CODEWITHSUNDEEP
objective-c
Bear
Bear

Reputation: 5152

What is the meaning of =! in objective c

!= in most language means not equal But how about =! in objective C? I found this in the code snippet below.
Thanks

- (void) toggleButton: (UIButton *) aButton
{
    if ((_isOn = !_isOn))
    {
        [self setBackgroundImage:BASEGREEN forState:UIControlStateNormal];
        [self setBackgroundImage:PUSHGREEN forState:UIControlStateHighlighted];
        [self setTitle:@"On" forState:UIControlStateNormal];
        [self setTitle:@"On" forState:UIControlStateHighlighted];
    }
    else
    {
        [self setBackgroundImage:BASERED forState:UIControlStateNormal];
        [self setBackgroundImage:PUSHRED forState:UIControlStateHighlighted];
        [self setTitle:@"Off" forState:UIControlStateNormal];
        [self setTitle:@"Off" forState:UIControlStateHighlighted];
    }

    [self relaxButton:self];
}

Upvotes: 0

Views: 1779

Answers (3)

Brian Tracy
Brian Tracy

Reputation: 6831

! is one of the fundamental boolean operators. Along with || and &&, ! is used to manipulate boolean values. Because booleans can only be true or false, boolean operators are easy to understand.

1) The not operator (!) is used to reverse the state of a boolean variable. True becomes false and false becomes true after not.

 BOOL a = NO;
 a = !a;
 if (a) {
 ...
 // will execute because a is now YES, or true
 }

2) The and operator (&&) is used on two boolean values. It return true if both values being compared are true. It returns false if any of the values being compared are false

BOOL a = NO;
BOOL b = YES;
if (a && b) {
...
// will not execute because NO && YES is NO
}

a = YES;
if (a && b) {
...
// will execute because YES && YES is YES
}

3) The or operator (||) is also used on two boolean variables. It return true if either one of the booleans in question is true.

BOOL a = YES;
BOOL b = NO;
if (a || b) {
...
// will execute because YES || NO is YES
}

A truth table is a valuable rescource when thinking about boolean operators

true || true = true
true || false = true
false || true = true
false || false = false

true && true = true
true && false = false
false && true = false
false && false = false

!true = false
!false = true

Upvotes: 1

pjs
pjs

Reputation: 19855

You're misreading it. It's not _isOn =! _isOn, it's _isOn = !_isOn. Notice the space between the = and the !. A single exclamation is logical negation, i.e., it is toggling the value of _isOn and assigning the result back to _isOn.

Upvotes: 0

Jsdodgers
Jsdodgers

Reputation: 5312

A =!B is basically a boolean saying "set A to the opposite of B".

For example, you could do:

BOOL a = NO;
BOOL b = !a;

and b would then be YES.

Your line of code is basically flipping the state of the BOOL is_On, and then the if statement block is being executed if the new state is YES, otherwise the else block is being executed.

Upvotes: 7

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