The Basics

The Basics

Work with common kinds of data and write basic syntax.

Swift provides many fundamental data types, including Int for integers, Double for floating-point values, Bool for Boolean values, and String for text. Swift also provides powerful versions of the three primary collection types, Array, Set, and Dictionary, as described in doc:CollectionTypes.

Swift uses variables to store and refer to values by an identifying name. Swift also makes extensive use of variables whose values can't be changed. These are known as constants, and are used throughout Swift to make code safer and clearer in intent when you work with values that don't need to change.

In addition to familiar types, Swift introduces advanced types such as tuples. Tuples enable you to create and pass around groupings of values. You can use a tuple to return multiple values from a function as a single compound value.

Swift handles the absence of a value using optional types. Optionals say either “there is a value, which is x” or “there isn't a value at all”. Optionals ensure that code always checks whether a value is missing before using the value, and non-optional values are guaranteed to never be missing.

Swift is a safe language, which means it makes it easier for you find and fix several categories of bugs as early as possible during the development process, and lets you guarantee that certain kinds of bugs can't happen. Type safety enables you to be clear about the types of values your code works with. If part of your code requires a String, type safety prevents you from passing it an Int by mistake. Memory safety ensures that you work with valid data only, not uninitialized memory or deinitialized objects, and ensures that you work with that data in safe ways --- even in programs that run multiple pieces of code at the same time. Swift performs most of its safety checks while building your code, and in some cases performs additional checks while your code is running.

Constants and Variables

Constants and variables associate a name (such as maximumNumberOfLoginAttempts or welcomeMessage) with a value of a particular type (such as the number 10 or the string "Hello"). The value of a constant can't be changed once it's set, whereas a variable can be set to a different value in the future.

Declaring Constants and Variables

Constants and variables must be declared before they're used. You declare constants with the let keyword and variables with the var keyword. Here's an example of how constants and variables can be used to track the number of login attempts a user has made:

let maximumNumberOfLoginAttempts = 10
var currentLoginAttempt = 0

This code can be read as:

“Declare a new constant called maximumNumberOfLoginAttempts, and give it a value of 10. Then, declare a new variable called currentLoginAttempt, and give it an initial value of 0.”

In this example, the maximum number of allowed login attempts is declared as a constant, because the maximum value never changes. The current login attempt counter is declared as a variable, because this value must be incremented after each failed login attempt.

If a stored value in your code won't change, always declare it as a constant with the let keyword. Use variables only for storing values that change.

When you declare a constant or a variable, you can give it a value as part of that declaration, like the examples above. Alternatively, you can assign its initial value later in the program, as long as it's guaranteed to have a value before the first time you read from it.

var environment = "development"
let maximumNumberOfLoginAttempts: Int
// maximumNumberOfLoginAttempts has no value yet.

if environment == "development" {
    maximumNumberOfLoginAttempts = 100
} else {
    maximumNumberOfLoginAttempts = 10
}
// Now maximumNumberOfLoginAttempts has a value, and can be read.

In this example, the maximum number of login attempts is constant, and its value depends on the environment. In the development environment, it has a value of 100; in any other environment, its value is 10. Both branches of the if statement initialize maximumNumberOfLoginAttempts with some value, guaranteeing that the constant always gets a value. For information about how Swift checks your code when you set an initial value this way, see doc:Declarations#Constant-Declaration.

You can declare multiple constants or multiple variables on a single line, separated by commas:

var x = 0.0, y = 0.0, z = 0.0

Type Annotations

You can provide a type annotation when you declare a constant or variable, to be clear about the kind of values the constant or variable can store. Write a type annotation by placing a colon after the constant or variable name, followed by a space, followed by the name of the type to use.

This example provides a type annotation for a variable called welcomeMessage, to indicate that the variable can store String values:

var welcomeMessage: String

The colon in the declaration means “…of type…,” so the code above can be read as:

“Declare a variable called welcomeMessage that's of type String.”

The phrase “of type String” means “can store any String value.” Think of it as meaning “the type of thing” (or “the kind of thing”) that can be stored.

The welcomeMessage variable can now be set to any string value without error:

welcomeMessage = "Hello"

You can define multiple related variables of the same type on a single line, separated by commas, with a single type annotation after the final variable name:

var red, green, blue: Double

Note: It's rare that you need to write type annotations in practice. If you provide an initial value for a constant or variable at the point that it's defined, Swift can almost always infer the type to be used for that constant or variable, as described in doc:TheBasics#Type-Safety-and-Type-Inference. In the welcomeMessage example above, no initial value is provided, and so the type of the welcomeMessage variable is specified with a type annotation rather than being inferred from an initial value.

Naming Constants and Variables

Constant and variable names can contain almost any character, including Unicode characters:

let π = 3.14159
let 你好 = "你好世界"
let 🐶🐮 = "dogcow"

Constant and variable names can't contain whitespace characters, mathematical symbols, arrows, private-use Unicode scalar values, or line- and box-drawing characters. Nor can they begin with a number, although numbers may be included elsewhere within the name.

Once you've declared a constant or variable of a certain type, you can't declare it again with the same name, or change it to store values of a different type. Nor can you change a constant into a variable or a variable into a constant.

Note: If you need to give a constant or variable the same name as a reserved Swift keyword, surround the keyword with backticks (`) when using it as a name. However, avoid using keywords as names unless you have absolutely no choice.

You can change the value of an existing variable to another value of a compatible type. In this example, the value of friendlyWelcome is changed from "Hello!" to "Bonjour!":

var friendlyWelcome = "Hello!"
friendlyWelcome = "Bonjour!"
// friendlyWelcome is now "Bonjour!"

Unlike a variable, the value of a constant can't be changed after it's set. Attempting to do so is reported as an error when your code is compiled:

let languageName = "Swift"
languageName = "Swift++"
// This is a compile-time error: languageName cannot be changed.

Printing Constants and Variables

You can print the current value of a constant or variable with the print(_:separator:terminator:) function:

print(friendlyWelcome)
// Prints "Bonjour!"

The print(_:separator:terminator:) function is a global function that prints one or more values to an appropriate output. In Xcode, for example, the print(_:separator:terminator:) function prints its output in Xcode's “console” pane. The separator and terminator parameter have default values, so you can omit them when you call this function. By default, the function terminates the line it prints by adding a line break. To print a value without a line break after it, pass an empty string as the terminator --- for example, print(someValue, terminator: ""). For information about parameters with default values, see doc:Functions#Default-Parameter-Values.

Swift uses string interpolation to include the name of a constant or variable as a placeholder in a longer string, and to prompt Swift to replace it with the current value of that constant or variable. Wrap the name in parentheses and escape it with a backslash before the opening parenthesis:

print("The current value of friendlyWelcome is \(friendlyWelcome)")
// Prints "The current value of friendlyWelcome is Bonjour!"

Note: All options you can use with string interpolation are described in doc:StringsAndCharacters#String-Interpolation.

Comments

Use comments to include nonexecutable text in your code, as a note or reminder to yourself. Comments are ignored by the Swift compiler when your code is compiled.

Comments in Swift are very similar to comments in C. Single-line comments begin with two forward-slashes (//):

// This is a comment.

Multiline comments start with a forward-slash followed by an asterisk (/*) and end with an asterisk followed by a forward-slash (*/):

/* This is also a comment
but is written over multiple lines. */

Unlike multiline comments in C, multiline comments in Swift can be nested inside other multiline comments. You write nested comments by starting a multiline comment block and then starting a second multiline comment within the first block. The second block is then closed, followed by the first block:

/* This is the start of the first multiline comment.
    /* This is the second, nested multiline comment. */
This is the end of the first multiline comment. */

Nested multiline comments enable you to comment out large blocks of code quickly and easily, even if the code already contains multiline comments.

Semicolons

Unlike many other languages, Swift doesn't require you to write a semicolon (;) after each statement in your code, although you can do so if you wish. However, semicolons are required if you want to write multiple separate statements on a single line:

let cat = "🐱"; print(cat)
// Prints "🐱".

Integers

Integers are whole numbers with no fractional component, such as 42 and -23. Integers are either signed (positive, zero, or negative) or unsigned (positive or zero), and their maximum and minimum value depends on their size (the number of bits used to store values). The integer types include their size and sign in their names --- for example, an 8-bit unsigned integer is of type UInt8, and a 32-bit signed integer is of type Int32. Like all types in Swift, these integer types have capitalized names. In most cases, when you don't need to specify the exact integer size, you use the Int type described below.

Integer types behave like most arithmetic you do by hand; integer math produces results without approximating. These characteristics make integers suitable for counting and other calculations that represent exact amounts --- for example, finding the longest line in a text file, applying a score multiplier in a game, or totaling prices on a receipt.

Although integers don't have a fractional component, you can use integers to represent quantities with fractions by counting a fractional part. For example, you can represent $1.23 by storing the number 123 in an integer that counts cents. This approach is known as fixed-point math because the decimal point is at a fixed position in the number. In the example above, the number 123 is understood to have a decimal point before the last two digits.

Note: For calculations in a regulated area like finance or construction, or in a domain that has an expectation of high-precision results, you might need a special-purpose numeric type that implements behaviors such as rounding and truncation, according to that area's requirements.

Integer Bounds

You can access the minimum and maximum values of each integer type with its min and max properties:

let minValue = UInt8.min  // minValue is equal to 0, and is of type UInt8
let maxValue = UInt8.max  // maxValue is equal to 255, and is of type UInt8

The values of these properties are of the appropriate-sized number type (such as UInt8 in the example above) and can therefore be used in expressions alongside other values of the same type.

Calculations that produce out-of-bounds results, like a number larger that the max property, stop the program's execution instead of storing an invalid result. You can explicitly make the operation overflow instead, as described in doc:AdvancedOperators#Overflow-Operators.

Int

In most cases, you don't need to pick a specific size of integer to use in your code. Swift provides an additional integer type, Int, which has the same size as the current platform's native word size:

• On a 32-bit platform, Int is the same size as Int32.
• On a 64-bit platform, Int is the same size as Int64.

Unless you need to work with a specific size of integer, always use Int for integer values in your code. This aids code consistency and interoperability. Even on 32-bit platforms, Int can store any value between -2,147,483,648 and 2,147,483,647, and is large enough for many integer ranges.

UInt

Swift also provides an unsigned integer type, UInt, which has the same size as the current platform's native word size:

• On a 32-bit platform, UInt is the same size as UInt32.
• On a 64-bit platform, UInt is the same size as UInt64.

Note: Use UInt only when you specifically need an unsigned integer type with the same size as the platform's native word size. If this isn't the case, Int is preferred, even when the values to be stored are known to be nonnegative. A consistent use of Int for integer values aids code interoperability, avoids the need to convert between different number types, and matches integer type inference, as described in doc:TheBasics#Type-Safety-and-Type-Inference.

Floating-Point Numbers

Floating-point numbers have a fractional component, such as 3.14159, 0.1, and -273.15. Swift provides a variety of floating-point types that support different sizes of numbers, just like it has different sizes of integers. If you don't need to specify an exact size, use Double. Otherwise, use the type that includes the needed size in its name, such as Float16 or Float80. Following common terminology for floating-point math, Float uses 32 bits and Double uses 64 bits. You can also write these types as Float32 or Float64. For example, graphics code often uses Float to match the GPU's fastest data type. Some floating-point types are supported only by certain platforms, but Float and Double are available on all platforms.

Floating-point numbers let you work with very small and very large numbers, but can't represent every possible value in that range. Unlike integer calculations, which always produce an exact result, floating-point math rounds results to the nearest representable number. For example, when storing the number 10,000 as a Float, the next largest number you can represent is 10,000.001 ---- values between these two numbers round to one or the other. The space between numbers is also variable; there are larger spaces between large numbers than between small numbers. For example, the next Float value after 0.001 is 0.0010000002, which is smaller than the spacing after 10,000.

Floating-point numbers have values for negative zero, infinity, and negative infinity, which represent overflow and underflow in calculations. They also have not-a-number (NaN) values to represent an invalid or undefined result, such as dividing zero by zero. This behavior is different from integers, which stop the program if they can't represent the result.

If you need the same spacing between all possible values, or if the calculations you're doing require exact results and don't call for the special values listed above, a floating-point number might not be the right data type. Consider using fixed-point numbers instead, as described in doc:TheBasics#Integers.

Type Safety and Type Inference

Every value in a Swift program has a type. Every place you store a value --- including constants, variables, and properties --- also has a type. You might write the type explicitly using a type annotation, or Swift might infer the type from an initial value. Every place in your code where you provide a value, that value's type must match the place you use it. For example, if part of your code requires a String, you can't pass it an Int by mistake. This kind of checking makes Swift a type-safe language.

A type safe language encourages you to be clear about the types of values your code works with. Values of one type are never implicitly converted to another type. However, some types can be explicitly converted. When building code, Swift checks the code for type safety and flags any mismatched types as errors.

Type checking helps you avoid errors when you're working with different types of values. However, this doesn't mean that you have to specify the type of every constant and variable that you declare. If you don't specify the type of value you need, Swift uses type inference to work out the appropriate type. Type inference enables a compiler to deduce the type of a particular expression automatically when it compiles your code, simply by examining the values you provide.

Because of type inference, Swift requires far fewer type declarations than languages such as C or Objective-C. Constants and variables are still explicitly typed, but much of the work of specifying their type is done for you.

Type inference is particularly useful when you declare a constant or variable with an initial value. This is often done by assigning a literal value (or literal) to the constant or variable at the point that you declare it. (A literal value is a value that appears directly in your source code, such as 42 and 3.14159 in the examples below.)

For example, if you assign a literal value of 42 to a new constant without saying what type it is, Swift infers that you want the constant to be an Int, because you have initialized it with a number that looks like an integer:

let meaningOfLife = 42
// meaningOfLife is inferred to be of type Int

Likewise, if you don't specify a type for a floating-point literal, Swift infers that you want to create a Double:

let pi = 3.14159
// pi is inferred to be of type Double

Swift always chooses Double (rather than Float) when inferring the type of floating-point numbers.

If you combine integer and floating-point literals in an expression, a type of Double will be inferred from the context:

let anotherPi = 3 + 0.14159
// anotherPi is also inferred to be of type Double

The literal value of 3 has no explicit type in and of itself, and so an appropriate output type of Double is inferred from the presence of a floating-point literal as part of the addition.

Numeric Literals

Integer literals can be written as:

• A decimal number, with no prefix
• A binary number, with a 0b prefix
• An octal number, with a 0o prefix
• A hexadecimal number, with a 0x prefix

All of these integer literals have a decimal value of 17:

let decimalInteger = 17
let binaryInteger = 0b10001       // 17 in binary notation
let octalInteger = 0o21           // 17 in octal notation
let hexadecimalInteger = 0x11     // 17 in hexadecimal notation

Floating-point literals can be decimal (with no prefix), or hexadecimal (with a 0x prefix). They must always have a number (or hexadecimal number) on both sides of the decimal point. Decimal floats can also have an optional exponent, indicated by an uppercase or lowercase e; hexadecimal floats must have an exponent, indicated by an uppercase or lowercase p.

For decimal numbers with an exponent of x, the base number is multiplied by 10ˣ:

• 1.25e2 means 1.25 x 10², or 125.0.
• 1.25e-2 means 1.25 x 10⁻², or 0.0125.

For hexadecimal numbers with an exponent of x, the base number is multiplied by 2ˣ:

• 0xFp2 means 15 x 2², or 60.0.
• 0xFp-2 means 15 x 2⁻², or 3.75.

All of these floating-point literals have a decimal value of 12.1875:

let decimalDouble = 12.1875
let exponentDouble = 1.21875e1
let hexadecimalDouble = 0xC.3p0

Numeric literals can contain extra formatting to make them easier to read. Both integers and floats can be padded with extra zeros and can contain underscores to help with readability. Neither type of formatting affects the underlying value of the literal:

let paddedDouble = 000123.456
let
let justOverOneMillion = 1_000_000.000_000_1

Numeric Type Conversion

Use the Int type for all general-purpose integer constants and variables in your code, even if they're known to be nonnegative. Using the default integer type in everyday situations means that integer constants and variables are immediately interoperable in your code and will match the inferred type for integer literal values.

Use other integer types only when they're specifically needed for the task at hand, because of explicitly sized data from an external source, or for performance, memory usage, or other necessary optimization. Using explicitly sized types in these situations helps to catch any accidental value overflows and implicitly documents the nature of the data being used.

Integer Conversion

The range of numbers that can be stored in an integer constant or variable is different for each numeric type. An Int8 constant or variable can store numbers between -128 and 127, whereas a UInt8 constant or variable can store numbers between 0 and 255. A number that won't fit into a constant or variable of a sized integer type is reported as an error when your code is compiled:

let cannotBeNegative: UInt8 = -1
// UInt8 can't store negative numbers, and so this will report an error
let tooBig: Int8 = Int8.max + 1
// Int8 can't store a number larger than its maximum value,
// and so this will also report an error

Because each numeric type can store a different range of values, you must opt in to numeric type conversion on a case-by-case basis. This opt-in approach prevents hidden conversion errors and helps make type conversion intentions explicit in your code.

To convert one specific number type to another, you initialize a new number of the desired type with the existing value. In the example below, the constant twoThousand is of type UInt16, whereas the constant one is of type UInt8. They can't be added together directly, because they're not of the same type. Instead, this example calls UInt16(one) to create a new UInt16 initialized with the value of one, and uses this value in place of the original:

let twoThousand: UInt16 = 2_000
let one: UInt8 = 1
let twoThousandAndOne = twoThousand + UInt16(one)

Because both sides of the addition are now of type UInt16, the addition is allowed. The output constant (twoThousandAndOne) is inferred to be of type UInt16, because it's the sum of two UInt16 values.

SomeType(ofInitialValue) is the default way to call the initializer of a Swift type and pass in an initial value. Behind the scenes, UInt16 has an initializer that accepts a UInt8 value, and so this initializer is used to make a new UInt16 from an existing UInt8. You can't pass in any type here, however --- it has to be a type for which UInt16 provides an initializer. Extending existing types to provide initializers that accept new types (including your own type definitions) is covered in doc:Extensions.

Integer and Floating-Point Conversion

Conversions between integer and floating-point numeric types must be made explicit:

let three = 3
let pointOneFourOneFiveNine = 0.14159
let pi = Double(three) + pointOneFourOneFiveNine
// pi equals 3.14159, and is inferred to be of type Double

Here, the value of the constant three is used to create a new value of type Double, so that both sides of the addition are of the same type. Without this conversion in place, the addition would not be allowed.

Floating-point to integer conversion must also be made explicit. An integer type can be initialized with a Double or Float value:

let integerPi = Int(pi)
// integerPi equals 3, and is inferred to be of type Int

Floating-point values are always truncated when used to initialize a new integer value in this way. This means that 4.75 becomes 4, and -3.9 becomes -3.

Note: The rules for combining numeric constants and variables are different from the rules for numeric literals. The literal value 3 can be added directly to the literal value 0.14159, because number literals don't have an explicit type in and of themselves. Their type is inferred only at the point that they're evaluated by the compiler.

Type Aliases

Type aliases define an alternative name for an existing type. You define type aliases with the typealias keyword.

Type aliases are useful when you want to refer to an existing type by a name that's contextually more appropriate, such as when working with data of a specific size from an external source:

typealias AudioSample = UInt16

Once you define a type alias, you can use the alias anywhere you might use the original name:

var maxAmplitudeFound = AudioSample.min
// maxAmplitudeFound is now 0

Here, AudioSample is defined as an alias for UInt16. Because it's an alias, the call to AudioSample.min actually calls UInt16.min, which provides an initial value of 0 for the maxAmplitudeFound variable.

Booleans

Swift has a basic Boolean type, called Bool. Boolean values are referred to as logical, because they can only ever be true or false. Swift provides two Boolean constant values, true and false:

let orangesAreOrange = true
let turnipsAreDelicious = false

The types of orangesAreOrange and turnipsAreDelicious have been inferred as Bool from the fact that they were initialized with Boolean literal values. As with Int and Double above, you don't need to declare constants or variables as Bool if you set them to true or false as soon as you create them.

Boolean values are particularly useful when you work with conditional statements such as the if statement:

if turnipsAreDelicious {
    print("Mmm, tasty turnips!")
} else {
    print("Eww, turnips are horrible.")
}
// Prints "Eww, turnips are horrible."

Conditional statements such as the if statement are covered in more detail in doc:ControlFlow.

Swift's type safety prevents non-Boolean values from being substituted for Bool. The following example reports a compile-time error:

let i = 1
if i {
    // this example will not compile, and will report an error
}

However, the alternative example below is valid:

let i = 1
if i == 1 {
    // this example will compile successfully
}

The result of the i == 1 comparison is of type Bool, and so this second example passes the type checking. Comparisons like i == 1 are discussed in doc:BasicOperators.

As with other examples of type safety in Swift, this approach avoids accidental errors and ensures that the intention of a particular section of code is always clear.

Tuples

Tuples group multiple values into a single compound value. The values within a tuple can be of any type and don't have to be of the same type as each other.

In this example, (404, "Not Found") is a tuple that describes an HTTP status code. An HTTP status code is a special value returned by a web server whenever you request a web page. A status code of 404 Not Found is returned if you request a webpage that doesn't exist.

let http404Error = (404, "Not Found")
// http404Error is of type (Int, String), and equals (404, "Not Found")

The (404, "Not Found") tuple groups together an Int and a String to give the HTTP status code two separate values: a number and a human-readable description. It can be described as “a tuple of type (Int, String)”.

You can create tuples from any permutation of types, and they can contain as many different types as you like. There's nothing stopping you from having a tuple of type (Int, Int, Int), or (String, Bool), or indeed any other permutation you require.

You can decompose a tuple's contents into separate constants or variables, which you then access as usual:

let (statusCode, statusMessage) = http404Error
print("The status code is \(statusCode)")
// Prints "The status code is 404".
print("The status message is \(statusMessage)")
// Prints "The status message is Not Found".

If you only need some of the tuple's values, ignore parts of the tuple with an underscore (_) when you decompose the tuple:

let (justTheStatusCode, _) = http404Error
print("The status code is \(justTheStatusCode)")
// Prints "The status code is 404".

Alternatively, access the individual element values in a tuple using index numbers starting at zero:

print("The status code is \(http404Error.0)")
// Prints "The status code is 404".
print("The status message is \(http404Error.1)")
// Prints "The status message is Not Found".

You can name the individual elements in a tuple when the tuple is defined:

let http200Status = (statusCode: 200, description: "OK")

If you name the elements in a tuple, you can use the element names to access the values of those elements:

print("The status code is \(http200Status.statusCode)")
// Prints "The status code is 200".
print("The status message is \(http200Status.description)")
// Prints "The status message is OK".

Tuples are particularly useful as the return values of functions. A function that tries to retrieve a web page might return the (Int, String) tuple type to describe the success or failure of the page retrieval. By returning a tuple with two distinct values, each of a different type, the function provides more useful information about its outcome than if it could only return a single value of a single type. For more information, see doc:Functions#Functions-with-Multiple-Return-Values.

Note: Tuples are useful for simple groups of rel

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