The_Swift_Programming_Language
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2 ++i // i now equals 1
Each time you call ++i, the value of i is increased by 1. Essentially, ++i is shorthand for saying i = i + 1. Likewise, --i can be used as shorthand for i = i - 1.
The ++ and -- symbols can be used as prefix operators or as postfix operators. ++i and i++ are both valid ways to increase the value of i by 1. Similarly, --i and i-- are both valid ways to decrease the value of i by 1.
Note that these operators modify i and also return a value. If you only want to increment or decrement the value stored in i, you can ignore the returned value. However, if you do use the returned value, it will be different based on whether you used the prefix or postfix version of the operator, according to the following rules:
If the operator is written before the variable, it increments the variable before returning its value.
If the operator is written after the variable, it increments the variable after returning its value.
For example:
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var a = 0
let b = ++a
//a and b are now both equal to 1 let c = a++
//a is now equal to 2, but c has been set to the pre-increment value of 1
In the example above, let b = ++a increments a before returning its value. This is why both a and b are equal to to the new value of 1.
However, let c = a++ increments a after returning its value. This means that c gets the old value of 1, and a is then updated to equal 2.
Unless you need the specific behavior of i++, it is recommended that you use ++i and --i in all cases, because they have the typical expected behavior of modifying i and returning the result.
Unary Minus Operator
The sign of a numeric value can be toggled using a prefixed -, known as the unary minus operator:
1 let three = 3
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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:
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let minusSix = -6
let alsoMinusSix = +minusSix // alsoMinusSix equals -6
Although the unary plus operator doesn’t actually do anything, you can use it to provide symmetry in your code for positive numbers when also using the unary minus operator for negative numbers.
Compound Assignment Operators
Like C, Swift provides compound assignment operators that combine assignment (=) with another operation. One example is the addition assignment operator (+=):
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var a = 1
a += 2
// a is now equal to 3
The expression a += 2 is shorthand for a = a + 2. Effectively, the addition and the assignment are combined into one operator that performs both tasks at the same time.
N O T E
The compound assignment operators do not return a value. You cannot write let b = a += 2, for example. This behavior is different from the increment and decrement operators mentioned above.
A complete list of compound assignment operators can be found in Expressions.
Comparison Operators
Swift supports all standard C 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)
N O T E
Swift also provides two identity operators (=== and !==), which you use to test whether two object references both refer to the same object instance. For more information, see Classes and Structures.
Each of the comparison operators returns a Bool value to indicate whether or not the statement is true:
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1 == 1 // true, because 1 is equal to 1
2 != 1 // true, because 2 is not 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 is not less than or equal to 1
Comparison operators are often used in conditional statements, such as the if statement:
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let name = "world"
if name == "world" {
println("hello, world")
} else {
println("I'm sorry \(name), but I don't recognize you")
}
// prints "hello, world", because name is indeed equal to "world"
For more on the if statement, see Control Flow.
Ternary Conditional Operator
The ternary conditional operator is a special operator with three parts, which takes the form question ? answer1 : answer2. It is 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:
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if question {
answer1
} else {
answer2
}
Here’s an example, which calculates the pixel height for a table row. The row height should be 50 pixels taller than the content height if the row has a header, and 20 pixels taller if the row doesn’t have a header:
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let contentHeight = 40
let hasHeader = true
let rowHeight = contentHeight + (hasHeader ? 50 : 20)
// rowHeight is equal to 90
The preceding example is shorthand for the code below:
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let contentHeight = 40
let hasHeader = true
var rowHeight = contentHeight
if hasHeader {
rowHeight = rowHeight + 50
} else {
rowHeight = rowHeight + 20
}
// rowHeight is equal to 90
The first example’s use of the ternary conditional operator means that rowHeight can be set to the correct value on a single line of code. This is more concise than the second example, and removes the need for rowHeight to be a variable, because its value does not need to be modified within an if statement.
The ternary conditional operator provides an efficient shorthand for deciding which of two expressions to consider. Use the ternary conditional operator with care, however. Its conciseness can lead to hard-to-read code if overused. Avoid combining multiple instances of the ternary conditional operator into one compound statement.
Range Operators
Swift includes two 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 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:
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for index in 1...5 {
println("\(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
For more on for-in loops, see Control Flow.
Half-Closed Range Operator
The half-closed range operator (a..b) defines a range that runs from a to b, but does not include b. It is said to be half-closed because it contains its first value, but not its final value.
Half-closed ranges are particularly useful when you work with zero-based lists such as arrays, where it is useful to count up to (but not including) the length of the list:
1 let names = ["Anna", "Alex", "Brian", "Jack"]
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let count = names.count
for i in 0..count {
println("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
Note that the array contains four items, but 0..count only counts as far as 3 (the index of the last item in the array), because it is a half-closed range. For more on arrays, see Arrays.
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 (!a) inverts a Boolean value so that true becomes false, and false becomes true.
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:
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let allowedEntry = false
if !allowedEntry {
println("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.
As in this example, careful choice of Boolean constant and variable names can help to keep code readable and concise, while avoiding double negatives or confusing logic statements.
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.
If either value is false, the overall expression will also be false. 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:
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let enteredDoorCode = true
let passedRetinaScan = false
if enteredDoorCode && passedRetinaScan {
println("Welcome!")
} else {
println("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.
Like the Logical AND operator above, the Logical OR operator uses short-circuit evaluation to consider its expressions. If the left side of a Logical OR expression is true, the right side is not evaluated, because it cannot 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:
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let hasDoorKey = false
let knowsOverridePassword = true
if hasDoorKey || knowsOverridePassword {
println("Welcome!")
} else {
println("ACCESS DENIED")
}
// prints "Welcome!"
Combining Logical Operators
You can combine multiple logical operators to create longer compound expressions:
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if enteredDoorCode && passedRetinaScan || hasDoorKey || knowsOverridePassword {
println("Welcome!")
} else {
println("ACCESS DENIED")
}
// prints "Welcome!"
This example uses multiple && and || operators to create a longer compound expression. However, the && and || operators still operate on only two values, so this is actually three smaller expressions chained together. It can be read as:
If we’ve entered the correct door code and passed the retina scan; or if we have a valid door key; or if we know the emergency override password, then allow access.
Based on the values of enteredDoorCode, passedRetinaScan, and hasDoorKey, the first two mini-
expressions are false. However, the emergency override password is known, so the overall compound expression still evaluates to true.
Explicit Parentheses
It is sometimes useful to include parentheses when they are not strictly needed, to make the intention of a complex expression easier to read. In the door access example above, it is useful to add parentheses around the first part of the compound expression to make its intent explicit:
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if (enteredDoorCode && passedRetinaScan) || hasDoorKey || knowsOverridePassword {
println("Welcome!")
} else {
println("ACCESS DENIED")
}
// prints "Welcome!"
The parentheses make it clear that the first two values are considered as part of a separate possible state in the overall logic. The output of the compound expression doesn’t change, but the overall intention is clearer to the reader. Readability is always preferred over brevity; use parentheses where they help to make your intentions clear.
Strings and Characters
A string is an ordered collection of characters, such as "hello, world" or "albatross". Swift strings are represented by the String type, which in turn represents a collection of values
of Character type.
Swift’s String and Character types provide a fast, Unicode-compliant way to work with text in your code. The syntax for string creation and manipulation is lightweight and readable, with a similar syntax to C strings. String concatenation is as simple as adding together two strings with the + operator, and string mutability is managed by choosing between a constant or a variable, just like any other value in Swift.
Despite this simplicity of syntax, Swift’s String type is a fast, modern string implementation. Every string is composed of encoding-independent Unicode characters, and provides support for accessing those characters in various Unicode representations.
Strings can also be used to insert constants, variables, literals, and expressions into longer strings, in a process known as string interpolation. This makes it easy to create custom string values for display, storage, and printing.
N O T E
Swift’s String type is bridged seamlessly to Foundation’s NSString class. If you are working with the Foundation framework in Cocoa or Cocoa Touch, the entire NSString API is available to call on any String value you create, in addition to the String features described in this chapter. You can also use a String value with any API that requires an NSString instance.
For more information about using String with Foundation and Cocoa, see Using Swift with Cocoa and Objective-C.
String Literals
You can include predefined String values within your code as string literals. A string literal is a fixed sequence of textual characters surrounded by a pair of double quotes ("").
A string literal can be used to provide an initial value for a constant or variable:
1 let someString = "Some string literal value"
Note that Swift infers a type of String for the someString constant, because it is initialized with a string literal value.