boolean operation trick

Question

I've seen this before in code, but forgotten it. Basically it toggles a boolean variable. If it's true, it'll set to false and vice-versa. But unfortunately forgot the syntax.

It's basically a one liner for this:

if (myVar) {  
    myVar = false;  
} else {  
    myVar = true;  
}  

It's something like this, but don't know what it's called or the correct syntax of it:

myVar = myVar : false ? true;

Answer 1

How about

myVar = !myVar

?

Answer 2

myVar = myVar ? false : true; is using the conditional operator.

You can just do this though

myVar = !myVar;

Answer 3

The smallest code I can think of at the moment. I don't know what its called (if it has a name, as you seem to suggest)

myVar = !myVar

Answer 4

What you are thinking of is the conditional operator:

myVar = myVvar ? false : true;

(As you see, a lot of people call this "the ternary operator", but that only means that it is an operator with three operands. As it happens, there is only one operator with three operands in this language, but it still says nothing about what the operator does.)

It's of course easier to use the negation operator:

myVar = !myVar;

Answer 5

There is a ternary operator (wikipedia). Which allows you to write a condensed if-else statement like in the second example.

In java:

myVar = (myVar) ? true : false;

There is also the NOT operator, which toggles a boolean variable. In java that is !. I believe that is what you want.

myVar = !myVar;

Answer 6

What you're talking about is the "ternary" or "conditional" operator, which does an inline substitution as per a condition.

The syntax is:

condition ? trueValue : falseValue

I usually throw parentheses around my condition, sometimes around the whole conditional operator. Depends on how much I'm trying to delineate it from everything else.

So for example, suppose you want to return the larger of two numbers:

public int max(int a, int b)
{
  return (a > b) ? a : b;
}

Notice that it can be substituted into the middle of something else.

---

Okay, now let's tackle your actual question about toggling a boolean type.

myVar = (myVar) ? false : true;

is how you would do it with the conditional operator. (Again, parentheses aren't required, I just favor them.)

But there's a simpler way to toggle the boolean... using the logical NOT ("!") operator:

myVar = !myVar;

Keep it simple. :-)

Answer 7

if(myVar == true)
{
    myVar = false;
}
else if (myVar == false)
{
    myVar = true;
}
else
{
    myVar = FILE_NOT_FOUND
}

Answer 8

This also works :P

v=v?!v:!v;

Answer 9

public boolean toggle(boolean bool)
{
  return !bool;
}

Answer 10

Another option is XOR:

myVar ^= true;

It's notable in that only the LHS of the assignment ever changes; the right side is constant and will toggle any boolean variable. Negation's more self-documenting IMO, though.

Answer 11

As you requested a trick I would offer this one:

myVar ^= true;

Answer 12

As others have noted, there are two ways to negate something: "lvalue = !lvalue;" and "lvalue ^= 1;". It's important to recognize the differences.

Saying "lvalue = !lvalue" will cause lvalue to be set to 1 if it was zero, and 0 if it was set to anything else. The lvalue will be evaluated twice; this is not a factor for simple variables, but saying "someArray[index1][index2][index3][index4] = !someArray[index1][index2][index3][index4]" could slow things down.

Saying "lvalue ^= 1;" will cause lvalue to be set to 1 if it was 0, 0 if it was 1, and something else if it was neither zero nor 1. The lvalue need only be specified or evaluated once, and if the value is known to be either zero or 1, this form is likely to be faster.

Too bad there's no auto-negate operator; there are times such a thing would be handy.

Answer 13

You can also use the binary form of negation as shown here.

if ((v == true) && !(v = false)) {
    v != true;   /* negate with true if true. */
} else {
    v =! false;  /* negate with false if false. */
}