swift - Exclamation mark prefix on variables during if statements and other as well? -

i'm confused after looking through similar questions of the(!) operator when prefixed on variable or other object in if statements, functions, etc?

example:    mutating func add(value: t)     {           if !contains(items, value)           {           items.append(value)             }    } 

the exclamation mark ! used 2 purposes. when see appear @ beginning of object, such in !contains(items, values), means "not". example...

let x = 10 let y = 5  if x == y { print("x equal y") } else if x != y { print("x not equal y") } 

the above code print => "x not equal y" .

the logical not (!) operator can used reverse boolean values. example...

    var falseboolvalue = false      falseboolvalue = !falseboolvalue     print(falseboolvalue) 

the above code print => "true"

in addition usage logical not operator, exclamation mark used implicitly unwrap optional values. whenever see exclamation mark appear @ end of object name, such in somevariable!, being used implicitly unwrap optional value. read optionals gain better understanding of how ! used optional values.