Conditions and While Loops

Boolean Operators


Use the || infix operator to use an inclusive or operator between two booleans

let myBool = false || true
myBool: Bool = true

Both or and and operators in Swift are short-circuit meaning that the operator will operate on its parameters until it has guaranteed result in which case it will return. This means, in some cases, they will not operate both conditions.

let anotherBool = true || 0 / 0 == 1

Due to short-cuircuit rules, we would expect this to evaluate to true. When an or operator reaches an expression that evaluates to true, it can stop evaluation because no matter what the other expression is, it will be true. Thus, it will never encounter the error.

I slightly lied... This is actually a special case because the compiler knows to look for division by 0, so this actually does not compile (but if we ran it, it would evaluate to true). We can silence this error by hiding the fact that we are dividing by 0 with a function:

func hiddenZero() -> Int { 0 }
// hiddenZero: () -> Int

let hiddenErrorBool = true || 0 / hiddenZero() == 1
// hiddenErrorBool: Bool = true

This compiles and runs without error. Although, if we used false instead of true for the first expression, the second expression would need to be evaluated, and the program would then throw an error.

You may think this is an odd piece of dynamic symantics (how code runs) to analyze, but this can produce puzzling behavior in object oriented programming. If the expressions change the state of your program, this is something you must keep in mind.


And is similar to the or operator–it's a short-circuit operator. But, we use the && symbol instead.

let marthaControlsTheWeather = true && true
marthaControlsTheWeather: Bool = true

In contrast, the and operator can only short-circuit if the first expression evaluates to false.

func hiddenZero() -> Int { 0 }
// hiddenZero: () -> Int

let hiddenErrorBool = false || 0 / hiddenZero() == 1
// hiddenErrorBool: Bool = false


Unlike or and and, not is a unary prefix operator. This means that we can apply it to a singular boolean expression by placing an exclamation point (or bang) ! in front of the expression:

let isWeatherBot = !false
// isWeatherBot: Bool = true

let multipleNots = !(true && true)
// multipleNots: Bool = false

Control Flow

If-Else Statements

If-else statements are standard in Swift. You just write if, followed by an expression, followed by an execution block. If you want, you can follow this with an else statement or an else if if you want another condition.

func welcomeMartha(name: String) -> String {
    if name == "Martha" {
        return "What should the weather be today?"
    } else if name == "Justin" {
        return "Shouldn't you be working on lecture?"
    } else {
        return "Who are you :|"


Ternary, for 3 expressions, can be though of as in-place if-else expression. Ternaries are written as <insert boolean expression> ? <expression 1> : <expression 2>. This only works if expression 1 and expression 2 have the same type. If the boolean is true, expression 1 is returned, otherwise, expression 2 is returned.

let color: UIColor = true ? .blue : .red
// color: UIColor = .blue

let anotherColor: UIColor = false ? .blue : .red
// color: UIColor = .red


You often use booleans to store states. In these cases, you will want to be toggling this state on and off. You could write that like this:

var myState: Bool
func toggle() {
    if myState {
        myState = false
    } else {
        myState = true

We could also do this with a ternary:

var myState: Bool
func toggle() {
    myState = myState ? false : true

We could even do it in fewer lines with not:

var myState: Bool
func toggle() {
    myState = !myState

But actually we don't need to do any of these because is already done in Swift. If we want to toggle a boolean, we can simply use the toggle method.

var myState: Bool = true
// myState: Bool = true

// myState: Bool = false

// myState: Bool = true


While loops exactly what you'd expect them to be. A loop the executes while some expression evaluate to be true, with the syntax while <insert expression> { <code block> }. Just make sure you're making progress towards termination!

func factorial(_ n: Int) -> Int {
    var result = 1
    var n = n // This is necessary because the parameter n is a let-constant
    while n > 0 {
        result *= n
        n -= 1
    return result
// factorial(10): Int = 3628800

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