W1035 Not a Value

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

— Merlin, The Coder

Paris, Notre Dame, Kilometer Null

Prerequisites

• W1031 Positive Integers
• W1032 Negative Integers
• W1033 Character Encoding

Introduction

There's often a need to represent the absence of a value. For example, consider a database tracking students' first name, middle initial, and last name. Not all students have a middle initial. So what value should we store in a variable representing a student's middle initial if this particular student lacks a middle initial?

The name of this special value, representing non-existence, varies from language to language. In Swift, it's called nil.

Nil

Nil represents the absence of value of the specified type. We indicate that we intend to accept such nil values by specifying the type explicitly and suffixing the type name with a question mark. For example, in the aforementioned case, rather than specify the type as:

let middleName : Character

we use:

let middleName : Character?

This enables us to specify either a character or the special place-holder for no value, nil:

let middleName : Character? = "A"

-or-

let middleName : Character? = nil