W1037 Expressions and Operators

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

— Merlin, The Coder

	Coming Soon
	Expressions

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"

References

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