Guard statements give us another way to avoid overly nested code. A guard statement has the following form:

func greetUser(isAuthenticated: Bool) {
    guard isAuthenticated else {
        print("Error: user is not authenticated")
        return
    }
    print("Hello user!")
}

One way to read this guard statement is: “Make sure that userAuthenticated is true. If it’s false, print “Error: user is not authenticated” and return. If it’s true, continue executing the rest of the code.”

Unlike if statements, a guard statement must have an else statement. The else statement after a guard has to exit the current scope. Guard statements can be used to exit the scope of loops as well:

var nums = [3,1,0,6]

for num in nums {
    guard num != 0 else {
        print("Can't divide by zero!")
        continue
    }
    print("10 / \(num) = \(10 / num)")
}

// prints
// 10 / 3 = 3
// 10 / 1 = 10
// Can't divide by zero!
// 10 / 6 = 1

Guard statements can also be used to bind optionals just like with if let statements. They also support multiple assignments:

func addOptionals(shouldPrintSum: Bool) {
    let a: Int? = 1
    let b: Int? = 2
    let c: Int? = 3

    guard let unwrappedA = a, let unwappedB = b, let unwrappedC = c, shouldPrintSum else {
        return
    }
    print(unwrappedA + unwappedB + unwrappedC)
}