An Introduction to
Programming with C++
Fifth Edition
Chapter 9
Value-Returning Functions
Objectives
•
•
•
•
Raise a number to a power
Generate random numbers
Create and invoke a function that returns a value
Pass information, by value, to a function
An Introduction to Programming with C++, Fifth Edition
2
Objectives (continued)
• Write a function prototype
• Understand a variable’s scope and lifetime
• Use the .NET C++ Math::Pow() method
• Generate random integers in .NET C++
An Introduction to Programming with C++, Fifth Edition
3
Concept Lesson
•
•
•
•
•
Functions
Value-Returning Functions
Raising a Number to a Power
Generating Random Numbers
Creating Program-Defined Value-Returning
Functions
• Processing a Function
An Introduction to Programming with C++, Fifth Edition
4
Functions
• A function is a block of code that performs a task
• A C++ program contains at least one function:
main()
– Most contain many more
• Programmers use functions because they:
– Allow programmer to avoid duplicating code
– Allow programs to be broken into manageable tasks
• Functions can be categorized into:
– Value-returning functions
– Void functions
An Introduction to Programming with C++, Fifth Edition
5
Value-Returning Functions
• A value-returning function returns precisely one
value after completing its assigned task
– Usually returned to the statement that called function
• Typically, statement displays return value, uses it in a
calculation, or assigns it to a variable
– Some value-returning functions are built into C++
• E.g., getline() and toupper()
An Introduction to Programming with C++, Fifth Edition
6
Raising a Number to a Power
• You use pow( )to raise a number to a power and
return result as a double number
An Introduction to Programming with C++, Fifth Edition
7
Generating Random Numbers
• A pseudo-random number generator produces a
sequence of numbers that meet certain statistical
requirements for randomness
– Numbers are chosen with equal probability from a
finite set of numbers
– Chosen numbers are sufficiently random for practical
purposes
• rand() returns an integer greater than or equal to
zero and less than or equal to RAND_MAX
– RAND_MAX is always at least 32,767
An Introduction to Programming with C++, Fifth Edition
8
Generating Random Numbers
(continued)
An Introduction to Programming with C++, Fifth Edition
9
Generating Random Numbers
(continued)
An Introduction to Programming with C++, Fifth Edition
10
Generating Random Numbers
(continued)
An Introduction to Programming with C++, Fifth Edition
11
Generating Random Numbers
(continued)
An Introduction to Programming with C++, Fifth Edition
12
Generating Random Numbers
(continued)
• You should initialize the random number generator
in each program in which it is used
– Otherwise, it will generate the same series of
numbers each time
– Use the srand() function
• You must provide a seed
– Integer that represents the starting point for the
random number generator
– Usually use time(): returns current time as
seconds elapsed since midnight January 1, 1970
An Introduction to Programming with C++, Fifth Edition
13
Generating Random Numbers
(continued)
An Introduction to Programming with C++, Fifth Edition
14
Random Numbers Program:
Displaying Random Addition Problems
• Program displays five random addition problems on
the screen
– After displaying an addition problem, it allows user to
enter the answer to the problem
– It checks whether the user’s answer is correct
• “Correct!”
• “Sorry, the sum is X”
An Introduction to Programming with C++, Fifth Edition
15
Random Numbers Program: Displaying
Random Addition Problems (continued)
An Introduction to Programming with C++, Fifth Edition
16
Random Numbers Program: Displaying
Random Addition Problems (continued)
An Introduction to Programming with C++, Fifth Edition
17
Random Numbers Program:
Displaying Random Addition Problems
(continued)
An Introduction to Programming with C++, Fifth Edition
18
Creating Program-Defined
Value-Returning Functions
• You can create your own value-returning functions
– Program-defined functions
• A function definition is composed of:
– Function header
– Function body
An Introduction to Programming with C++, Fifth Edition
19
Creating Program-Defined
Value-Returning Functions (continued)
An Introduction to Programming with C++, Fifth Edition
20
Function Header
• Function header: first line in a function definition
– Does not end with a semicolon
• Not considered a C++ statement
– Begins with returnDataType
– Next comes the name of the function
• Naming rules are the same as for naming variables
• Name usually begins with a verb
– Also has an optional parameterList enclosed in ()
• Lists data type and name of formal parameters
– Formal parameters store the information passed
to the function when it is invoked
An Introduction to Programming with C++, Fifth Edition
21
Passing Information to a Function
• Items passed to a function are actual arguments
– Can be a variable, named constant, literal constant,
or keyword
• You can pass a variable:
– by value (a copy of the value is passed), or
default
– by reference (the variable address is passed)
– Examples
• calcRectangleArea(2, 3)
• calcRectangleArea(length, width)
An Introduction to Programming with C++, Fifth Edition
pass by value
22
Function Body
• Function body contains instructions the function
follows to perform its assigned task
– Begins with { and ends with }
– The last statement is usually return expression;
• expression represents the value the function returns to
the statement that called it
– return statement alerts computer that the function
has completed its task
• Data type of expression must agree with the
returnDataType specified in function header
An Introduction to Programming with C++, Fifth Edition
23
Using a Function Prototype
• If a function definition appears below main(), you
must enter a function prototype above main()
– Prototype specifies function’s name, data type of
return value and of each formal parameter
– Usually after #includes and usings
An Introduction to Programming with C++, Fifth Edition
24
Processing a Function
•
When the computer processes a statement
containing a program-defined function:
1. It locates the function’s code
2. Passes values of actual arguments to function
3. Function receives values and stores them in the
memory locations listed in its parameterList
4. Function code is processed
5. Return value is returned to calling function
An Introduction to Programming with C++, Fifth Edition
25
Processing a Function (continued)
An Introduction to Programming with C++, Fifth Edition
26
Processing a Function (continued)
An Introduction to Programming with C++, Fifth Edition
27
Processing a Function (continued)
An Introduction to Programming with C++, Fifth Edition
28
Processing a Function (continued)
An Introduction to Programming with C++, Fifth Edition
29
The Scope and Lifetime of a Variable
• A variable’s scope indicates where in the program
it can be used
– Local or global
• Local variables are declared within a function or
appear in a function’s parameterList
• Global variables are declared outside of any function
and remain in memory until program ends
– Avoid using global variables in your programs
• A variable’s lifetime indicates how long it remains
in memory
An Introduction to Programming with C++, Fifth Edition
30
The Bonus Calculation Program
• Program calculates and displays a 5% bonus
amount
– Based on the amount of sales entered by the user
• Program uses two value-returning functions:
– getSales()
– calcBonus()
An Introduction to Programming with C++, Fifth Edition
31
The Bonus Calculation Program
(continued)
An Introduction to Programming with C++, Fifth Edition
32
The Bonus Calculation Program
(continued)
An Introduction to Programming with C++, Fifth Edition
33
The Bonus Calculation Program
(continued)
An Introduction to Programming with C++, Fifth Edition
34
The Bonus Calculation Program
(continued)
An Introduction to Programming with C++, Fifth Edition
35
The Bonus Calculation Program
(continued)
An Introduction to Programming with C++, Fifth Edition
36
The Bonus Calculation Program
(continued)
An Introduction to Programming with C++, Fifth Edition
37
Summary
• Programmers use functions because they:
– Allow programmer to avoid duplicating code
– Allow programs to be broken into manageable tasks
• Functions can be value-returning or void
– E.g., pow() and rand() are value-returning functions
• A function definition has a function header and a body
– Header specifies return data type, function name, and
(optional) parameterList enclosed in parentheses
– By default, variables are passed to a function by value
An Introduction to Programming with C++, Fifth Edition
38
Summary (continued)
• The function body contains instructions for a task
– Enclosed in braces
• You must include a function prototype for each
function defined below main()
• A variable’s scope can be either local or global
– Indicates where in a program a variable can be used
• If multiple memory locations have same name, and
it appears in a statement, the computer uses the
position of the statement within the program to
determine which memory location to use
An Introduction to Programming with C++, Fifth Edition
39
Application Lesson: Using ValueReturning Functions in a C++ Program
• Lab 9.1: Stop and Analyze
• Lab 9.2:
– Create a program to calculate and display the
monthly payments of a loan
• Lab 9.3:
– Modified program should allow user to enter interest
rates either as a whole or as a decimal number
• Lab 9.4: Desk-Check Lab
• Lab 9.5: Debugging Lab
An Introduction to Programming with C++, Fifth Edition
40