1. Chapter 6: Functions Starting Out with C++ Early Objects
Seventh Edition
by Tony Gaddis, Judy Walters, and Godfrey Muganda
Revised 2015 EL Jones, FAMU,

2. Topics
6.1 Modular Programming 6.2 Defining and Calling Functions 6.3 Function Prototypes 6.4 Sending Data into a Function 6.5 Passing Data by Value 6.6 The return Statement 6.7 Returning a Value from a Function 6.8 Returning a Boolean Value

3. Topics (continued)
6.9 Using Functions in a Menu-Driven Program 6.10 Local and Global Variables 6.11 Static Local Variables 6.12 Default Arguments 6.13 Using Reference Variables as Parameters 6.14 Overloading Functions 6.15 The exit() Function 6.16 Stubs and Drivers

4. (Modular Programming)
STOP / START HERE
Chapter 6 - Functions
Part 1
(Modular Programming)
See pr8-09.cpp

5. 6.1 Modular Programming
Modular programming: breaking a program up into smaller, manageable functions or modules
Function: a collection of statements to perform a specific task
Motivation for modular programming
Simplifies the process of writing programs
Improves maintainability of programs

6. A First Picture Poly-lithic (future) Mono-lithic Poly-lithic (now)
#include <lib1> // others
#include <lib2> …
int main //yours {
ssssss }
#include <lib1> //others
#include <lib2> …
module (… ) //yours
{ … }
int main //yours {
ssssss }
int main //yours {
ssssss }

7. Using Modules Using your own modules Using C++ library modules
#include <lib1> // others
#include <lib2> …
#include <cmath>
#include <iostream
//Compute sum of two integers.
int Sum (int a, int b) // yours
{ return a+b; }
int main //yours {
float pi = , sqrtPI;
sqrtPI = sqrt(pi);
cout << “square root of PI = “;
<< sqrtPI << endl; }
int main //yours
{ int x, y;
cin >> x >> y;
cout << Sum(x,y) << endl; }

8. Using Modules Using your own modules Using C++ library modules
#include <lib1> // others
#include <lib2> …
#include <cmath>
#include <iostream
//Display string.
void Show (string myStr) // yours {
cout <<endl<< myStr << endl; }
int main //yours {
string S;
getline(cin, S);
cout << “String= “ << S << endl; }
int main //yours {
string q = “HELLO, Joe”;
Show(q); }

9. Modular Programming - Terminology
A module has these characteristics
A job description (what it does)
Raw materials (called parameters) that must be provided to for the module to do its job.
Results produced by the function
C++ functions that implement modules require:
Documentation of the job the function performs
An interface specification identifying the raw materials and results
A body that implements an algorithm to transform raw materials into results

10. Your First Function // Compute sum of two integers.
int Sum (int a, int b) // interface – 2 integers, one int result. {
return a+b;
} // end of body
int main //yours
{ int a,b, x, y, s;
cin >> x >> y >> a >> b;
s = Sum(x,y); // CALL, invoke or activate function..
cout << “SUM1 = “ << s << endl;
cout << “S2 = “ << Sum(a,b) << endl; // Invocation of function.. }

11. Function Prototype
The compiler must know the following about a function before it can be called
name
return type
number of parameters
data type of each parameter
int Big (int x , int y);
int Sum (int , int);
void printHeading( );

12. Function Definition Definition includes Header:
return type: data type of the value the function returns to the part of the program that called it
name: name of the function. Function names follow same rules as variable names
parameter list: variables that hold the values passed to the function
body: block of statements that perform the function’s task

13. Function Definition

14. Function Definition = header + body
The function header consists of *
the function return type
the function name
the function parameter list
Examples:
int main()
int Big (int x, int y)
Note: no semicolon (;) after header … because … a body follows!!

15. Scope Rules for Functions
Same as for variables: declare BEFORE using.
A function call can PRECEDE the function definition.
Possible order of code sections:
prototypes definitions prototypes
calls (main) calls (main) definitions
definitions calls (main)
See pr6-05.cpp

16. Prototype Notes Place prototypes near top of program
Program must include either prototype or full function definition before any call to the function, otherwise a compiler error occurs
When using prototypes, function definitions can be placed in any order in the source file. Traditionally, after the main.

17. (Function Calls, Passing Parameters by Value)
STOP / START HERE
Chapter 6 - Functions
Part 2
(Function Calls, Passing Parameters by Value)
See pr8-09.cpp

18. Calling a Function
main is automatically called when the program starts
main can call any number of functions
A function can call other functions
Call a function by its NAME
Sum(3,2) …
NOT int Sum(3,2); // this is a declaration!!
See pr6-03.cpp and pr6-04.cpp

19. Calling a Function
When a function is called by a module, the program executes the body of the function while the calling module is temporarily suspended.
Only after the called function completes does execution resume in the calling module at the point following the call.
See pr6-01.cpp and pr6-02.cpp

20. Function Return Type If a function returns a value
the data type of the value is the first word in the header (or prototype)
int main()
The function body must contain a return statement
int P(…){ … return int_express;}

21. Function Return Type *
If a function returns a value, the type of the value must be indicated
int main()
If a function does not return a value, its return type is void
void printHeading() {
cout << "Monthly Sales“ << endl; }

22. Return Type Affects How To Call Function
If a function returns a value, the function call must be an expression
var = Sum(3,x) ; not Sum(a,b);
If a function does not returns a value, the function call must be a statement
printHeading() ;

23. Calling and Defining Functions
Function call: expression or statement that causes a function to execute
root = pow(x, 0.5); // call is an expression
getline(cin, myName); // call is a statement
Function definition: statements that carry out the work of the function
int main( ) int Big(int x, int y) // header
{ {
cout << “HELLO” << endl; if (x > y) return x;
return 0; else return y;
} }

24. Providing Data for a Function
Can pass values into a function at time of call
c = sqrt(a*a + b*b);
Values passed to function are called arguments
Variables in function that hold values passed as arguments are parameters
Alternate names:
argument: actual argument, actual parameter
parameter: formal argument, formal parameter

25. Function Call Notes A function can have zero or more parameters.
Value of each argument is assigned to the corresponding parameter when the function is called.
Each function parameter must be declared in the parameter list ( … ) in the function header.
Passed arguments will be promoted or demoted as necessary to match data type of parameters.

26. Parameters, Prototypes, and Function Headers
For each function argument,
the prototype must include the data type of each parameter in its ()
void evenOrOdd(int); //prototype
the header must include a declaration, with variable type and name, for each parameter in its ()
void evenOrOdd(int num) //header
The function call for the above function would look like this: evenOrOdd(val); //call
See pr6-06.cpp and pr6-07.cpp

27. Passing Data to Parameters by Value
Example: int val = 5;
evenOrOdd(val);
evenOrOdd can change parameter num, but it will have no effect on variable val
val
num 5 5
argument in
calling function
parameter in
evenOrOdd function
See pr6-09.cpp

28. Passing Data by Value
Pass by value: when argument is passed to a function, its value is assigned to the corresponding parameter
The function cannot access the original argument
Changes to the parameter in the function do not affect the value of the argument in the calling function

29. Calling Functions with Multiple Arguments
When calling a function with multiple arguments
the number of arguments in the call must match the number in the function prototype and definition
the value of the first argument will be assigned to the first parameter, the value of the second assigned to the second parameter, etc.

30. Calling Functions with Multiple Arguments Illustration *
displayData(height, weight); // call void displayData(int h, int w)// header { cout << "Height = " << h << endl; cout << "Weight = " << w << endl; }
See pr6-08.cpp

31. (Functions Returning Values, Overloading)
STOP / START HERE
Chapter 6 - Functions
Part 3
(Functions Returning Values, Overloading)
See pr8-09.cpp

32. Functions that Return Values
Using your own modules
Using C++ library modules
#include <lib1> // others
#include <lib2> …
#include <cmath>
#include <iostream
//Compute sum of two integers.
int Sum (int a, int b) // yours
{ return a+b; }
int main //yours {
float pi = , sqrtPI;
sqrtPI = sqrt(pi);
cout << “square root of PI = “;
<< sqrtPI << endl; }
int main //yours
{ int x, y;
cin >> x >> y;
cout << Sum(x,y) << endl; }

33. Returning a Value From a Function
return statement can be used to return a value from the function to the calling module
Prototype and definition indicate the data type of returned value (not void)
The calling function should “catch” the returned value, e.g.,
assign it to a variable
send it to an output stream (cout or a file stream)
use it in an expression (arithmetic, relational, logical)
use it as an argument to a function call
See pr6-11.cpp and pr6-12.cpp

34. The return Statement Used to end execution of a function
Can be placed anywhere in a function
Statements that follow the return statement will not be executed
Can be used to prevent abnormal termination (crash) of program
Without a return statement, the function ends at its last }
See pr6-10.cpp

35. Returning a Value – the return Statement
Format: return expression;
expression may be a variable, a literal value, or an expression.
expression should be of the same data type as the declared return type of the function (will be converted if not)
See pr6-11.cpp and pr6-12.cpp

36. Returning a Boolean Value
Function can return true or false
Declare return type in function prototype and header as bool
bool Odd(int num)
Function body must contain return statement(s) that return true or false
The calling function can use return value in a relational or logical expression.

37. Boolean return Example *
bool isValid(int); // function prototype
// Validate val in range [0,100].
bool isValid(int val) // function header {
if (val >= min && val <= max)
return true;
else return false; }
// In calling module …
cin >> score;
if (isValid(score)) // function call
See pr6-13.cpp

38. An Input Function
int ReadNum(); // function prototype int ReadNum() // function header { int v; cout << “Enter whole number: “; cin >> v; return v; } // Calling module … int num; num = ReadNum(); // function call
See pr6-13.cpp

39. An Input Function
int ReadNum(); // function prototype int ReadNum() // function header { int v; cout << “Enter whole number: “; cin >> v; return v; } // Calling module … int num; num = ReadNum(); // function call
See pr6-13.cpp

40. Function Overloading
Overloaded functions are two or more functions that have the same name, but different parameter lists (number, or data types)
Can be used to create functions that perform the same task, but take different parameter types or different number of parameters
Compiler will determine which version of function to call based on the argument list.

41. Overloaded Functions Example
If a program has these overloaded functions,
void getDimensions(int); // 1
void getDimensions(int, int); // 2
void getDimensions(int, float); // 3
void getDimensions(double, double);// 4
then the compiler will use them as follows:
int length, width;
double base, height;
getDimensions(length); // 1
getDimensions(length, width); // 2
getDimensions(length, height); // 3
getDimensions(height, base); // 4
See pr6-27.cpp and pr6-28.cpp

42. The exit() Function Terminates execution of a program
Can be called from any function
Passes a value to the operating system to indicate status of program execution
Usually used for abnormal termination of program (positive value; zero means error free)
Use carefully: NOT the same as return.
See pr6-29.cpp

43. (Parameters and Variables [global, local, static] )
STOP / START HERE
Chapter 6 - Functions
Part 4
(Parameters and Variables [global, local, static] )
See pr8-09.cpp

44. Default Arguments
Values passed automatically if arguments are missing from the function call
Must be a constant declared in prototype
void evenOrOdd(int = 0);
evenOrOdd(); same as … evenOrOdd(0);
Multi-parameter functions may have default arguments for some or all of them
int getSum(int, int=0, int=0);
See pr6-22.cpp

45. Default Arguments *
If not all parameters to a function have default values, the ones without defaults must be declared first in the parameter list
int getSum(int, int=0, int=0);// OK
int getSum(int, int=0, int); // wrong!
When a default argument is omitted from a function call, all arguments after it must also be omitted
sum = getSum(num1, num2); // OK
sum = getSum(num1, , num3); // wrong!

46. Local and Global Variables
Local variable: defined within a function or block; accessible only within the function or block
Other functions and blocks can define variables with the same name
When a function is called, local variables in the calling function are not accessible by name from within the called function
See pr6-15.cpp

47. Local and Global Variables *
Global variable: defined outside all functions and blocks; it is accessible to all functions within its scope
Easy but dangerous way to share large amounts of data between functions
Scope of a global variable is from its point of definition to the program end
Use sparingly

48. Local Variable Lifetime
A local variable only exists while its defining function is executing
Local variables are destroyed when the function terminates
Data cannot be retained in local variables between calls to the function in which they are defined

49. Static Local Variables *
Only exist while the function is executing
Are redefined/recreated each time function is called
Lose their contents when function terminates
static local variable
is defined with key word static
static int counter = 0;
is defined and initialized only the first time the function is executed
Retains its value between function calls
See pr6-20.cpp and pr6-21.cpp

50. Initializing Local and Global Variables
Local variables must be initialized by the programmer
Global variables are initialized to 0 (numeric) or NULL (character) when the variable is defined
See pr6-16.cpp and pr6-17.cpp

51. Global Variables – Why Use Sparingly?
Programs that use global variables are difficult to understand (e.g., what is the global variable used for?) and debug
Functions that use global variables cannot easily be re-used in other programs (that may not have the same global variables)

52. Local and Global Variable Names *
Local variables can have same names as global variables
When a function contains a local variable that has the same name as a global variable, the global variable is unavailable from within the function. The local definition "hides" or "shields" the global definition.
See pr6-18.cpp and pr6-19.cpp

53. Chapter 6 - Functions STOP / START HERE Part 5
(Passing Variables as Parameters – call by reference; Menu-Driven Program Pattern)
See pr8-09.cpp

54. Another Input Function *
void ReadNum(int &); // function prototype void ReadNum(int & num) // function header { cout << “Enter whole number: “; cin >> num; } // Calling module … int num = 5; cout << num << endl; // Output is 5. ReadNum(num); // function call; input is 17. cout << num << endl; // Output is 17.
See pr6-13.cpp

55. Using Reference Parameters
Mechanism that allows a function to access and modify the original variable from the calling module.
The parameter is an ALIAS for the variable passed in the function call.
The argument in the function call must be a variable.
Provides a way for the function to produce multiple results (delivered in the modified variables)

56. Reference Parameters
A reference parameter is an alias for another variable
Defined with an ampersand (&)
void getDimensions(int&, int&);
Changes to a reference parameter are made to the variable it refers to!!
Use reference variables to implement passing parameters by reference

57. Pass by Reference – Producing Multiple Results *
void SquareCube(float x, float & sqr, float & cube) { sqr = x * x; cube = x * x * x; }
See pr6-23.cpp and pr6-24.cpp

58. Pass by Reference – I/O Streams *
void Read(istream & inF, string & s) { inF >> s; } // CALLS. ifstream myF(“mydata”); Read(myF, name); // From file. Read(cin, name); // From keyboard.
See pr6-23.cpp and pr6-24.cpp

59. Reference Parameter Notes
Each reference parameter must contain &
Argument passed to reference parameter must be a variable (cannot be an expression or constant)
Use only when appropriate, such as when the function must input or change the value of the argument passed to it
File stream objects must always be passed by reference
See pr6-25.cpp and pr6-26.cpp

60. Testing Functions: Stubs and Drivers
Stub: dummy function in place of actual function
Usually displays a message indicating it was called. May also display parameters.
Place-holder for the real function.
Driver: a module that exists to test a function by calling it with arguments and examining the results.
Stubs and drivers are useful for testing and debugging program logic and design
See pr6-30.cpp

61. Using Functions in a Menu-Driven Program
Functions can be used
to implement the display of the menu and the input of the user menu choice
to implement the central switch for the menu
to implement a case for a specific menu choice
See pr6-14.cpp

62. Menu-Driven Program Pattern
char GetMenuChoice(){ … } void PerformMenuChoice(char choice, … ) { switch ( ) … }//PerformMenuCoice int main() do choice = GetMenuChoice(); PerformMenuChoice(choice, … ); } while (choice != QUITCHAR);
See pr6-23.cpp and pr6-24.cpp

63. STOP / START HERE
Chapter 6 - Functions
See pr8-09.cpp

64. Chapter 6: Functions Starting Out with C++ Early Objects
Seventh Edition
by Tony Gaddis, Judy Walters, and Godfrey Muganda
Revised 2015 EL Jones, FAMU,