Conditionals Guard in

About Conditionals Guard

The guard statement in Swift is used for early returns from Swift functions when a necessary condition which needs to be met for further processing to continue is not met, e.g.:

guard myValue = 0 else { return 0 }
let root = myValue.squareRoot()

Here, the guard checks if the Boolean expression following the guard keyword evaluates as true. If it does, then processing continues with the code following the guard statement (here let root = myValue.squareRoot(). Otherwise it will execute the code in the else clause. Unlike an if statement, a guard statement must have an else clause, and unlike the else clause of an if-else, the else clause of a guard must exit the scope of the guard statement. I.e. it must use a control transfer statement such as return, continue, break, or it must throw an error or exit the program.

An example of its use is the sinc function, which is equal to sin(x)/x with sinc(0) defined to be 1, avoiding issues with division by 0. This function can be written in Swift, using a guard as:

func sinc(_ x: Double) -> Double {
    guard x != 0 else { return 1 }
    return sin(x) / x
}

sinc(0)              // returns 1
sinc(Double.pi / 2)  // returns 0.6366197723675814