Another neat feature available for the cases of a switch statement is the where clause.

The where clause allows for additional pattern matching for a given expression. It can also be used with loops and other conditionals such as if statements.

Assume we’re creating a program that determines if a random integer between 0 and 10 is even or odd. We can write the following program:

var randomNumber = Int.random(in: 0...10)

switch randomNumber {
  case let x where x % 2 == 0:
    print("\(randomNumber) is even")
  case let x where x % 2 == 1:
    print("\(randomNumber) is odd")
  default:
    print("Invalid")
}

Let’s dive into what’s happening on the first line:

var randomNumber = Int.random(in: 0...10)

• We’re generating a random integer, Int, using the built-in Swift method, .random() which returns an arbitrary value from a range of numbers. Notice how we’re using the closed range operator, ..., to denote a numerical range.

• We then assign the randomly generated value to the variable, randomNumber. We’ll be working more with .random() in the following lessons.

Following the variable declaration is a standard switch statement that checks the value of randomNumber:

switch randomNumber {
  case let x where x % 2 == 0:
    print("\(randomNumber) is even")
  case let x where x % 2 == 1:
    print("\(randomNumber) is odd")
  default:
    print("Invalid")
}

// Prints: 7 is odd

• Each case contains a variable declaration followed by a where clause and a condition. When a condition is true, the code for that case will execute.

• The let keyword followed by the x creates a temporary binding to the randomNumber value. This means that the value of x temporarily becomes the value of randomNumber. If randomNumber is 5, then x is 5!

• The let keyword is specifically used here instead of var since the value of x will not be reassigned at any point throughout the switch statement, thus its value will always be constant. If var is used, Swift will display a compiler warning recommending us to use let instead:

Numbers.swift:6:12: warning: variable 'x' was never mutated; consider changing to 'let' constant

Note: a compiler warning is not an error. Your program should still run even with a warning.

• Lastly, the where condition checks if x is divisible by 2 with or without a remainder and determines if the randomNumber is even or odd.

If you run this code, chances are your output will be different from ours since the number generated each time is random!