How to get the opposite value of a bool variable in C ++

Question

For me, the bool variable indicates either true or false .

Some bool variable was defined and initialized with a value that we do not know. I just want to get the opposite value. How to do it in C ++?

Answer 1

Just use the operator ! :

 bool x = // something bool y = !x; // Get the opposite. 

Answer 2

 bool b = false; 

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Correct solution:

 b = !b; 

Interesting solutions that you should not use:

 b = b?false:true; b ^= 1; b = (b+1)%2; b = 1>>b; b = 1-b; b = b-1; b = -1*(b-1); b = b+'0'+'1'-'b'; 

As an exercise, try to find out why the above solutions work.

Answer 3

OPERATORS for logical not

You can choose whether you want to write ! , or not , or some kind of mixture.

However, as of version 10.0, Visual C ++ still does not have the reserved word not .

So, for Visual C ++, if you want to use not , you must include the header [ iso646.h ], which is the header from the C standard library, guaranteed to be available in C ++ as well. However, for the standard C ++ compiler that includes this header, it does not work (as indicated in footnote 176 of the C ++ 11 standard). So you can just turn it on anyway:

 #include <iostream> #include <iso646.h> // Visual C++ is not quite standard and requires this. int main() { using namespace std; // Display false and true as "false" and "true": cout << boolalpha; cout << "!false = " << !false << endl; cout << "not false = " << not false << endl; } 

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ON COMPARING Boolean Values:

Some newbies write things like

  v != true 

This is not just a verbose, but a completely dangerous habit. The reason this is dangerous is because many APIs define logical types where the possible values ​​are not limited to only 0 and 1 (the main example is the Window & rsquo; s BOOL ). Then the value can mean the logical value True, without being numerically equal to True.

So, for maximum safety, use the habit ! or not , and just don’t compare directly with a literal boolean value.

An example where it is usually necessary to compare logical values ​​is where you need a logical Xor (exclusive or, or-or). In C ++, there is no operator for this at the BOOL level. But you can write a != b to achieve the same as the hypothetical BOOL level operator Xor.

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Logical Xor as a CONTROLLED INVERSION

In some cases, you need an inversion (logical Not application) if some condition is true, for example & hellip;

  if( condition ) { x = !x; } 

Otherwise, it can be written as & hellip;

  x = ((condition) != x); 

It looks like a complete obfuscation, but it has two functions that may be useful in certain situations:

• it can be used as a kind of "masked" inversion for an array a values ​​with the values ​​of some other array, which serves to control the inversion of each element a and

• it is a calculation of a pure expression that does not include the choice of the execution path, and therefore it can apparently be used as optimization (however, I understand that the modern compiler will probably do this for you at the level of machine code, if appropriate).

Cheers and hth.,

Answer 4

You need a NOT operator, which ! in C, C ++ and many other related languages.

 bool t = true; bool f = !t; // f = false 

Answer 5

Given that bool value; set accordingly:

You should use for this:

  !value 

where ! logically not here.

In addition, there are a number of other ways to achieve the same effect that are all effectively obfuscated:

• std::not_equal_to<bool>()(true,value); ( #include <functional> required)
• std::count(&value, &value+1, false); ( #include <algorithm> required)
• std::count_if(&value,&value+1,std::not1(std::bind2nd(std::greater_equal<bool>(), true)));

Answer 6

you need to use the not operator so that it gives you the opposite

  ! 

this should give you what you need.

Answer 7

 bool opposite = (someboolvar) ? 0 : 1 ;