Conditionals Switch in

About Conditionals Switch

if str == "apple" {
    print("Let's bake an apple crumble")
} else if str == "lemon" {
    print("Let's bake a lemon meringue pie!")
} else if str == "peach" {
    print("Let's bake a peach pie!")
} else {
    print("Let's buy ice cream.")
}

While an else-if variant of if statements, like the one above, cleans things up considerably compared to a series of if statements without else clauses, there is still a lot of noise in the code from all of the curly braces. This is where the switch statement comes into play. In conditional statements with many possible branches, the switch statement shines. Note, however, that switch statements do work a bit differently from if statements.

Rather than evaluating a Boolean expression and using the value of that expression to choose the code branch that is run, a simple switch statement takes an input value (or expression which it evaluates to obtain the input value) of some type and compares against one or more values of the same type. If a case is found that matches the input value, the corresponding block of code is run.

Note that all possible cases must be covered in a switch statement. In cases like the above where all possible strings cannot be enumerated, a default case can be used to match all remaining cases.

switch str {
case "apple":
    print("Let's bake an apple crumble")
case "lemon":
    print("Let's bake a lemon meringue pie!")
case "peach":
    print("Let's bake a peach pie!")
default:
    print("Let's buy ice cream.")
}

Additionally, note that if multiple cases of a switch statement match the input value, only the first matching case is used. An underscore (_) can be used to match all values.

Unlike in some other languages, switch cases in Swift do not fall through to the next case unless that behavior is explicitly called for with the fallthrough keyword. this is the opposite behavior from C which requires explicit break statements to prevent fallthrough.

Binding and where statements

The values being matched in switch statements can also be bound to names which can be used in the body of the case. They can also be used in where clauses, which are additional boolean expressions that must evaluate as true for the case to match.

let x = 1337
switch sumOfDivisors(of: x) {
case let total where total == x:
  print(total, "is a perfect number")
default:
  print(x, "is not a perfect number")
}