I have a character array and I'm trying to figure out if it matches a string literal, for example:
char value[] = "yes";
if(value == "yes") {
// code block
} else {
// code block
}
This resulted in the following error: comparison with string literal results in unspecified behavior. I also tried something like:
char value[] = "yes";
if(strcmp(value, "yes")) {
// code block
} else {
// code block
}
This didn't yield any compiler errors but it is not behaving as expected.
Check the documentation for strcmp. Hint: it doesn't return a boolean value.
ETA: == doesn't work in general because cstr1 == cstr2 compares pointers, so that comparison will only be true if cstr1 and cstr2 point to the same memory location, even if they happen to both refer to strings that are lexicographically equal. What you tried (comparing a cstring to a literal, e.g. cstr == "yes") especially won't work, because the standard doesn't require it to. In a reasonable implementation I doubt it would explode, but cstr == "yes" is unlikely to ever succeed, because cstr is unlikely to refer to the address that the string constant "yes" lives in.
std::strcmp returns 0 if strings are equal.
strcmp returns a tri-state value to indicate what the relative order of the two strings are. When making a call like strcmp(a, b), the function returns
a value < 0 when a < b
0 when a == b
a value > 0 when a > b
As the question is tagged with c++, in addition to David Seilers excellent explanation on why strcmp() did not work in your case, I want to point out, that strcmp() does not work on character arrays in general, only on null-terminated character arrays (Source).
In your case, you are assigning a string literal to a character array, which will result in a null-terminated character array automatically, so no problem here. But, if you slice your character array out of e. g. a buffer, it may not be null-terminated. In such cases, it is dangerous to use strcmp() as it will traverse the memory until it finds a null byte ('\0') to form a string.
Another solution to your problem would be (using C++ std::string):
char value[] = "yes";
if (std::string{value} == "yes")) {
// code block
} else {
// code block
}
This will also only work for null-terminated character arrays. If your character array is not null-terminated, tell the std::string constructor how long your character array is:
char value[3] = "yes";
if (std::string{value, 3} == "yes")) {
// code block
} else {
// code block
}
I keep seeing code that does checks like this
if (IsGood == false)
{
DoSomething();
}
or this
if (IsGood == true)
{
DoSomething();
}
I hate this syntax, and always use the following syntax.
if (IsGood)
{
DoSomething();
}
or
if (!IsGood)
{
DoSomething();
}
Is there any reason to use '== true' or '== false'?
Is it a readability thing? Do people just not understand Boolean variables?
Also, is there any performance difference between the two?
I follow the same syntax as you, it's less verbose.
People (more beginner) prefer to use == true just to be sure that it's what they want. They are used to use operator in their conditional... they found it more readable. But once you got more advanced, you found it irritating because it's too verbose.
I always chuckle (or throw something at someone, depending on my mood) when I come across
if (someBoolean == true) { /* ... */ }
because surely if you can't rely on the fact that your comparison returns a boolean, then you can't rely on comparing the result to true either, so the code should become
if ((someBoolean == true) == true) { /* ... */ }
but, of course, this should really be
if (((someBoolean == true) == true) == true) { /* ... */ }
but, of course ...
(ah, compilation failed. Back to work.)
I would prefer shorter variant. But sometimes == false helps to make your code even shorter:
For real-life scenario in projects using C# 2.0 I see only one good reason to do this: bool? type. Three-state bool? is useful and it is easy to check one of its possible values this way.
Actually you can't use (!IsGood) if IsGood is bool?. But writing (IsGood.HasValue && IsGood.Value) is worse than (IsGood == true).
Play with this sample to get idea:
bool? value = true; // try false and null too
if (value == true)
{
Console.WriteLine("value is true");
}
else if (value == false)
{
Console.WriteLine("value is false");
}
else
{
Console.WriteLine("value is null");
}
There is one more case I've just discovered where if (!IsGood) { ... } is not the same as if (IsGood == false) { ... }. But this one is not realistic ;) Operator overloading may kind of help here :) (and operator true/false that AFAIK is discouraged in C# 2.0 because it is intended purpose is to provide bool?-like behavior for user-defined type and now you can get it with standard type!)
using System;
namespace BoolHack
{
class Program
{
public struct CrazyBool
{
private readonly bool value;
public CrazyBool(bool value)
{
this.value = value;
}
// Just to make nice init possible ;)
public static implicit operator CrazyBool(bool value)
{
return new CrazyBool(value);
}
public static bool operator==(CrazyBool crazyBool, bool value)
{
return crazyBool.value == value;
}
public static bool operator!=(CrazyBool crazyBool, bool value)
{
return crazyBool.value != value;
}
#region Twisted logic!
public static bool operator true(CrazyBool crazyBool)
{
return !crazyBool.value;
}
public static bool operator false(CrazyBool crazyBool)
{
return crazyBool.value;
}
#endregion Twisted logic!
}
static void Main()
{
CrazyBool IsGood = false;
if (IsGood)
{
if (IsGood == false)
{
Console.WriteLine("Now you should understand why those type is called CrazyBool!");
}
}
}
}
}
So... please, use operator overloading with caution :(
According to Code Complete a book Jeff got his name from and holds in high regards the following is the way you should treat booleans.
if (IsGood)
if (!IsGood)
I use to go with actually comparing the booleans, but I figured why add an extra step to the process and treat booleans as second rate types. In my view a comparison returns a boolean and a boolean type is already a boolean so why no just use the boolean.
Really what the debate comes down to is using good names for your booleans. Like you did above I always phrase my boolean objects in the for of a question. Such as
IsGood
HasValue
etc.
The technique of testing specifically against true or false is definitely bad practice if the variable in question is really supposed to be used as a boolean value (even if its type is not boolean) - especially in C/C++. Testing against true can (and probably will) lead to subtle bugs:
These apparently similar tests give opposite results:
// needs C++ to get true/false keywords
// or needs macros (or something) defining true/false appropriately
int main( int argc, char* argv[])
{
int isGood = -1;
if (isGood == true) {
printf( "isGood == true\n");
}
else {
printf( "isGood != true\n");
}
if (isGood) {
printf( "isGood is true\n");
}
else {
printf( "isGood is not true\n");
}
return 0;
}
This displays the following result:
isGood != true
isGood is true
If you feel the need to test variable that is used as a boolean flag against true/false (which shouldn't be done in my opinion), you should use the idiom of always testing against false because false can have only one value (0) while a true can have multiple possible values (anything other than 0):
if (isGood != false) ... // instead of using if (isGood == true)
Some people will have the opinion that this is a flaw in C/C++, and that may be true. But it's a fact of life in those languages (and probably many others) so I would stick to the short idiom, even in languages like C# that do not allow you to use an integral value as a boolean.
See this SO question for an example of where this problem actually bit someone...
isalpha() == true evaluates to false??
I agree with you (and am also annoyed by it). I think it's just a slight misunderstanding that IsGood == true evaluates to bool, which is what IsGood was to begin with.
I often see these near instances of SomeStringObject.ToString().
That said, in languages that play looser with types, this might be justified. But not in C#.
Some people find the explicit check against a known value to be more readable, as you can infer the variable type by reading. I'm agnostic as to whether one is better that the other. They both work. I find that if the variable inherently holds an "inverse" then I seem to gravitate toward checking against a value:
if(IsGood) DoSomething();
or
if(IsBad == false) DoSomething();
instead of
if(!IsBad) DoSomething();
But again, It doen't matter much to me, and I'm sure it ends up as the same IL.
Readability only..
If anything the way you prefer is more efficient when compiled into machine code. However I expect they produce exactly the same machine code.
From the answers so far, this seems to be the consensus:
The short form is best in most cases. (IsGood and !IsGood)
Boolean variables should be written as a positive. (IsGood instead of IsBad)
Since most compilers will output the same code either way, there is no performance difference, except in the case of interpreted languages.
This issue has no clear winner could probably be seen as a battle in the religious war of coding style.
I prefer to use:
if (IsGood)
{
DoSomething();
}
and
if (IsGood == false)
{
DoSomething();
}
as I find this more readable - the ! is just too easy to miss (in both reading and typing); also "if not IsGood then..." just doesn't sound right when I hear it, as opposed to "if IsGood is false then...", which sounds better.
It's possible (although unlikely, at least I hope) that in C code TRUE and FALSE are #defined to things other than 1 and 0. For example, a programmer might have decided to use 0 as "true" and -1 as "false" in a particular API. The same is true of legacy C++ code, since "true" and "false" were not always C++ keywords, particularly back in the day before there was an ANSI standard.
It's also worth pointing out that some languages--particularly script-y ones like Perl, JavaScript, and PHP--can have funny interpretations of what values count as true and what values count as false. It's possible (although, again, unlikely on hopes) that "foo == false" means something subtly different from "!foo". This question is tagged "language agnostic", and a language can define the == operator to not work in ways compatible with the ! operator.
I've seen the following as a C/C++ style requirement.
if ( true == FunctionCall()) {
// stuff
}
The reasoning was if you accidentally put "=" instead of "==", the compiler will bail on assigning a value to a constant. In the meantime it hurts the readability of every single if statement.
Occasionally it has uses in terms of readability. Sometimes a named variable or function call can end up being a double-negative which can be confusing, and making the expected test explicit like this can aid readability.
A good example of this might be strcmp() C/C++ which returns 0 if strings are equal, otherwise < or > 0, depending on where the difference is. So you will often see:
if(strcmp(string1, string2)==0) { /*do something*/ }
Generally however I'd agree with you that
if(!isCached)
{
Cache(thing);
}
is clearer to read.
I prefer the !IsGood approach, and I think most people coming from a c-style language background will prefer it as well. I'm only guessing here, but I think that most people that write IsGood == False come from a more verbose language background like Visual Basic.
Only thing worse is
if (true == IsGood) {....
Never understood the thought behind that method.
The !IsGood pattern is eaiser to find than IsGood == false when reduced to a regular expression.
/\b!IsGood\b/
vs
/\bIsGood\s*==\s*false\b/
/\bIsGood\s*!=\s*true\b/
/\bIsGood\s*(?:==\s*false|!=\s*true)\b/
For readability, you might consider a property that relies on the other property:
public bool IsBad => !IsGood;
Then, you can really get across the meaning:
if (IsBad)
{
...
}
I prefer !IsGood because to me, it is more clear and consise. Checking if a boolean == true is redundant though, so I would avoid that. Syntactically though, I don't think there is a difference checking if IsGood == false.
In many languages, the difference is that in one case, you are having the compiler/interpreter dictate the meaning of true or false, while in the other case, it is being defined by the code. C is a good example of this.
if (something) ...
In the above example, "something" is compared to the compiler's definition of "true." Usually this means "not zero."
if (something == true) ...
In the above example, "something" is compared to "true." Both the type of "true" (and therefor the comparability) and the value of "true" may or may not be defined by the language and/or the compiler/interpreter.
Often the two are not the same.
You forgot:
if(IsGood == FileNotFound)
It seems to me (though I have no proof to back this up) that people who start out in C#/java type languages prefer the "if (CheckSomething())" method, while people who start in other languages (C++: specifically Win32 C++) tend to use the other method out of habit: in Win32 "if (CheckSomething())" won't work if CheckSomething returns a BOOL (instead of a bool); and in many cases, API functions explicitly return a 0/1 int/INT rather than a true/false value (which is what a BOOL is).
I've always used the more verbose method, again, out of habit. They're syntactically the same; I don't buy the "verbosity irritates me" nonsense, because the programmer is not the one that needs to be impressed by the code (the computer does). And, in the real world, the skill level of any given person looking at the code I've written will vary, and I don't have the time or inclination to explain the peculiarities of statement evaluation to someone who may not understand little unimportant bits like that.
Ah, I have some co-worked favoring the longer form, arguing it is more readable than the tiny !
I started to "fix" that, since booleans are self sufficient, then I dropped the crusade... ^_^ They don't like clean up of code here, anyway, arguing it makes integration between branches difficult (that's true, but then you live forever with bad looking code...).
If you write correctly your boolean variable name, it should read naturally:
if (isSuccessful) vs. if (returnCode)
I might indulge in boolean comparison in some cases, like:
if (PropertyProvider.getBooleanProperty(SOME_SETTING, true) == true) because it reads less "naturally".
For some reason I've always liked
if (IsGood)
more than
if (!IsBad)
and that's why I kind of like Ruby's unless (but it's a little too easy to abuse):
unless (IsBad)
and even more if used like this:
raise InvalidColor unless AllowedColors.include?(color)
Cybis, when coding in C++ you can also use the not keyword. It's part of the standard since long time ago, so this code is perfectly valid:
if (not foo ())
bar ();
Edit: BTW, I forgot to mention that the standard also defines other boolean keywords such as and (&&), bitand (&), or (||), bitor (|), xor (^)... They are called operator synonyms.
If you really think you need:
if (Flag == true)
then since the conditional expression is itself boolean you probably want to expand it to:
if ((Flag == true) == true)
and so on. How many more nails does this coffin need?
If you happen to be working in perl you have the option of
unless($isGood)
I do not use == but sometime I use != because it's more clear in my mind. BUT at my job we do not use != or ==. We try to get a name that is significat if with hasXYZ() or isABC().
Personally, I prefer the form that Uncle Bob talks about in Clean Code:
(...)
if (ShouldDoSomething())
{
DoSomething();
}
(...)
bool ShouldDoSomething()
{
return IsGood;
}
where conditionals, except the most trivial ones, are put in predicate functions. Then it matters less how readable the implementation of the boolean expression is.
We tend to do the following here:
if(IsGood)
or
if(IsGood == false)
The reason for this is because we've got some legacy code written by a guy that is no longer here (in Delphi) that looks like:
if not IsNotGuam then
This has caused us much pain in the past, so it was decided that we would always try to check for the positive; if that wasn't possible, then compare the negative to false.
The only time I can think where the more vebose code made sense was in pre-.NET Visual Basic where true and false were actually integers (true=-1, false=0) and boolean expressions were considered false if they evaluated to zero and true for any other nonzero values. So, in the case of old VB, the two methods listed were not actually equivalent and if you only wanted something to be true if it evaluated to -1, you had to explicitly compare to 'true'. So, an expression that evaluates to "+1" would be true if evaluated as integer (because it is not zero) but it would not be equivalent to 'true'. I don't know why VB was designed that way, but I see a lot of boolean expressions comparing variables to true and false in old VB code.