Is it not a good practice to put long condition in if statement like
if(((FIO2PIN & 0x00001000)>>12))
which will give result as 0/1 at the end in ARM7?
Is that so that I can only check for 0 or 1 in if condition?
For example
if(x!=0)
or
if(x==1)??
indirectly (FIO2PIN & 0x00001000)>>12 will also give some value at the end which might be 0/1 depending on FIO2PIN status right?
The expression ((FIO2PIN & 0x00001000)>>12) is an integer expression and is implicitly cast to a boolean by the if(...), where zero is false and non-zero is true.
There is nothing wrong with that in the sense that it is entirely unambiguous as far as the compiler and language definition are concerned, but I prefer to use only explicitly boolean expressions in conditional statements - in order to make the intent of the programmer clear. That is easily done by explicitly comparing the result with zero; in this case:
if( ((FIO2PIN & 0x00001000) >> 12) != 0 )
However, the shift is entirely unnecessary in either case, because any non-zero value will be accepted as true (which is why you should always compare with zero - or nothing at all). So:
if( FIO2PIN & 0x00001000 )
or
if( (FIO2PIN & 0x00001000) != 0 )
are equally valid - the latter being my preference.
As mentioned, any non-zero value will be accepted as true while only zero is false, so where x is an integer expression, the test x == 1 is a dangerous one, and you should use x != 0 instead.
The if statement will be true if the expression is non-zero. So shifting right by twelve bits is not necessary in your example. Because (FIO2PIN & 0x00001000) is non-zero whenever ((FIO2PIN & 0x00001000) >> 12) is non-zero. In other words, it doesn't matter which bit is non-zero. The if statement will test true if any bit is non-zero.
In my opinion, using a complex expression within an if statement could be bad practice if the expression is so complex that it is difficult for a developer to understand or maintain. But otherwise, as long as the expression is correct, then the compiler should sort it out and you shouldn't need to worry whether it is too complex for the compiler.
I'm looking over the diffs submitted to a project by another developer, and they have a lot of code that does !!<some BOOL value>. In fact, this seems to be their standard pattern for implementing boolean getters and setters. They've implemented their code like:
- (BOOL) hasId {
return !!hasId_;
}
- (void) setHasId:(BOOL) value {
hasId_ = !!value;
}
I've never seen this pattern before, and am wondering if there is any benefit in using it. Is the double-negation doing anything useful?
The double boolean operator just makes sure that the value returned is either a 1 or a 0. That's all : )
! is a logical negation operator. So if setHasId: was passed, eg., 0x2 then the double negation would store 0x1.
It is equivalent to:
hasId_ = value ? 1 : 0;
It is useful in some cases because if you do this:
BOOL x = y & MY_FLAG;
You might get 0 if MY_FLAG is set, because the result gets truncated to the size of a BOOL (8 bits). This is unexpected. For the same reasons, people sometimes prefer that BOOL is either 0 or 1 (so bit operations work as expected). It is usually unnecessary.
In languages with a built-in bool type such as C (as of C99) and C++, converting an integer to bool does this automatically.
It makes more sense in some other cases for example where you are returning BOOL but don't want to put an if statement in.
- (BOOL)isMyVarSet
{
return !!myVar;
}
In this case I can't just return myVar because it's not a BOOL (this is a very contrived example - I can't dig out a decent one from my projects).
I've used this before and I believe:
if (!!myVar)
is equivalent to:
if (myVar != nil)
Basically, I use it to verify the value of SOMETHING.
I will admit... this is probably not the best practice or most-understood way to accomplish this goal.
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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 I have a Boolean expression to check
(A && B)
If A is found to be false will the language bother to check B? Does this vary from language to language?
The reason I ask is that I'm wondering if it's the case that B is checked even if A is false then wouldn't
if (A) {
if(B) {
} else {
// code x
}
} else {
// code x
}
be marginally quicker than
if (A && B) {
} else {
// code x
}
This depends on the language. Most languages will implement A && B as a short-circuit operator, meaning that if A evaluates to false, B will never be evaluated. There's a detailed list on Wikipedia.
Almost every language implements something called short-circuit evaluation, which means that yes, (A && B) will not evaluate B if A is false. This also takes effect if you write:
if (A || B) {
...
}
and A is true. This is worth remembering if B may take a long time to load, but generally it's not something to worry about.
As a bit of history, in my mind this is a bit of a sore part of LISP because code like this:
(if (and (= x 5) (my-expensive-query y)) "Yes" "No")
is not made of functions, but rather so-called "special forms" (that is, "and" could not be defun'd here).
This would depend 100% on how the language compiles said code. Anything is possible :)
Is there a specific language you're wondering about?
In short, no. A double branch involves various forms of branch prediction. If A and B are simple to evaluate, it may be faster to do if (A && B) in a non-short-circuit way than if (A) if (B). In addition, you've duplicated code in the second form. This is virtually always (exception to every rule .. I guess) bad and far worse than any gain.
Secondly, this is the kind of micro-optimization that you give to the language interpreter, JIT or compiler.
Many languages (including almost all curly-brace languages, like C/C++/Java/C#) offer short-circuit boolean evaluation. In those languages, if A is false then B won't be evaluated. You'll need to see (or ask) whether this is true for your specific language, or whether there's a way to do it (VB has AndAlso, for example).
If you find your language doesn't support it, you'll also need to consider whether the cost of evaluating B is worth having to maintain two identical pieces of code -- and the potential doubling in cache footprint (not to mention all the extra branching) that'd come from doing that duplication every time.
As others have said, it depends on the language and/or compiler. For me, I don't care how fast or slow it might be to short-circuit or not, the need to duplicate code is a deal-killer.
If A and B are actually calls that have side-effects (i.e. they do more than simply return a value suitable for comparison), then I would argue that those calls should be made into variable assignments that are then used in your comparison. It doesn't matter whether or not you always require those side-effects or only require them conditionally, the code will be more readable if you don't depend on whether or not short-circuit exists.
That last bit about readability is based on my feeling that reducing the need to refer to external documentation improves readability. Reading a book with a bunch of new words that require dictionary look-ups is much more challenging than reading that same book when you already have the necessary vocabulary. In this case, short-circuit is invisible, so anybody that needs to look it up won't even know that they need to look it up.
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.