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02.BasicOperators.md

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Basic Operators

An operator is a special symbol or phrase that you use to check, change, or combine values. For example, the addition operator (+) adds two numbers, as in let i = 1 + 2, and the logical AND operator (&&) combines two Boolean values, as in if enteredDoorCode && passedRetinaScan.

Terminology

  • Unary operators operate on a single target (such as -a). Unary prefix operators appear immediately before their target (such as !b), and unary postfix operators appear immediately after their target (such as c!).
  • Binary operators operate on two targets (such as 2 + 3) and are infix because they appear in between their two targets.
  • Ternary operators operate on three targets. Like C, Swift has only one ternary operator, the ternary conditional operator (a ? b : c).

Assignment Operator

The assignment operator (a = b) initializes or updates the value of a with the value of b:

let b = 10
var a = 5
a = b
// a is now equal to 10

If the right side of the assignment is a tuple with multiple values, its elements can be decomposed into multiple constants or variables at once:

let (x, y) = (1, 2)
// x is equal to 1, and y is equal to 2

Unlike the assignment operator in C and Objective-C, the assignment operator in Swift doesn’t itself return a value. The following statement isn’t valid:

if x = y {
    // This isn't valid, because x = y doesn't return a value.
}

Arithmetic Operators

Swift supports the four standard arithmetic operators for all number types:

  • Addition (+)
  • Subtraction (-)
  • Multiplication (*)
  • Division (/)
1 + 2       // equals 3
5 - 3       // equals 2
2 * 3       // equals 6
10.0 / 2.5  // equals 4.0

The addition operator is also supported for String concatenation:

"hello, " + "world" // equals "hello, world"

Remainder Operator

The remainder operator (a % b) works out how many multiples of b will fit inside a and returns the value that’s left over (known as the remainder).

Here’s how the remainder operator works. To calculate 9 % 4, you first work out how many 4s will fit inside 9:

You can fit two 4s inside 9, and the remainder is 1 (shown in orange).

In Swift, this would be written as:

9 % 4 // equals 1

The same method is applied when calculating the remainder for a negative value of a:

-9 % 4 // equals -1

Unary Minus Operator

The sign of a numeric value can be toggled using a prefixed -, known as the unary minus operator:

let three = 3
let minusThree = -three       // minusThree equals -3
let plusThree = -minusThree   // plusThree equals 3, or "minus minus three"

The unary minus operator (-) is prepended directly before the value it operates on, without any white space.

Unary Plus Operator

The unary plus operator (+) simply returns the value it operates on, without any change:

let minusSix = -6
let alsoMinusSix = +minusSix  // alsoMinusSix equals -6

Compound Assignment Operators

Like C, Swift provides compound assignment operators that combine assignment (=) with another operation. One example is the addition assignment operator (+=):

var a = 1
a += 2   
// => a = a + 2
// a is now equal to 3

Comparison Operators

Swift supports the following comparison operators:

  • Equal to (a == b)
  • Not equal to (a != b)
  • Greater than (a > b)
  • Less than (a < b)
  • Greater than or equal to (a >= b)
  • Less than or equal to (a <= b)

Each of the comparison operators returns a Bool value to indicate whether or not the statement is true:

1 == 1   // true because 1 is equal to 1
2 != 1   // true because 2 isn't equal to 1
2 > 1    // true because 2 is greater than 1
1 < 2    // true because 1 is less than 2
1 >= 1   // true because 1 is greater than or equal to 1
2 <= 1   // false because 2 isn't less than or equal to 1

Comparison operators are often used in conditional statements, such as the if statement:

let name = "world"
if name == "world" {
    print("hello, world")
} else {
    print("I'm sorry \(name), but I don't recognize you")
}
// Prints "hello, world", because name is indeed equal to "world".

You can compare two tuples if they have the same type and the same number of values. Tuples are compared from left to right, one value at a time, until the comparison finds two values that aren’t equal. Those two values are compared, and the result of that comparison determines the overall result of the tuple comparison. If all the elements are equal, then the tuples themselves are equal.

(1, "zebra") < (2, "apple")   // true because 1 is less than 2; "zebra" and "apple" aren't compared
(3, "apple") < (3, "bird")    // true because 3 is equal to 3, and "apple" is less than "bird"
(4, "dog") == (4, "dog")      // true because 4 is equal to 4, and "dog" is equal to "dog"

when the tuples’ first elements are the same, their second elements are compared—this is what happens on the second and third line.

As demonstrated in the code below, you can compare two tuples of type (String, Int) because both String and Int values can be compared using the < operator. In contrast, two tuples of type (String, Bool) can’t be compared with the < operator because the < operator can’t be applied to Bool values.

("blue", -1) < ("purple", 1)        // OK, evaluates to true
("blue", false) < ("purple", true)  // Error because < can't compare Boolean values

Ternary Conditional Operator

The ternary conditional operator is a special operator with three parts, which takes the form question ? answer1 : answer2. It’s a shortcut for evaluating one of two expressions based on whether question is true or false. If question is true, it evaluates answer1 and returns its value; otherwise, it evaluates answer2 and returns its value.

The ternary conditional operator is shorthand for the code below:

if question {
    answer1
} else {
    answer2
}
let contentHeight = 40
let hasHeader = true
let rowHeight = contentHeight + (hasHeader ? 50 : 20)
// rowHeight is equal to 90

The example above is shorthand for the code below:

let contentHeight = 40
let hasHeader = true
let rowHeight: Int
if hasHeader {
    rowHeight = contentHeight + 50
} else {
    rowHeight = contentHeight + 20
}
// rowHeight is equal to 90

Nil-Coalescing Operator

The nil-coalescing operator (a ?? b) unwraps an optional a if it contains a value, or returns a default value b if a is nil. The expression a is always of an optional type. The expression b must match the type that’s stored inside a.

The nil-coalescing operator is shorthand for the code below: a != nil ? a! : b

The example below uses the nil-coalescing operator to choose between a default color name and an optional user-defined color name:

let defaultColorName = "red"
var userDefinedColorName: String?   // defaults to nil

var colorNameToUse = userDefinedColorName ?? defaultColorName
// userDefinedColorName is nil, so colorNameToUse is set to the default of "red"

If you assign a non-nil value to userDefinedColorName and perform the nil-coalescing operator check again, the value wrapped inside userDefinedColorName is used instead of the default:

userDefinedColorName = "green"
colorNameToUse = userDefinedColorName ?? defaultColorName
// userDefinedColorName isn't nil, so colorNameToUse is set to "green"

Range Operators

Swift includes several range operators, which are shortcuts for expressing a range of values.

Closed Range Operator

The closed range operator (a...b) defines a range that runs from a to b, and includes the values a and b. The value of a must not be greater than b.

The closed range operator is useful when iterating over a range in which you want all of the values to be used, such as with a for-in loop:

for index in 1...5 {
    print("\(index) times 5 is \(index * 5)")
}
// 1 times 5 is 5
// 2 times 5 is 10
// 3 times 5 is 15
// 4 times 5 is 20
// 5 times 5 is 25

Half-Open Range Operator

The half-open range operator (a..<b) defines a range that runs from a to b, but doesn’t include b. It’s said to be half-open because it contains its first value, but not its final value. As with the closed range operator, the value of a must not be greater than b. If the value of a is equal to b, then the resulting range will be empty.

Half-open ranges are particularly useful when you work with zero-based lists such as arrays, where it’s useful to count up to (but not including) the length of the list:

let names = ["Anna", "Alex", "Brian", "Jack"]
let count = names.count
for i in 0..<count {
    print("Person \(i + 1) is called \(names[i])")
}
// Person 1 is called Anna
// Person 2 is called Alex
// Person 3 is called Brian
// Person 4 is called Jack

One-Sided Ranges

The closed range operator has an alternative form for ranges that continue as far as possible in one direction—for example, a range that includes all the elements of an array from index 2 to the end of the array. In these cases, you can omit the value from one side of the range operator. This kind of range is called a one-sided range because the operator has a value on only one side. For example:

for name in names[2...] {
    print(name)
}
// Brian
// Jack

for name in names[...2] {
    print(name)
}
// Anna
// Alex
// Brian

The half-open range operator also has a one-sided form that’s written with only its final value. Just like when you include a value on both sides, the final value isn’t part of the range. For example:

for name in names[..<2] {
    print(name)
}
// Anna
// Alex

One-sided ranges can be used in other contexts, not just in subscripts. You can’t iterate over a one-sided range that omits a first value, because it isn’t clear where iteration should begin. You can iterate over a one-sided range that omits its final value; however, because the range continues indefinitely, make sure you add an explicit end condition for the loop. You can also check whether a one-sided range contains a particular value, as shown in the code below.

let range = ...5
range.contains(7)   // false
range.contains(4)   // true
range.contains(-1)  // true

Logical Operators

Logical operators modify or combine the Boolean logic values true and false. Swift supports the three standard logical operators found in C-based languages:

  • Logical NOT (!a)
  • Logical AND (a && b)
  • Logical OR (a || b)

Logical NOT Operator

The logical NOT operator is a prefix operator, and appears immediately before the value it operates on, without any white space. It can be read as “not a”, as seen in the following example:

let allowedEntry = false
if !allowedEntry {
    print("ACCESS DENIED")
}
// Prints "ACCESS DENIED"

The phrase if !allowedEntry can be read as “if not allowed entry.” The subsequent line is only executed if “not allowed entry” is true; that is, if allowedEntry is false.

Logical AND Operator

The logical AND operator (a && b) creates logical expressions where both values must be true for the overall expression to also be true.

In fact, if the first value is false, the second value won’t even be evaluated, because it can’t possibly make the overall expression equate to true. This is known as short-circuit evaluation.

This example considers two Bool values and only allows access if both values are true:

let enteredDoorCode = true
let passedRetinaScan = false
if enteredDoorCode && passedRetinaScan {
    print("Welcome!")
} else {
    print("ACCESS DENIED")
}
// Prints "ACCESS DENIED"

Logical OR Operator

The logical OR operator (a || b) is an infix operator made from two adjacent pipe characters. You use it to create logical expressions in which only one of the two values has to be true for the overall expression to be true.

If the left side of a Logical OR expression is true, the right side isn’t evaluated, because it can’t change the outcome of the overall expression.

In the example below, the first Bool value (hasDoorKey) is false, but the second value (knowsOverridePassword) is true. Because one value is true, the overall expression also evaluates to true, and access is allowed:

let hasDoorKey = false
let knowsOverridePassword = true
if hasDoorKey || knowsOverridePassword {
    print("Welcome!")
} else {
    print("ACCESS DENIED")
}
// Prints "Welcome!"

##Combining Logical Operators You can combine multiple logical operators to create longer compound expressions:

if enteredDoorCode && passedRetinaScan || hasDoorKey || knowsOverridePassword {
    print("Welcome!")
} else {
    print("ACCESS DENIED")
}
// Prints "Welcome!"

The Swift logical operators && and || are left-associative, meaning that compound expressions with multiple logical operators evaluate the leftmost subexpression first.

Explicit Parentheses

It’s sometimes useful to include parentheses when they’re not strictly needed, to make the intention of a complex expression easier to read. In the door access example above, it’s useful to add parentheses around the first part of the compound expression to make its intent explicit:

if (enteredDoorCode && passedRetinaScan) || hasDoorKey || knowsOverridePassword {
    print("Welcome!")
} else {
    print("ACCESS DENIED")
}
// Prints "Welcome!"