Intro To C++ - Class #19: Functions

An Introduction To Software
Development Using C++
Class #19:
Functions

Let Us Imagine An
Automobile Factory…
• When an auto is manufactured, it is not made
from raw parts.
• It is put together from previously
manufactured parts.
• Some parts are made by
the auto company.
• Some parts are made by
other firms.
Image Credit: www.porschepedia.org

What Are C++ Functions?
• You can think of functions as being like building blocks.
• They let you divide complicated programs into manageable
pieces.
• Advantages of functions:
– Working on one function allows you to just focus on it and
construct and debug it.
– Different people can work on different functions simultaneously.
– If a function is needed in more than one place in a program or in
different programs, you can write it once and use it multiple
times.
– Using functions greatly enhances
a program’s readability.
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Predefined C++ Functions
• Examples: pow(x,y), sqrt(x), floor(x)
• The power function calculates xy.
pow(2.0,3) = 2.03 = 8. Is of type double and has two
parameters.
• Square Root calculates the square root:
sqrt(2.25) = 1.5. Is of type double, only has one parameter.
• floor(x) calculates the largest whole number that is less than
or equal to x.
floor(48.79) = 48.0. Is of type double with one parameter.
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How To Use Predefined Functions
• You must include the header file that contains
the function’s specifications via the include
statement.
• To use the pow function, a program must
include:
#include <cmath>
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User-Define Functions
• C++ does not provide every function that you
will ever need.
• Designer’s can’t know a user’s specific needs.
• This means that you need to learn to write
your own functions.
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Two Types Of User-Defined Functions
• Value-Returning Functions: These are
functions that have a return type. They return
a value of a specific type using the return
statement.
• Void Functions: Functions that do not have a
return type. These functions DO NOT use a
return statement to return a value.
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Value-Returning Functions
• pow, sqrt, and floor are examples of value-returning
functions
• You need to include the header file in your program
using the header statement.
• You need to know the following items:
– The name of the function
– The parameters, if any
– The data type of each parameter
– The data type of the value returned by the function.
– The code required to accomplish the task.
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What Do You Do With The Value Returned
By A Value-Returning Function?
• Save the value for further calculation:
Ex: x = pow(3.0, 2.5);
• Use the value in some calculation:
Ex. Area = PI * pow(radius, 2.0);
• Print the value:
Ex. cout << abs(-5) << endl;
Image Credit: www.fenwayfrank.com

Defining A Value-Returning Function
• You need to know the following items:
1. The name of the function
2. The parameters, if any
3. The data type of each parameter
4. The data type of the value returned by the function.
5. The code required to accomplish the task.
int abs (int number)
1 234
“formal parameter” Image Credit: www.platform21.nl

Suntax Of A Value-Returning Function
Type of the value that the function returns
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Function Call
• A function’s formal parameter list can be empty.
• The empty parentheses are still needed ()
• In a function call, the number of actual
parameters (and their data types) must match
with the formal parameters in the order given.
This is called a one-to-one correspondence.
A variable or an expression listed
in the call to a function.
Image Credit: www.behance.net

Let’s Create A Function!
• Create a function that will determine which of two numbers is
bigger.
• Function has two parameters. Both parameters are numbers.
• Assume that the data type of the numbers is floating point –
double.
• Because the larger number will be double,
the type of the function will need to be
double also.
• Let’s call our function larger
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Our Function!
Image Credit: obdtooles.blogger.ba

Other Ways To Code Our Function
Note: no need for a
local variable!
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Using Our Function
• Assume num, num1, and num2 are double variables.
• Assume num1 = 45.75, num2 = 35.50
• Various ways that we can call larger:
Image Credit: obdtooles.blogger.ba

What Order Should User-Define
Functions Appear In A Program?
• Should we place our function larger before or
after the main function?
• You must declare an identifier before you use it.
We use larger in main. We must declare larger
before main.
• C++ programmers often place the main function
before all other user-defined functions.
• This will create a compilation error.
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Function Prototype
• To prevent compilation errors when we place main first, we
place function prototypes before any function definition
(including the definition of main).
• The function prototype is NOT a definition.
• It gives the compiler the name of the function, the number
and data types of the parameters, and the data type of the
returned value – just enough to let C++ use the function.
• The prototype is a promise that a full definition of the
function will occur later on in the program. Bad things may
happen if this does not occur.
Image Credit: manuals.deere.com

Example Of A Function Prototype
1 Image Credit: thecoolimages.net

Example Of Multiple
Return Statements// File: Return.cpp
// Description: Use of "return"
#include <iostream>
using namespace std;
char cfunc(int i) {
if(i == 0)
return 'a';
if(i == 1)
return 'g';
if(i == 5)
return 'z';
return 'c';
}
int main() {
cout << "type an integer: ";
int val;
cin >> val;
cout << cfunc(val) << endl;
} // main
type an integer: 10
c
• In cfunc( ), the first if that evaluates to true exits the
function via the return statement.
• Notice that a function declaration is not necessary
because the function definition appears before it is used
in main( ), so the compiler knows about it from that
function definition.
Image Credit: gallerygogopix.net

Value-Returning Functions:
Some Weird Stuff
• What’s wrong with this code?
Image Credit: www.enginebasics.com

Value-Returning Functions:
Some Weird Stuff
• This is a legal return statement – what will get
returned?
• This is legal, but don’t do it!
return x,y;
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2 Types Of Function Parameters
• Value Parameters: When a function is called, the value of a
value parameter is copied into the corresponding formal
parameter. After the copying, there is no relationship
between the value parameter and the formal parameter.
Value parameters only provide a one-way link from the value
parameter to the formal parameter.
• Reference Parameter: The reference parameter received the
address (memory location) of the actual parameter. Reference
parameters can pass one or more values from a function and
can change the value of the actual parameter.
Image Credit: mathinsight.org

When Are Reference Parameters
Useful?
1. When the value of the actual parameter needs
to be changed
2. When you want to return more than one value
from a function (note that the return statement
can only return one value)
3. When passing the address would save memory
space and time relative to copying a large
amount of data.
Image Credit: www.amazon.com

How Do You Define A Reference
Parameter?
• When you attach a & after a data type for a variable name in
the formal parameter list of a function, the variable becomes
a reference parameter.

Void Functions
• Void functions and value returning functions have a
similar structure. Both have a heading and a body.
• A void function does not have a data type.
• A void function may or may not have formal
parameters.
• You can use a return statement to exit a void function.
• A call to a void statement has to
be a stand-alone statement.
Image Credit: www.cnn.com

What Do Void Functions Look Like?
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Scope Of An Identifier
• Are you allowed to access an identifier
(variable) anywhere in a program?
• Answer: no
• The scope of an identifier refers to where in
the program the variable is accessible.
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A Couple Of Definitions
• Local Identifier: Identifiers declared within a
function (or block). Note: local identifiers are
not accessible outside of the function.
• Global Identifier: Identifiers declared outside
of every function.
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Important Note
• C++ does not allow the nesting of functions.
• You cannot include the definition of a function
within the body of another function.
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Rules For Accessing A Variable
• Global variables are accessible if:
– The variable is declared before the function
definition.
– The function name is different than the variable.
– All parameters of the function are different than
the name of the variable.
– All local variables have names that are different
than the name of the global variable.
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• Nested block: An identifier declared within a
block is accessible if:
– Only within the block from the point that it was
declared until the end of the block.
– By those blocks that are nested within that block if
the nested blocks do not have a variable with the
same name as that of the outside block.
Image Credit: www.123rf.com

• The scope of a function name is similar to the
scope of a variable declared outside of any
block. The scope of a function name is the
same as the scope of a global variable.
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Constants In C++
• Constants are variables that have values that cannot be
changed during execution.
• Named constants placed before the main function are called
global named constants.
• Placing them here can improve the readability of your
program.

Static & Automatic Variables
• The rules so far:
– Memory for global variables remains allocated as
long as the program executes. (these are called
static variables)
– Memory for variables allocated within a block is
allocated at block entry and deallocated at block
exit. (these are called automatic variables)
Image Credit: southbostontoday.com

• It turns out that you can declare a static
variable within a function by using the
reserved word static.
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• Static variables declared within a function are local to
that function. Their scope is the same as any other
local variable in that function.
• Because the memory for static variables remains
allocated between function calls, static variables allow
you to use the value of a variable from one function
call to another.
• The difference between this an using global variables is
that other functions cannot manipulate the variable’s
value.
2 Image Credit: www.iconfinder.com

In Class Programming Challenge:
Drywall Calculations
• You are to create a program that will calculate the materials that will be needed in order to drywall
a new house that is being built.
• Your program will read in data on the dimensions of the room and calculate the total number of
sheets of drywall needed and the boxes of screws that will be required.
• All rooms will be rectangular, 10’ tall, and will be measured in meters
(1m = 3.28 ft).
• Drywall comes in sheets that are 4 x 8, 10, 12, 14, or 16
• Screws come in boxes of 5 and 25 pounds. One pound of screws contains 185 screws. A ratio of one
screw per square foot of drywall is assumed.
• Your program should read in the room data, report the size of each room, the drywall sheets
needed for each room and the number of screws needed.
• Three rules: (1) rooms have 4 walls, not 2, (2) Ignore doors and windows, (3) drywall can be shared
within a room but not between rooms.
• Required functions: convert_to_feet, calculate_drywall,
calculate_screws, print_output. You may create other
functions if you wish.
Image Credit: www.1drywall.com
You must make your main function be the first function in your file.

What’s In Your C++ Toolbox?
cout / cin #include if/else/
Switch
Math Class String getline While
For do…While Break /
Continue
Arrays Functions

Intro To C++ - Class #19: Functions

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