Chapter 9 Functions

Chapter 9 Functions

Spring 2014

Chapter 9

Functions

9.1 What are Functions?

Functions - group of related statements that perform a specific task or job

Terminology:• Function header: Start of the function• Function call: Act of invoking the function• Parameters: Variables that provide information

to a function• Return: A value returned from a function

9.1 What are Functions?Functions can be treated as a “black box”

Black box - implies we can regard this function as a standalone unit

With predefined functions, don’t need to understand its implementation - just its purpose

Inputs OutputMagic

HappensHere

9.1.1 Advantages & Disadvantages

Advantages of functions• Modularization aids in the process of step-wise

refinement

• By breaking code into individual components it becomes easier to maintain and understand by others

• Focusing energies in a specific area results in a shorter time to find and correct errors

9.1.1 Advantages and Disadvantages

Advantages of functions (continued)• Once written, that code can be used again

simply by calling

• Provides the ability of many programmers all writing key functions and integrating these functions into a working and cohesive project


Other function characteristics:• Calling a function has a little cost in relation to

performance

• When called, flow of the program is transferred to that function

• When completed, control returns to where the function was called


How does the computer know where to go back to and how does it maintain the state of all of the variables when the function was called?

• This information is saved prior to the flow being transferred to the function and restored when control is returned to the calling function

9.1.1 Advantages and Disadvantages• Because of overhead costs, don’t write

functions that are one or two lines long unless it is going to be reused repeatedly

• If short and not going to be called again, the overhead would be detrimental to the program’s performance

• Expense of using functions is minor in comparison to the benefits they provide (saving developer time in debugging, maintaining, and extending the code)

9.1.2 What About main?

• Takes on the role of the director

• Controls flow of the program - and little else

• Always the entry point

9.1.2 What About main?

• May have a conditional, a loop, and function calls, but that is about it

• Includes a minimum of processing

• Physically place main as the first function within your source code

9.2 Function Components

Three components to every function (including predefined functions): • Declaration

• Definition (body)

• Call

9.2 Function Components

Two of the predefined function components, the declaration and definition, are hidden• Declaration exists in the header files

• Definition is in the C++ library

Third component - function call - provided by the programmer to invoke the function

9.2.1 Function Declaration

Function declaration - provides compiler with information so it can verify the existence of the function name and the required parameters

Function prototype and function declaration often used interchangeably in C++


• Should be placed above main making them global

• Provides each function the ability to call any other function

Syntax:

<return-type> <function-name> ( <parameter-list> );


<return-type> specifies data type of value returned from the function

• If no value returned, the function must be specified as void

•void - in C and C++ literally means nothing


<function-name> represents a valid identifier name• Must adhere to all naming rules as defined for

variable names

• Function name should begin with a strong verb signifying what task the function will perform (read, write, calculate, etc.)


<parameter-list> represents a comma delimited list of the variables passed to the function

• If no parameters are passed to the function, place the void keyword in the parentheses OR leave the parentheses empty


// Returns nothing, is passed nothingvoid DisplayMenu();

// Returns nothing, is passed nothingvoid PrintInstructions( void );

// Returns an integer and is passed nothingint ReadData();

// Returns an integer and is passed two integersint FindMaximum( int value1, int value2 );

// Returns nothing, passed a float, a double and a boolvoid DisplayResults( float, double, bool );

9.2.2 Function Definition

Function definition - defines what the function doesTwo components

1. Function header 2. Function body

Function header - start of the function definition• Exactly like the function declaration, minus semicolon

Function body - statements that will be executed when the function is invoked


When a function body has finished program flow is returned to the place where the function was invoked

// Returns nothing, is passed nothingvoid PrintInstructions()// Function header-notice no ‘;’{// Beginning of function body cout << "Staple your loan application.\n"; cout << "In addition, sign your form!\n\n"; cout << "\t\t\t ***** Have a great day ****";}// End of function body


Must be placed outside of any other function

Syntactically incorrect to define a function within another function

9.2.3 Function Call

Function call - transfers control of the program to a specific function

If a function returns a value, immediately assign it to a variable or use it in an expression -- if not, it will be lost

9.2.3 Function Call

// Function returns no valuePrintInstructions();

// Function returns an integercout << "Maximum number is: " << FindMaximum( val_1, val_2 ); // Function returns an integerint max_value = FindMaximum( val_1, val_2 );

// Returns an integer but value is never usedFindMaximum( val_1, val_2 );

9.3 return

All predefined math functions gave a value back after doing the calculations ( i.e., function returned a value)

In both the function header and declaration a return type other than void specifies the function will return a value

9.3 return

return - immediately ends execution of the function and sends the value back to where it was called

bool DisplayLogoffMsg( bool done ){ cout << "Staple all pages of your application.\n"; cout << "In addition date and sign your form!\n\n"; if ( done ) return true; else return false; // Unreachable code! cout << "\t\t * Have a great day *\n\n"; cout << "\t\t\t\t ** Logged off **"; }

9.3 return

If a function returns a value, all paths through the function must have a return statement

bool DetermineHonorRollStatus(){ double gpa = 2.4; if ( gpa > 3.50 ) { cout << "Excellent Work" << endl; return true; } else if ( gpa >= 3.0 ) { // Will produce compiler warning: Not all control paths // return a value cout << "Nice job" << endl; } else { cout << "Keep Trying" << endl; return false; }}

9.3 return

Considered poor style to place a return in the body of a loop

Can be used without an actual value to return• Generally used to abnormally terminate the

function

• Avoid both of these usages of the return

9.3 return

int SumTheValues() { int sum = 0, value = 0; for ( int i = 0; i < 5; i++ ) { cout << "Enter value " << i + 1 << ": "; cin >> value;

sum += value; } return sum;}

9.3 returnNot necessary to return a value from a

function• Without a return the function will end anyway• Flow transfers back to where called from when

last statement in the body has executed• Signature of this type of function will have void

as its return typeIf return type other than void, and no value is

returned, will have unknown value for that function.

9.4 Passing Parameters

Passing a value or parameter - giving a value to a function• Calling function provides information to the

called function

• Value sent is placed within the function call’s parentheses


Example passing two values to the pow function

double base = 3.0, power = 4.0, exponent;

...

exponent = pow( base, power ); // Calling pow function


Every value in the call has a corresponding value in the function header and declaration• First parameter caught in the first parameter in

the header • Second parameter caught in the second

parameter and so on• Data types of parameters must all match


• Data types of parameters must all matchbool Nums( int value1, char value2, double value3 );

bool i = Nums(val1, val2, val3);

//Types for val1 must be int, val2 must be char, and val3 must be double. Order is important.

bool Nums(int value1, char value2, double value3)

{

bool something = true;

return something;

}


Order of parameters determines where values go - not the name of individual parameters • Could call function foo and pass the values a, b, and c

• In the function header, you could catch those values as x, y, and z •x must be same type as a, y same as b, and z same

as c.

9.4 Passing parameters

// Declaration/Prototypeint CalculateBoxVolume( int len, int wid, int h );

int main(){ int length = 3, width = 4; ... // Call int box_vol = CalculateBoxVolume( length, width, 2 ); ... return 0;}// Definitionint CalculateBoxVolume( int len, int width, int height ) { return len * width * height;}

9.4.1 Formal and Actual Parameters

Formal parameters appear in function header int CalculateBoxVolume( int len, int width, int height )

Actual parameters appear in function callint box_vol = CalculateBoxVolume( length, width, 2 );

Names of actual parameters do not have to match names in function prototype or the formal parameters - but the data types should match

9.4.2 Scope

Scope - refers to where within a program an identifier can be seen

Local variables - variables declared within the body of a specific function• Visibility is only within that particular function

• Lifetime limited to only that function

Global variables - variable declared outside of any function

9.4.3 Passing by Value

Passing by value - a copy of the value is made and passed to the function• Function manipulates the copy, not the original

• When called function is done, function's actual parameter retains original value


// Includes and constants. . .// Prototype int CalcDays( int age );int main(){ int age = 20; int days = 0; days = CalcDays( age ); . . . return 0;}

// CalcDays functionint CalcDays( int age ){ int days; // Since Ralph lied about // his age age += 35; days= age * DAYS_IN_YEAR; return days;}

Addresses

20 0

age days

1000 1004

main Before Call

55 20075

age* days*

2000 2004

20 20075

age days

1000 1004* Values after calculations

CalcDays main After Call


Following functions have all of their parameters passed by value

double CalcAverage( double first_val, double second_val );

int FindTestTotal( int test1, int test2, int test3 );

char FindLetterGrade( int overall_score );

9.4.4 Passing by Reference

Passing by reference - pass an alias that directly references the variable• Changes made to formal parameter are

reflected in the actual parameter when the function is done

• To pass by reference place an ampersand (&) after the formal parameter’s data type


// Includes and constants. . .// Prototype int CalcDays( int & age );int main(){ int age = 20; int days = 0; days = CalcDays( age ); . . . return 0;}

// CalcDays functionint CalcDays( int & age ){ int days; // Since Ralph lied about // his age age += 35; days= age * DAYS_IN_YEAR; return days;}

20 0

age days

1000 1004

main Before Call

20075

age* days*

2000 2004

* Values after calculations

CalcDays

55 20075

age days

1000 1004

main After Call

Addresses


Following example passes parameters both by value and by reference

// Two parameters passed by referencevoid Swap( int & a, int & b );

// Two parameters passed by value, one by referencevoid FindAverage( double a, double b, double &average );

// Two parameters passed by referenceint EnterData( int &id, char &code );


Use only if the original value needs to be changed• Use pass by value for all other occasions

Regardless how arguments are passed into the function, the call statement remains the same

9.5 Default Arguments

Default argument - value the programmer provides that will automatically be inserted if no value is provided for that specific parameter in the function call

Two types of arguments or parameters: mandatory and default


Mandatory arguments - must be specified in the function call• Mandatory arguments must come before any

default arguments in the parameter list


Default arguments - provided starting with the rightmost variable in parameter list

Default values can continue from right to left, but once stopped, can not start again later


Put default values only in the function declaration or prototype - do NOT include them in the function header


// Function declarations ONLYvoid DisplayMenu( int times = 1 );void PrintInstructions( int length, int height = 7 );int ReadData( int &records, double units = 45.5, int size = 11 );

// Examples of different calling optionsDisplayMenu( times );DisplayMenu();DisplayMenu( 3 );PrintInstructins( 5, 10 );PrintInstructions( 7 );ReadData( records );ReadData( records, 50.5 );ReadData( records, 50.2, 11 );

9.6 Putting It All Together

// Chapter 9 - Section 9.6// Filename: Example9_6_1#include <iostream>#include <cmath>using std::cout;using std::endl;int Fun1( int a, int b );void Fun2( int & a, int b );void Fun3( int & c, int & d );void PrintOutput( int a, int b );int main(){ int a = 2, b = 10; PrintOutput( a, b ); a = Fun1( a, b ); cout << a << "\t" << b << endl; Fun2( a, b ); PrintOutput( a, b ); return 0;}void PrintOutput( int a, int b ){ cout << a << "\t" << b << endl;}

int Fun1( int a, int b ){ int c; c = a + b; a++; --b; cout << a << "\t" << b << "\t" << c << endl;

return c;}void Fun2( int & a, int b ){ a += 5; double temp = pow(static_cast<double>(a),2); b = static_cast<int>( temp ); PrintOutput( a, b ); Fun3( a, b ); PrintOutput( a, b );}void Fun3( int & c, int & d ){ c = 25; d = 10; PrintOutput( c, d );}

9.6 Putting It All Together

// Output

2 10

3 9 12

12 10

17 289

25 10

25 10

25 10

9.7 Debugging - Call Stack

Call Stack window - allows viewing where in the hierarchy of function calls the current line of execution is located

Also shows values of each of the parameters passed to the function


To view the Call Stack window while debugging access the following menu items• Debug -> Windows -> Call Stack• OR – Ctrl + Alt + C (to display Call Stack

window)


From Call Stack window

Can see there are other functions that called main • Responsible for displaying the console window• Also for loading the program into memory

9.8 More Predefined Functions

Chapter 6 discussed some of the predefined mathematical functions

The following functions manipulate individual characters

9.8.1 Character CaseTo change the case of a single character

Function declarations•int toupper( int c );•int tolower( int c );

These functions are passed an integer by value and return an integer

Don’t forget ASCII values

Can pass a character and catch it as an integer because this would be a widening conversion - no type casting is needed

9.8.1 Character Case

To use toupper and tolower - include <cctype>

#include <cctype>...char again;

cout << "\nDo you wish to multiply two numbers (y/n)? ";cin >> again;

// Notice the function callwhile ( toupper( again ) == 'Y' ) { ... cout << "\n wish to multiply two more numbers (y/n)? "; cin >> again;}

9.8.1 Character Case

Parameters passed to the toupper and tolower functions were passed by value - original parameter is not changed

Converted value is returned

Like mathematical functions, toupper and tolower are not in a namespace

9.8.2 The “is” Functions

Group of functions to test characters to determine what classification the character fits into• Example: ispunct - determines if the

character is punctuation

• All have the following syntax:

int <is-function>( int c );


The <cctype> header file is required for the “is” functions

Function Description

isalnum Is the character alphanumeric? (A-Z, a-z, 0-9)

isalpha Is the character a letter? (A-Z, a-z)

iscntrlIs the character a control character? (ASCII values 0-31 and 127)

isdigit Is the character a digit? (0-9)

ispunct Is the character punctuation?

isspace Is the character whitespace? (ASCII values 9-13 and 32)

http://www.cplusplus.com/reference/cctype/

http://www.cplusplus.com/reference/cctype/


All of the “is” functions can be used in a similar manner as shown below

// Example 1if ( isalpha( grade ) == 0 ) { cout << "Error: Not a letter grade!";}// Example 2if ( !isalpha( grade ) ) { cout << "Error: Not a letter grade!";}

9.9 Structure Charts

Structure Chart - tool to help better visualize the organization of our solution or program

• Like a flowchart, uses symbols and connectors

• Higher level approach versus flowchart

• Graphically shows the tasks or functions needed as well as the relationship of these tasks to each other

9.9 Structure Charts

PrintOutput

main

Fun1 Fun2

Fun3PrintOutput

PrintOutput

• Allows visualizing which functions a specific function calls

• The symbol for PrintOutput was used to show that function is called from more than one function – parameters are NOT shown

9.11 C – The DifferencesUnlike C++, there is a difference between a void parameter and an empty parameter list

• void parameter list specifies there are no parameters passed to the function

• Empty parameter list specifies an indeterminate number of parameters that could be passed to a function

9.11 C – The DifferencesCan not specify default arguments in C

The functionality of passing by reference is accomplished in a much different manner called passing by pointer• Since this is covered in Chapter 12, refer to that

chapter• In C, passing by pointer is often called passing

by reference

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Chapter 9 Functions