Study Guide - Chapter 4 void Functions
ITP 134 C++ Study Guide
Instructions: Use these notes to give you the important points of this chapter. See the book for lots of great
programming examples. Important concepts, keywords, statements and functions are shown in emphasized font.
Programming code syntax and examples are shown in code font.
Chapter 4 void Functions
4.1 Modularizing a Program with
Functions
There are two general types of functions: those that
return a value, and those that do not. In C++, a function
that does not return a value is known as a void
function. In this chapter, we will demonstrate how void
functions can be used to break down a long program
into smaller, manageable pieces. This approach is
sometimes called divide and conquer because a large
program is divided into several smaller programs which
are easily solved. (pg 139)
CONCEPT: A large program can be broken up into
manageable functions that each performs a major task.
(pg 139)
Benefits of Using Functions (pages 140 – 141)
 Simpler code
 Code reuse
 Better testing
 Faster development
 Easier facilitation of teamwork
4.2 Defining and Calling a void Function
CONCEPT: The code for a function is called a function
definition. To execute a function you write a statement
that calls it. (pg 141)
Function Names
C++ rules for naming a function: (pg 142)




Must be 1 word. Cannot contain spaces.
First character must be a-z, A-Z, or _
(underscore)
After first character can also be a number, but
cannot be a special character such as $.
Names are case sensitive.
ITP 134 – Chapter 4
Mrs. Eaton
Defining a void Function – Function Definition
 To create a function you write the function
definition. This specifies what a function does,
but it does NOT cause the function to execute.
 The first line in a function definition is known as
the function header. (page 142)
 Notice that the function header does NOT end
with a semicolon
 This is the general syntax of a void function
definition:
void functionName()
{
statement1;
statement2;
}
Calling a Function
 To execute a function, you must call it.
 When a function is called, the program jumps to
the function definition code and executes it.
After the function ends, the programs jumps
back to the statement that called it and
continues executing with the very next
statement.
 Notice that you do NOT write the word void in
the function call statement.
The general form of a call statement is: (pg 143)
functionName();
Function Prototypes
A function prototype is a statement that declares the
existence of a function, but does not define the
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Study Guide - Chapter 4 void Functions
function. It tells the C++ compiler that a particular
function exists, and the definition appears later in the
program. (pg 145 – 147).
Eaton note: This is also called a function declaration in
some books and languages.
The general syntax of a function prototype is:
void functionName();
4.3 Designing a Program to Use Functions
CONCEPT: Programmers commonly use a technique
known as top-down design to break down a program
into functions that each performs a single task. (pg 148)
Top-down design process (pg 148- 149)
1. Break down the overall task into a series of
subtasks.
2. Determine if each subtask can be further
broken down into more subtasks. Repeat this
step until no more subtasks can be identified.
3. Once you identify all the subtasks, then write
the code.
Hierarchy Charts
Programmers commonly use hierarchy charts to
visually represent the relationships between functions
in a program. This is also known as a structure chart,
and shows boxes that represent each function. The
boxes are connected to show how the functions are
called. (pg 150-151) See Figure 4-8 on page 151 for an
example hierarchy chart for drawing the City Skyline.
4.4 Local Variables
Duplicate Variable Names
You cannot have 2 different variables with the same
name inside the same function. (pg 154)
4.5 Passing Arguments to Functions
CONCEPT: A piece of data is an argument passed into a
function when the function is called. When an argument
is passed into a function, a special variable known as a
parameter in the function receives the argument. (pg
155) Eaton Note: Remember that the calling statement
contains arguments. Mnemonic call, call, get into a fight
and argument. The function definition contains
parameters.
Parameter Variable Scope
A parameter variable’s scope is the entire function
where the parameter is declared. It is visible (and
accessible) only to statements inside the function.
Eaton Note: In other words a parameter is a local
variable to the function definition. (pg 159)
Passing Multiple Arguments
If a function needs multiple arguments passed, you also
need multiple parameters to receive the values passed.
Eaton note: The arguments and parameters must be in
the same order and the same type. (pg 160)
Passing Arguments by Value
When you pass an argument by value you pass only a
copy of the value. Therefore if you change the value,
the value is only changed inside the function. (pg 163)
CONCEPT: A local variable is declared inside function
and cannot be accessed by statements that are outside
the function. Different functions can have local
variables with the same names because the functions
cannot see inside other functions for the local variables.
(pg 151)
Passing Arguments by Reference
When you pass an argument by reference, you pass the
memory location of the value (not just a copy).
Therefore if you change the value, the value is changed
inside the function and also in the calling program or
function. (pg 165)
Scope and Local Variables
Scope describes the portion of the overall program
where a variable may be accessed. A local variable’s
scope begins at the variable’s declaration and ends at
the end of the function where the variable is declared.
(pg 154)
4.6 Global Variables and Constants
ITP 134 – Chapter 4
Mrs. Eaton
CONCEPT: A global variable is available to every
function in a program, and is declared outside all the
functions in a program. (pg 169)
Global Constants
A global constant is a named constant that is available
to every function in a program. Global constants are
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Study Guide - Chapter 4 void Functions
typically used to represent unchanging values that are
needed throughout a program. (pg 171)
Programs in this Chapter
4.1 FunctionDemo.cpp (page 143)
 This program displays “I am Arthur, King of the
Britons.”
 Notice the functions are given before the
DarkGDK function, so function prototypes are
not needed.
 This is not a good programming practice.
 This program demonstrates function calls and
function definitions for 2 very simple functions:
displayMessage()
pauseProgram()
4.2 Prototypes.cpp (page 147)

This program displays “I am Arthur, King of the
Britons.”
 Notice the DarkGDK is the first function
followed by the other function, so function
prototypes are required.
 This is a good programming practice.
 This program rewrites the previous program
using function prototypes.
displayMessage()
pauseProgram()
Program 4-3 ModularCitySkyline.cpp (page 149 –
150)
 This program draws a city skyline.
 Compare this program with functions to
Program 2-9 (pages 55-56) which draws the
same drawing without using functions.
 The functions are:
drawBuildings()
drawWindows()
drawStars()
Program 4-4 ErrorLocalVars.cpp (page 152)
 This program should draw 2 boxes, but it has
errors.
 This program has errors on lines 34 and 46
before you are trying to use local variables
outside the functions where they are declared.
The next program corrects these errors.
Program 4-5 LocalVars.cpp (page 153)
 This program draws 2 boxes.
 This program demonstrates the correct use of
local variables and fixes the errors from the
previous program.
ITP 134 – Chapter 4
Mrs. Eaton
Program 4-6 CenterCircle.cpp (page 156)
 This program draws a circle in the center of the
screen.
 This program demonstrates passing the radius
of the circle as arguments or parameters by
value. Passes a copy.
Program 4-7 ConcentricCircles.cpp (page 158)
 This program is the same as the previous expect
the function drawCircle is called 4 times.
Program 4-8 Sum.cpp (page 160)
 This program calculates the sum of 2 numbers.
 This program demonstrates passing 2
arguments or parameters by value. Passes a
copy.
Program 4-9 RandomBricks.cpp (page 162-163)
 This program draws random bricks.
 This program demonstrates calling the
drawBrick function 3 times and passing 2
arguments or parameters x and y.
Program 4-10 PassByValue.cpp (page 164)
 This program demonstrates passing arguments
or parameters by value.
 The value does not change in the calling
program because you are passing a copy which
you pass by value.
Program 4-11 PassByReference(pages 166 – 167)
 This program prints some numbers.
 This program demonstates pass by reference and
shows how the numbers also change in the calling
program because you are passing a memory
location not just a copy.
Program 4-12 WindowSizes.cpp (page 168)
 This program prints the height and width of the
program window.
 It is supposed to demonstrate passing by
reference, but it is a terrible example because
the arguments are not initialized before they
are passed.
Program 4-13 GlobalVar.cpp (page 169)
 This program prompts the user for a number
and then displays it.
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Study Guide - Chapter 4 void Functions

The program demonstrates a global variable
called number.
Program 4-14 ItalianFlag3.cpp (page 171-173)
 This program draws the Italian flag.
 This program demonstrates using global
constants: GREEN, WHITE, RED and BLACK.
ITP 134 – Chapter 4
Mrs. Eaton

This program also demonstrates the use of
functions. Compare this program to to Program
3-17 ItalianFlag2.cpp on page 130 that draws
the same image.
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