W1201 Scope

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

— Merlin, The Coder

Scope

Prerequisites

• W1102 Type Overview

Research

• Read Scope (Wikipedia)
• Read Computer Science I Textbook (by Bourke) Chapter 2.2.4. Scoping

Introduction

The lifetime of a variable is the time period in which the variable has memory allocated to it and may legitimately be referenced.

The scope of a declaration is that part of the code in which the declaration is in effect.

The visibility of a declaration refers to that part of the code in which the declaration is accessible (not hidden) by an identical declaration in another scope.

To best understand these terms, we'll look at several examples below. Beforehand, however, let's revisit our discussion about L-Values.

Not All L-Values are Created Equal

Typical Memory Layout

Review our previous discussion about W1038 L-Values and R-Values. It's clear that the defining difference is that L-Values persist beyond a single expression, and are addressable somewhere in main memory. The big question is, where?

Experiment

Create a directory within your "project" directory.

cd ~/projects
mkdir project-1201
cd project-1201

Edit a new file named "main.swift"

emacs main.swift

For this project you'll repeatedly edit the same file. It's probably easiest to test this file within emacs by typing: M-& (async-shell-command) and observing the results. For each question, be sure to understand the behavior before proceeding to the next question. It will be helpful to record your answers and reasoning for later review.

Helpful Hint

The async-shell-command can be executed with ALT-SHIFT-7. The command to execute each time is: swift main.swift

1

let x = 10
print(x)

What is output? (Ignore whitespace)

	23
	21
	10
	Compile or runtime error

2

let x = 10
print(x)
print(y)

What is output? (Ignore whitespace)

	10 21
	Compile or runtime error
	10 10
	21 23

3

let x = 10
print(x)
let x = 21
print(x)

What is output? (Ignore whitespace)

	Compile or runtime error
	10 10
	21 23
	10 21

4

let x = 10
print(x)
var x = 21
print(x)

What is output? (Ignore whitespace)

	21 23
	10 21
	Compile or runtime error
	10 10

5

let x = 10
print(x)
do {
    let x = 21
    print(x)
} //

	21 21
	10 21
	Compile or runtime error
	10 10

6

let x = 10
print(x)
do {
    let x = 21
    print(x)
} //
print(x)

	Compile or runtime error
	10 21 21
	10 10 10
	10 21 10

7

let x = 10
print(x)
repeat {
    let x = 21
    print(x)
} while x == 21
print(x)

	Infinite loop
	10 21 21
	10 21 10
	Compile or runtime error

8

let x = 10
print(x)
while x == 21 {
    let x = 21
    print(x)
} //
print(x)

	10 21 10
	Infinite loop
	10 10
	Compile or runtime error

9

let x = 10
print(x)
for x in 11 ... 11 {
    print(x)
    let x = x + 10
    print(x)
} //
print(x)

	Infinite loop
	10 11 21 10
	Compile or runtime error
	10 10 20 10

10

let x = 10
print(x)
for x in 11 ... 11 {
    print(x)
    x = x + 10
    print(x)
} //
print(x)

	10 11 21 10
	Infinite loop
	10 10 20 10
	Compile or runtime error