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? To better understand this, we'll need to take a deeper look at the typical memory layout of a program.

Program Memory Layout

TEXT

Program memory is divided into several different sections, often called segments. Logically, every program must have a least one segment, the executable code itself. This is typically stored as read-only (i.e. it may not be modified) and is referred to as "TEXT".

func f() {
    print("Hello, World!")
}

DATA

The next segment is referred to as "DATA". It contains initialized memory, generally global or static variables assigned a specific value. The values themselves are actually stored in the TEXT segment and copied to the DATA segment during program startup. This segment is read-write.

let x = 7

BSS

The "BSS" segment contains uninitialized memory, generally global or static variables which have not been assigned a specific value. This segment is read-write.

let x : Int

The remainder of memory is read-write and managed dynamically, likely changing frequently as the program executes. It's divided into two main areas.

The Heap

The heap area begins where the DATA and BSS segments end. Memory is allocated dynamically within this area, as needed.

The Stack

The stack area begins from a high-point in memory and usually grows downward, toward the heap. It's a very important structure and is responsible for the orderly execution of function invocations. The stack area is organized into a series of frames, one for each function invocation. The frame contains the return address, i.e. the location in memory to which the function will return when it completes execution, as well as any local variables.

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 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

For this project you'll repeatedly edit the same file. It's probably easiest to test this file within emacs by using the async-shell-command and observing the results. 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
	10
	21
	Compile or runtime error

2

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

What is output? (Ignore whitespace)

	21 23
	10 21
	Compile or runtime error
	10 10

3

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

What is output? (Ignore whitespace)

	21 23
	Compile or runtime error
	10 10
	10 21

4

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

What is output? (Ignore whitespace)

	Compile or runtime error
	10 21
	10 10
	21 23

5

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

	10 21
	21 21
	10 10
	Compile or runtime error

6

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

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

7

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

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

8

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

	10 10
	Infinite loop
	10 21 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)

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

10

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

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