W1037 Expressions and Operators

Within these castle walls be forged Mavens of Computer Science ...

— Merlin, The Coder

The expression of the emotions in man and animals (1872)

Prerequisites

• W1031 Positive Integers
• W1032 Negative Integers
• W1033 Character Encoding
• W1038 L-Values and R-Values

Introduction

An expression is a finite series of symbols that is well-formed according to agreed upon rules. The symbols may include constants, variables, operators, and brackets. A valid expression will yield a value of a specific type according to the rules of the language.

Type Inference

Integers

All expressions, when evaluated, yield a particular type. For example, in the expression:

let x = 3

x will be of type Int, which is an integer whose size depends on the size of the underlying platform. (On 32-bit platforms, Int will be equivalent to Int32, and on 64-bit platforms, Int will be equivalent to Int64.) On the other hand, in the expression:

let y = 3.0

y will be of type Double, which is a double-precision, floating-point type. Because Swift is a type-safe language, all expressions will always have a specific type. Type-safety enables the compiler to warn us when we make a mistake, such as by using an Int when we intended a Double. Like many other type-safe languages, we're able to explicitly specify the type of a constant of variable:

let x : Int = 3

We're able to use explicit typing to ensure that we're evaluating the expression as the desired type as in the example above. However, in most cases, Swift allows us to be more brief, and rely on type inference. Type inference will rely on the literal value(s) or named value(s) to determine the type. Let's look at a few examples:

let maximumClassCount = 8
// maximumClassCount will be of type Int

let maximumGPA = 5.0
// maximumGPA will be of type Double

In addition to decimal, literal integer expressions Swift will also infer the Int type from binary, octal, and hexadecimal:

let n = 0b101 // An integer formed from a binary, literal constant
let m = 0o17  // An integer formed from an octal, literal constant
let p = 0x2F  // An integer formed from a hexadecimal, literal constant

Caution

Regardless of the base used in an integer expression, the ultimate value is simply an integer.

Floating Points

Floating point numbers may be specified in decimal or in hexadecimal. Decimal numbers may have an optional exponent (indicated by e or E), which means that the base number is multiplied by $10^{n}$.

let f = 1.23e5 // f is a Double of value 123,000

Hexadecimal numbers may also have an optional exponent (indicated by p or P), which means that the base number is multiplied by $2^{n}$.

let g = 0xFp3 // g is a Double of value 120

Note that for literal constants, Swift will interpret the expression as a Double if any of the component expressions evaluate to a Double. ⚠ This does not apply to named constants.

Readability

Both integers and floating point numbers may have leading zeroes and embedded underscores for easier reading (by humans).

let oneBillion = 1_000_000_000
let tenBillion = 010_000_000_000

Strings

A series of characters embedded within quotes will be inferred to be of type String:

let name = "Captain Picard"

	Coming Soon
	String length

Operators

In programming, operators provide us with the ability to write many expressions in a manner generally familiar to us from algebra. Operators can be grouped by arity, that is, the number of operands on which the operator will function. For example, the expression 4+5 makes use of a binary operator (addition) which takes two operands, in this case a 4 and a 5. (An arity of one is often called unary while an arity of two is often called binary.)

Some common operators are listed in the table below:

Name	Symbol (Swift)	Arity	Type
Assignment	=	2	General
Unary Plus (Positive)	+	1	Arithmetic
Unary Minus (Negative)	-	1	Arithmetic
Addition	+	2	Arithmetic
Subtraction	-	2	Arithmetic
Multiplication	*	2	Arithmetic
Division	/	2	Arithmetic
Remainder	%	2	Arithmetic
Logical NOT	!	1	Boolean
Logical AND	&&	2	Boolean
Logical OR	||	2	Boolean
Equal to	==	2	Comparison
Not equal to	!=	2	Comparison
Greater than	>	2	Comparison
Greater than or equal to	>=	2	Comparison
Less than	<	2	Comparison
Less than or equal to	<=	2	Comparison
Concatenation	+	2	String

Some languages also support compound assignment operators, that is, they perform an operation on an L-Value and then store the result in the same L-Value. For example, the statements:

var x = 9
x += 2

assign the integer value of 9 to "x", and then increment "x" by 2, resulting in a new value for "x" of 11. This is exactly equivalent to:

var x = 9
x=x+2

Expressions with Operators

We can use one or more operators with appropriate operands to form more complex expressions and assign the resultant value to a constant or variable:

let areaOfRectangle = 4.0 * 5.2
let radius = 2.7
let areaOfCircle = Double.pi * radius * radius

The concatenation operator can be used to form more complex strings from simpler strings:

let firstName = "William"
let lastName = "Riker"
let fullName = firstName + " " + lastName

	Coming Soon
	Additional examples of expressions using operators

Key Concepts

Key Concepts

• An expression is a finite series of symbols that is well-formed according to agreed upon rules
• A valid expression will yield a value of a specific type according to the rules of the language
• All expressions, when evaluated, yield a particular type
• On 32-bit platforms, Int will be equivalent to Int32
• On 64-bit platforms, Int will be equivalent to Int64
• Swift is a type-safe language, all expressions will always have a specific type
  • Type-safety enables the compiler to warn us when we make a mistake, such as by using an Int when we intended a Double
  • We're able to use explicit typing to ensure that we're evaluating the expression as the desired type
• In most cases, Swift allows us to be more brief, and rely on type inference
  • Type inference will rely on the literal value(s) or named value(s) to determine the type
• Integers may be specified in decimal, binary, octal, or hexadecimal
• Floating point numbers may be specified in decimal or hexadecimal
  • Exponents may be specified by using e for decimal numbers and p for hexadecimal numbers
• Both integers and floating point numbers may have leading zeroes and embedded underscores
• A series of characters embedded within quotes will be inferred to be of type String
  • The concatenation operator can be used to form more complex strings from simpler strings
• We can use one or more operators with appropriate operands to form more complex expressions

References

• Expressions (Wikipedia.org)
• Expressions (Swift.org)