Lexical Structure

Question 1

What are the rules for the characters allowed in an identifier?

Answer 1

Identifiers begin with an upper case or lower case letter A through Z, an underscore (_), a noncombining alphanumeric Unicode character in the Basic Multilingual Plane, or a character outside the Basic Multilingual Plane that isn’t in a Private Use Area. After the first character, digits and combining Unicode characters are also allowed.

Question 2

How can one use a reserved word as an identifier?

Answer 2

To use a reserved word as an identifier, put a backtick () before and after it. For example, class is not a valid identifier, but class ` is valid. The backticks are not considered part of the identifier; ` x ` and x have the same meaning.

Question 3

Describe the rule for implicit identifiers within closures.

Answer 3

Inside a closure with no explicit parameter names, the parameters are implicitly named $0, $1, $2, and so on. These names are valid identifiers within the scope of the closure.

Question 4

List all the keywords that can appear in declarations.

Answer 4

class, deinit, enum, extension, func, import, init, let, protocol, static, struct, subscript, typealias, and var.

Question 5

List all the keywords that can appear in statements.

Answer 5

break, case, continue, default, do, else, fallthrough, if, in, for, return, switch, where, and while.

Question 6

List all the keywords that can appear in expressions and types.

Answer 6

as, dynamicType, is, new, super, self, Self, Type, __COLUMN__, __FILE__, __FUNCTION__, and __LINE__.

Question 7

List all the keywords that are only reserved in particular contexts in the grammar.

Answer 7

associativity, didSet, get, infix, inout, left, mutating, none, nonmutating, operator, override, postfix, precedence, prefix, right, set, unowned, unowned(safe), unowned(unsafe), weak and willSet. Outside the context in which they appear in the grammar, these can be used as identifiers.

Question 8

State the two uses of whitespace in Swift

Answer 8

Whitespace has two uses: to separate tokens in the source file and to help determine whether an operator is a prefix or postfix, but is otherwise ignored.

Question 9

Which characters are considered whitespace?

Answer 9

The following characters are considered whitespace: space (U+0020), line feed (U+000A), carriage return (U+000D), horizontal tab (U+0009), vertical tab (U+000B), form feed (U+000C) and null (U+0000).

Question 10

What rules govern comments?

Answer 10

Single line comments begin with // and continue until the end of the line. Multiline comments begin with /* and end with */. Nesting is allowed, but the comment markers must be balanced.

Question 11

How can you use a Swift keyword as an identifier.

Answer 11

A keyword in Swift can be used as an identifier if it escaped with backticks. The backticks are not considered part of the identifier; x and x have the same meaning.

Question 12

What are the kinds of literals described as being part of Swift’s lexical structure?

Answer 12

literal → integer-literal­ | floating-point-literal­ | string-literal­

Question 13

What is an integer literal?

Answer 13

Integer literals represent integer values of unspecified precision. By default, integer literals are expressed in decimal. Decimal literals contain the digits 0 through 9. Negative integers literals are expressed by prepending a minus sign (-) to an integer literal, as in -42.

Question 14

How do you specify an alternate base of an integer literal?

Answer 14

You can specify an alternate base using a prefix. Binary literals begin with 0b, octal literals begin with 0o, and hexadecimal literals begin with 0x.

Question 15

What characters can appear in an integer literal?

Answer 15

Binary literals contain 0 and 1, octal literals contain 0 through 7, and hexadecimal literals contain 0 through 9 as well as A through F in upper- or lowercase. Underscores (_) are allowed between digits for readability, but are ignored and therefore don’t affect the value of the literal. Integer literals can begin with leading zeros (0), but are likewise ignored and don’t affect the base or value of the literal.

Question 16

What is a floating point literal?

Answer 16

Floating-point literals represent floating-point values of unspecified precision. By default, floating-point literals are expressed in decimal (with no prefix), but they can also be expressed in hexadecimal (with a 0x prefix).

Question 17

How do you express a decimal floating point literal?

Answer 17

Decimal floating-point literals consist of a sequence of decimal digits followed by either a decimal fraction, a decimal exponent, or both. The decimal fraction consists of a decimal point (.) followed by a sequence of decimal digits. The exponent consists of an upper- or lowercase e prefix followed by sequence of decimal digits that indicates what power of 10 the value preceding the e is multiplied by.

Question 18

How do you express a hexidecimal floating point literal?

Answer 18

Hexadecimal floating-point literals consist of a 0x prefix, followed by an optional hexadecimal fraction, followed by a hexadecimal exponent. The hexadecimal fraction consists of a decimal point followed by a sequence of hexadecimal digits. The exponent consists of an upper- or lowercase p prefix followed by sequence of decimal digits that indicates what power of 2 the value preceding the p is multiplied by.

Question 19

What is the lexical rule for negative floating point literals?

Answer 19

Unlike with integer literals, negative floating-point numbers are expressed by applying the unary minus operator (-) to a floating-point literal, as in -42.0. The result is an expression, not a floating-point integer literal.

Question 20

What is the type of an integer literal?

Answer 20

Unless otherwise specified, the default type of an integer literal is the Swift standard library type Int. The Swift standard library also defines types for various sizes of signed and unsigned integers.

Question 21

What is the type of a floating point literal?

Answer 21

Unless otherwise specified, the default type of a floating-point literal is the Swift standard library type Double, which represents a 64-bit floating-point number. The Swift standard library also defines a Float type, which represents a 32-bit floating-point number.

Question 22

What is a string literal?

Answer 22

A string literal is a sequence of characters surrounded by double quotes, with the following form: “characters”

Question 23

What unescaped characters cannot be contained in a string literal?

Answer 23

String literals cannot contain an unescaped double quote (“), an unescaped backslash (), a carriage return, or a line feed.

Question 24

What special character escape sequences can appear in string literals?

Answer 24

Null Character (\0) Backslash (\) Horizontal Tab (\t) Line Feed (\n) Carriage Return (\r) Double Quote (") Single Quote (')

Question 25

How can you include unicode characters in a string literal?

Answer 25

Characters can also be expressed by \x followed by two hexadecimal digits, \u followed by four hexadecimal digits, or \U followed by eight hexadecimal digits. The digits in these escape sequences identify a Unicode codepoint.

Question 26

How can the value of an expression be inserted into a string literal?

Answer 26

The value of an expression can be inserted into a string literal by placing the expression in parentheses after a backslash ().

Question 27

What characters must an interpolated expression never contain?

Answer 27

The interpolated expression must not contain an unescaped double quote (“), an unescaped backslash (), a carriage return, or a line feed.

Question 28

What types can an interpolated expression evaluate to?

Answer 28

An interpolated expression must evaluate to a value of a type that the String class has an initializer for.

Question 29

What is the default type of a string literal?

Answer 29

The default type of a string literal is String. The characters that make up a string are of type Character.

Question 30

Which characters can be used in operators?

Answer 30

Operators are made up of one or more of the following characters: /, =, -, +, !, *, %, , &, |, ^, ~, and .

Question 31

Which tokens cannot be used as operators?

Answer 31

The tokens =, ->, //, /*, */, ., and the unary prefix operator & are reserved. These tokens can’t be overloaded, nor can they be used to define custom operators.

Question 32

What determines whether an operator is used as a prefix, postfix, or binary operator?

Answer 32

The whitespace around an operator is used to determine whether an operator is used as a prefix operator, a postfix operator, or a binary operator.

Question 33

What is special about the following tokens: =, ->, //, /*, */, ., and the unary prefix operator &

Answer 33

They are reserved, they cannot be overloaded, nor can they be used to define custom operators.

Question 34

When is an operator treated as a binary operator?

Answer 34

If an operator has whitespace around both sides or around neither side, it is treated as a binary operator. As an example, the + operator in a+b and a + b is treated as a binary operator.

Question 35

When is an operator treated as a prefix unary operator?

Answer 35

If an operator has whitespace on the left side only, it is treated as a prefix unary operator. As an example, the ++ operator in a ++b is treated as a prefix unary operator.

Question 36

When is an operator treated as a postfix unary operator?

Answer 36

If an operator has whitespace on the right side only, it is treated as a postfix unary operator. As an example, the ++ operator in a++ b is treated as a postfix unary operator. If an operator has no whitespace on the left but is followed immediately by a dot (.), it is treated as a postfix unary operator. As an example, the ++ operator in a++.b is treated as a postfix unary operator (a++ . b rather than a ++ .b).

Question 37

What additional characters are considered ‘whitespace’ in the context of the rules governing operator treatment?

Answer 37

In rules regarding the treatment of operators, the characters ( [and { before an operator, the characters )] and } after an operator, and the characters , ; and : are also considered whitespace.

Question 38

What are the caveats regarding the operator whitespace rules?

Answer 38

If the ! or ? operator has no whitespace on the left, it is treated as a postfix operator, regardless of whether it has whitespace on the right. To use the ? operator as syntactic sugar for the Optional type, it must not have whitespace on the left. To use it in the conditional (? :) operator, it must have whitespace around both sides.