is mostly because these programming languages are designed to increase the programmers productivity, while the older programming languages' main goal was application performance and not so much programmer productivity.
It's Natural, but Language-Dependent
Your approach would indeed seem more natural but that really depends on the language you use for the implementation.
Rationale for the Mess
C being a systems programming language, and fairly close to the hardware (funny though, as we used to consider a "high-level" language, as opposed to writing machine code), it's not exactly expressive.
Modern higher-level languages (again, arguable, lisp is not that modern, historically speaking, but would allow you to do that nicely) allow you to do such things by using built-in constructs or library support (for instances, using Ranges, Tuples or equivalents in languages like Python, Ruby, Groovy, ML-languages, Haskell...).
Possible Solutions
Option 1
One option for you would be to implement a function or subroutine taking an array of values and checking them.
Here's a basic prototype, and I leave the implementation as an exercise to you:
/* returns non-zero value if check is in values */
int is_in(int check, int *values, int size);
However, as you will quickly see, this is very basic and not very flexible:
it works only on integers,
it works only to compare identical values.
Option 2
One step higher on the complexity ladder (in terms of languages), an alternative would be to use pre-processor macros in C (or C++) to achieve a similar behavior, but beware of side effects.
Other Options
A next step could be to pass a function pointer as an extra parameter to define the behavior at call-point, define several variants and aliases for this, and build yourself a small library of comparators.
The next step then would be to implement a similar thing in C++ using templates to do this on different types with a single implementation.
And then keep going from there to higher-level languages.
Pick the Right Language (or learn to let go!)
Typically, languages favoring functional programming will have built-in support for this sort of thing, for obvious reasons.
Or just learn to accept that some languages can do things that others cannot, and that depending on the job and environment, that's just the way it is. It mostly is syntactic sugar, and there's not much you can do. Also, some languages will address their shortcomings over time by updating their specifications, while others will just stall.
Maybe a library implements such a thing already and that I am not aware of.
that was a lot of interesting alternatives. I am surprised nobody mentioned switch...case - so here goes:
switch(x) {
case 1:
case 2:
// do your work
break;
default:
// the else part
}
it is more readable than having a
bunch of x == 1 || x == 2 || ...
more optimal than having a
array/set/list for doing a
membership check
I doubt I'd ever do this, but to answer your question, here's one way to achieve it in C# involving a little generic type inference and some abuse of operator overloading. You could write code like this:
if (x == Any.Of(1, 2)) {
Console.WriteLine("In the set.");
}
Where the Any class is defined as:
public static class Any {
public static Any2<T> Of<T>(T item1, T item2) {
return new Any2<T>(item1, item2);
}
public struct Any2<T> {
T item1;
T item2;
public Any2(T item1, T item2) {
this.item1 = item1;
this.item2 = item2;
}
public static bool operator ==(T item, Any2<T> set) {
return item.Equals(set.item1) || item.Equals(set.item2);
}
// Defining the operator== requires these three methods to be defined as well:
public static bool operator !=(T item, Any2<T> set) {
return !(item == set);
}
public override bool Equals(object obj) { throw new NotImplementedException(); }
public override int GetHashCode() { throw new NotImplementedException(); }
}
}
You could conceivably have a number of overloads of the Any.Of method to work with 3, 4, or even more arguments. Other operators could be provided as well, and a companion All class could do something very similar but with && in place of ||.
Looking at the disassembly, a fair bit of boxing happens because of the need to call Equals, so this ends up being slower than the obvious (x == 1) || (x == 2) construct. However, if you change all the <T>'s to int and replace the Equals with ==, you get something which appears to inline nicely to be about the same speed as (x == 1) || (x == 2).
Err, what's wrong with it? Oh well, if you really use it a lot and hate the looks do something like this in c#:
#region minimizethisandneveropen
public bool either(value,x,y){
return (value == x || value == y);
}
#endregion
and in places where you use it:
if(either(value,1,2))
//yaddayadda
Or something like that in another language :).
In php you can use
$ret = in_array($x, array(1, 2));
As far as I know, there is no built-in way of doing this in C. You could add your own inline function for scanning an array of ints for values equal to x....
Like so:
inline int contains(int[] set, int n, int x)
{
int i;
for(i=0; i<n; i++)
if(set[i] == x)
return 1;
return 0;
}
// To implement the check, you declare the set
int mySet[2] = {1,2};
// And evaluate like this:
contains(mySet,2,x) // returns non-zero if 'x' is contained in 'mySet'
In T-SQL
where x in (1,2)
In COBOL (it's been a long time since I've even glanced briefly at COBOL, so I may have a detail or two wrong here):
IF X EQUALS 1 OR 2
...
So the syntax is definitely possible. The question then boils down to "why is it not used more often?"
Well, the thing is, parsing expressions like that is a bit of a bitch. Not when standing alone like that, mind, but more when in compound expressions. The syntax starts to become opaque (from the compiler implementer's perspective) and the semantics downright hairy. IIRC, a lot of COBOL compilers will even warn you if you use syntax like that because of the potential problems.
In .Net you can use Linq:
int[] wanted = new int{1, 2};
// you can use Any to return true for the first item in the list that passes
bool result = wanted.Any( i => i == x );
// or use Contains
bool result = wanted.Contains( x );
Although personally I think the basic || is simple enough:
bool result = ( x == 1 || x == 2 );
Thanks Ignacio! I translate it into Ruby:
[ 1, 2 ].include?( x )
and it also works, but I'm not sure whether it'd look clear & normal. If you know about Ruby, please advise. Also if anybody knows how to write this in C, please tell me. Thanks. -Nathan
Perl 5 with Perl6::Junction:
use Perl6::Junction 'any';
say 'yes' if 2 == any(qw/1 2 3/);
Perl 6:
say 'yes' if 2 == 1|2|3;
This version is so readable and concise I’d use it instead of the || operator.
Pascal has a (limited) notion of sets, so you could do:
if x in [1, 2] then
(haven't touched a Pascal compiler in decades so the syntax may be off)
A try with only one non-bitwise boolean operator (not advised, not tested):
if( (x&3) ^ x ^ ((x>>1)&1) ^ (x&1) ^ 1 == 0 )
The (x&3) ^ x part should be equal to 0, this ensures that x is between 0 and 3. Other operands will only have the last bit set.
The ((x>>1)&1) ^ (x&1) ^ 1 part ensures last and second to last bits are different. This will apply to 1 and 2, but not 0 and 3.
You say the notation (x==1 || x==2) is "awkward and hard to read". I beg to differ. It's different than natural language, but is very clear and easy to understand. You just need to think like a computer.
Also, the notations mentioned in this thread like x in (1,2) are semantically different then what you are really asking, they ask if x is member of the set (1,2), which is not what you are asking. What you are asking is if x equals to 1 or to 2 which is logically (and semantically) equivalent to if x equals to 1 or x equals to 2 which translates to (x==1 || x==2).
In java:
List list = Arrays.asList(new Integer[]{1,2});
Set set = new HashSet(list);
set.contains(1)
I have a macro that I use a lot that's somewhat close to what you want.
#define ISBETWEEN(Var, Low, High) ((Var) >= (Low) && (Var) <= (High))
ISBETWEEN(x, 1, 2) will return true if x is 1 or 2.
Neither C, C++, VB.net, C#.net, nor any other such language I know of has an efficient way to test for something being one of several choices. Although (x==1 || x==2) is often the most natural way to code such a construct, that approach sometimes requires the creation of an extra temporary variable:
tempvar = somefunction(); // tempvar only needed for 'if' test:
if (tempvar == 1 || tempvar == 2)
...
Certainly an optimizer should be able to effectively get rid of the temporary variable (shove it in a register for the brief time it's used) but I still think that code is ugly. Further, on some embedded processors, the most compact and possibly fastest way to write (x == const1 || x==const2 || x==const3) is:
movf _x,w ; Load variable X into accumulator
xorlw const1 ; XOR with const1
btfss STATUS,ZERO ; Skip next instruction if zero
xorlw const1 ^ const2 ; XOR with (const1 ^ const2)
btfss STATUS,ZERO ; Skip next instruction if zero
xorlw const2 ^ const3 ; XOR with (const2 ^ const3)
btfss STATUS,ZERO ; Skip next instruction if zero
goto NOPE
That approach require two more instructions for each constant; all instructions will execute. Early-exit tests will save time if the branch is taken, and waste time otherwise. Coding using a literal interpretation of the separate comparisons would require four instructions for each constant.
If a language had an "if variable is one of several constants" construct, I would expect a compiler to use the above code pattern. Too bad no such construct exists in common languages.
(note: Pascal does have such a construct, but run-time implementations are often very wasteful of both time and code space).
return x === 1 || x === 2 in javascript
If a BOOL has a nice short name, it's easy enough to write:
myBOOL = !myBOOL;
But what if the BOOL has a long name?
objectWithLongishName.memberWithLongishName.submember.myBOOL = !(objectWithLongishName.memberWithLongishName.submember.myBOOL);
. . . does not look so pretty.
I'm wondering if there is an easy way to toggle the BOOL without entering its name twice?
Here's another:
MyBooleanYaddaYadda ^= YES;
This is kinda brittle - it will break on legacy C code that implies that any nonzero integer evaluates to true. But then again, so will Apple framework code - I encountered cases in Cocoa where a nonzero, non-one int, when passed as a BOOL, would not cause the same effect as passing a YES.
However, it does not rely on the bit pattern of YES - only on NO being 0. Which is pretty much a given, considering the way C interprets integers as logical values. Also, it does not assume the actual datatype of BOOL (which on Cocoa is signed char, by the way).
The bit pattern of YES on Cocoa is 1. But that's not a universal convention. On some platforms with no built-in boolean datatype, the integer constant that serves as a logical TRUE is -1 - all one bits. That's 0xFFFFFFFF if interpreted as unsigned. This coding has a vague advantage that bitwize NOT (the ~ operator in C ) is equivalent to logical NOT (the ! operator in C). That is, ~0xFFFFFFFF is 0, i. e. ~TRUE is FALSE. Doesn't work that way if TRUE is defined as 1.
#define NOT(b) (b) = !(b)
NOT(MyBooleanVariableWithAFreakishlyLongName);
Or, if it's Objective C++:
inline void NOT(BOOL &b)
{
b = !b;
}
No there isn't an obvious way in (Objective-)C to do what you describe (without using a preprocessor macro), but see Seva's answer for a possible (though potentially brittle) solution. More importantly, something like objectWithLongishName.memberWithLongishName.submember.myBOOL indicates a Law of Demeter violation; you should be providing submember directly to any code unit that needs to access submember.myBOOL.
Write a method for the submember class that toggles it for you?
- (void) toggleMyBOOL {
self.myBool = !self.myBool;
}
Then you can do:
[objectWithLongishName.memberWithLongishName.submember toggleMyBOOL];
Use XOR. In C, this is ^.
BOOL x = YES;
x ^= YES; // it's now NO
x ^= YES; // it's now YES
x ^= YES; // it's now NO
x ^= YES; // it's now YES
x ^= YES; // it's now NO
x ^= YES; // it's now YES
...
Edit: someone posted this already, apparently. I guess I should say I've never actually used this in code. :-)
You have a lovely set of answers focused on flipping a YES to a NO or vice-versa, but no answers that touched on what would appear to be an architectural issue in the code.
Well, some answers. I'm blind.
Namely, you have this:
objectWithLongishName.memberWithLongishName.submember.myBOOL =
!(objectWithLongishName.memberWithLongishName.submember.myBOOL);
That smells like a potential encapsulation violation. In particular (and assuming this is a model layer), it means that the connectivity of the sub-graph of objects is being overtly exposed -- flattened into, effectively -- the entry point of that path; whatever objectWithLongishName is must now have rather intimate knowledge of the innards of the objects along the rest of the path.
Typically, you don't reach deeply into the model layer along key paths to edit state outside of the Cocoa Bindings layer (and even that is a bit fragile).
Sometimes such long-ish paths do make sense. In such a case, I would leave the über-verbose form you have above as a visual indication that encapsulation is being purposefully shred.
I want to check whether a value is equal to 1. Is there any difference in the following lines of code
Evaluated value == 1
1 == evaluated value
in terms of the compiler execution
In most languages it's the same thing.
People often do 1 == evaluated value because 1 is not an lvalue. Meaning that you can't accidentally do an assignment.
Example:
if(x = 6)//bug, but no compiling error
{
}
Instead you could force a compiling error instead of a bug:
if(6 = x)//compiling error
{
}
Now if x is not of int type, and you're using something like C++, then the user could have created an operator==(int) override which takes this question to a new meaning. The 6 == x wouldn't compile in that case but the x == 6 would.
It depends on the programming language.
In Ruby, Smalltalk, Self, Newspeak, Ioke and many other single-dispatch object-oriented programming languages, a == b is actually a message send. In Ruby, for example, it is equivalent to a.==(b). What this means, is that when you write a == b, then the method == in the class of a is executed, but when you write b == a, then the method in the class of b is executed. So, it's obviously not the same thing:
class A; def ==(other) false end; end
class B; def ==(other) true end; end
a, b = A.new, B.new
p a == b # => false
p b == a # => true
No, but the latter syntax will give you a compiler error if you accidentally type
if (1 = evaluatedValue)
Note that today any decent compiler will warn you if you write
if (evaluatedValue = 1)
so it is mostly relevant for historical reasons.
Depends on the language.
In Prolog or Erlang, == is written = and is a unification rather than an assignment (you're asserting that the values are equal, rather then testing that they are equal or forcing them to be equal), so you can use it for an assertion if the left hand side is a constant, as explained here.
So X = 3 would unify the variable X and the value 3, whereas 3 = X would attempt to unify the constant 3 with the current value of X, and be equivalent of assert(x==3) in imperative languages.
It's the same thing
In general, it hardly matters whether you use,
Evaluated value == 1 OR 1 == evaluated value.
Use whichever appears more readable to you. I prefer if(Evaluated value == 1) because it looks more readable to me.
And again, I'd like to quote a well known scenario of string comparison in java.
Consider a String str which you have to compare with say another string "SomeString".
str = getValueFromSomeRoutine();
Now at runtime, you are not sure if str would be NULL. So to avoid exception you'll write
if(str!=NULL)
{
if(str.equals("SomeString")
{
//do stuff
}
}
to avoid the outer null check you could just write
if ("SomeString".equals(str))
{
//do stuff
}
Though this is less readable which again depends on the context, this saves you an extra if.
For this and similar questions can I suggest you find out for yourself by writing a little code, running it through your compiler and viewing the emitted asembler output.
For example, for the GNU compilers, you do this with the -S flag. For the VS compilers, the most convenient route is to run your test program in the debugger and then use the assembeler debugger view.
Sometimes in C++ they do different things, if the evaluated value is a user type and operator== is defined. Badly.
But that's very rarely the reason anyone would choose one way around over the other: if operator== is not commutative/symmetric, including if the type of the value has a conversion from int, then you have A Problem that probably wants fixing rather than working around. Brian R. Bondy's answer, and others, are probably on the mark for why anyone worries about it in practice.
But the fact remains that even if operator== is commutative, the compiler might not do exactly the same thing in each case. It will (by definition) return the same result, but it might do things in a slightly different order, or whatever.
if value == 1
if 1 == value
Is exactly the same, but if you accidentally do
if value = 1
if 1 = value
The first one will work while the 2nd one will produce an error.
They are the same. Some people prefer putting the 1 first, to void accidentally falling into the trap of typing
evaluated value = 1
which could be painful if the value on the left hand side is assignable. This is a common "defensive" pattern in C, for instance.
In C languages it's common to put the constant or magic number first so that if you forget one of the "=" of the equality check (==) then the compiler won't interpret this as an assignment.
In java, you cannot do an assignment within a boolean expression, and so for Java, it is irrelevant which order the equality operands are written in; The compiler should flag an error anyway.