Conditionals

Introduction#

Conditional expressions, involving keywords such as if, else if, and else, provide Swift programs with the ability to perform different actions depending on a Boolean condition: True or False. This section covers the use of Swift conditionals, Boolean logic, and ternary statements.

Remarks#

For more information about conditional statements, see The Swift Programming Language.

Using Guard

Guard checks for a condition, and if it is false, it enters the branch. Guard check branches must leave its enclosing block either via return, break, or continue (if applicable); failing to do so results in a compiler error. This has the advantage that when a guard is written it’s not possible to let the flow continue accidentally (as would be possible with an if).

Using guards can help keep nesting levels low, which usually improves the readability of the code.

func printNum(num: Int) {
    guard num == 10 else {
        print("num is not 10")
        return
    }
    print("num is 10")
}

Guard can also check if there is a value in an optional, and then unwrap it in the outer scope:

func printOptionalNum(num: Int?) {
    guard let unwrappedNum = num else {
        print("num does not exist")
        return
    }
    print(unwrappedNum)
}

Guard can combine optional unwrapping and condition check using where keyword:

func printOptionalNum(num: Int?) {
guard let unwrappedNum = num, unwrappedNum == 10 else {
    print("num does not exist or is not 10")
    return
}
print(unwrappedNum)
}

Basic conditionals: if-statements

An if statement checks whether a Bool condition is true:

let num = 10

if num == 10 {
    // Code inside this block only executes if the condition was true.
    print("num is 10")
}

let condition = num == 10   // condition's type is Bool
if condition {
    print("num is 10")
}

if statements accept else if and else blocks, which can test alternate conditions and provide a fallback:

let num = 10
if num < 10 {  // Execute the following code if the first condition is true.
    print("num is less than 10")
} else if num == 10 {  // Or, if not, check the next condition...
    print("num is 10")
} else {  // If all else fails...
    print("all other conditions were false, so num is greater than 10")
}

Basic operators like && and || can be used for multiple conditions:

The logical AND operator

let num = 10
let str = "Hi"
if num == 10 && str == "Hi" {
    print("num is 10, AND str is \"Hi\"")
}

If num == 10 was false, the second value wouldn’t be evaluated. This is known as short-circuit evaluation.

The logical OR operator

if num == 10 || str == "Hi" {
    print("num is 10, or str is \"Hi\")
}

If num == 10 is true, the second value wouldn’t be evaluated.

The logical NOT operator

if !str.isEmpty {
    print("str is not empty")
}

Optional binding and “where” clauses

Optionals must be unwrapped before they can be used in most expressions. if let is an optional binding, which succeeds if the optional value was not nil:

let num: Int? = 10 // or: let num: Int? = nil

if let unwrappedNum = num {
    // num has type Int?; unwrappedNum has type Int
    print("num was not nil: \(unwrappedNum + 1)")
} else {
    print("num was nil")
}

You can reuse the same name for the newly bound variable, shadowing the original:

// num originally has type Int?
if let num = num {
    // num has type Int inside this block
}

Combine multiple optional bindings with commas (,):

if let unwrappedNum = num, let unwrappedStr = str {
    // Do something with unwrappedNum & unwrappedStr
} else if let unwrappedNum = num {
    // Do something with unwrappedNum
} else {
    // num was nil
}

Apply further constraints after the optional binding using a where clause:

if let unwrappedNum = num where unwrappedNum % 2 == 0 {
    print("num is non-nil, and it's an even number")
}

If you’re feeling adventurous, interleave any number of optional bindings and where clauses:

if let num = num                           // num must be non-nil
    where num % 2 == 1,                    // num must be odd
    let str = str,                         // str must be non-nil
    let firstChar = str.characters.first   // str must also be non-empty
    where firstChar != "x"                 // the first character must not be "x"
{
    // all bindings & conditions succeeded!
}

In Swift 3, where clauses have been replaced (SE-0099): simply use another , to separate optional bindings and boolean conditions.

if let unwrappedNum = num, unwrappedNum % 2 == 0 {
    print("num is non-nil, and it's an even number")
}

if let num = num,                           // num must be non-nil
    num % 2 == 1,                           // num must be odd
    let str = str,                          // str must be non-nil
    let firstChar = str.characters.first,   // str must also be non-empty
    firstChar != "x"                        // the first character must not be "x"
{
    // all bindings & conditions succeeded!
}

Ternary operator

Conditions may also be evaluated in a single line using the ternary operator:

If you wanted to determine the minimum and maximum of two variables, you could use if statements, like so:

let a = 5
let b = 10
let min: Int

if a < b {
    min = a 
} else {
    min = b 
}

let max: Int

if a > b {
    max = a 
} else {
    max = b 
}

The ternary conditional operator takes a condition and returns one of two values, depending on whether the condition was true or false. The syntax is as follows: This is equivalent of having the expression:

(<CONDITION>) ? <TRUE VALUE> : <FALSE VALUE>

The above code can be rewritten using ternary conditional operator as below:

let a = 5
let b = 10
let min = a < b ? a : b
let max = a > b ? a : b

In the first example, the condition is a < b. If this is true, the result assigned back to min will be of a; if it’s false, the result will be the value of b.

Note: Because finding the greater or smaller of two numbers is such a common operation, the Swift standard library provides two functions for this purpose: max and min.

Nil-Coalescing Operator

The nil-coalescing operator <OPTIONAL> ?? <DEFAULT VALUE> unwraps the <OPTIONAL> if it contains a value, or returns <DEFAULT VALUE> if is nil. <OPTIONAL> is always of an optional type. <DEFAULT VALUE> must match the type that is stored inside <OPTIONAL>.

The nil-coalescing operator is shorthand for the code below that uses a ternary operator:

a != nil ? a! : b

this can be verified by the code below:

(a ?? b) == (a != nil ? a! : b) // ouputs true

Time For An Example

let defaultSpeed:String = "Slow"
var userEnteredSpeed:String? = nil

print(userEnteredSpeed ?? defaultSpeed) // ouputs "Slow"

userEnteredSpeed = "Fast"
print(userEnteredSpeed ?? defaultSpeed) // ouputs "Fast"