Parameters

When a function is executed, it uses the arguments given to it in the function call.

C++ supports two kinds of parameters

Value parameters - the function receives a copy of the argument’s value

Reference parameters (& after data type) - the function receives the memory address of the caller’s argument

Examples:

void Example (int& param1,    // reference

int param2,    // value

      float param3 ) // value

Value Parameters

Because value parameters are passed copies of their arguments, anything that has a value may be passed to a value parameter, including constants, variables and complicated expressions.

Examples

PrintLines(3);

PrintLines(lineCount);

PrintLines(2 * abs(10 – someInt));

There must be the same number of arguments in a function call as there are parameters in the function heading. Also, each argument should have the same data type as the parameter in the same position (if not, they are coerced).

Important Note

When a function returns, the contents of its value parameters are destroyed, along with the contents of its local variables.

The difference between value parameters and local variables is that the values of local variables are undefined when a function starts to execute, whereas value parameters are automatically initialized to the values of the corresponding arguments.

Reference Parameters

A function is allowed to inspect and modify the value of a reference parameter.

For a reference parameter, there is only one copy of the information which is used by both the caller and the callee function.

In essence, the same memory location is known by two different names.

Reference Parameter Example -
Activity Problem

Main Print Activity

Get temperature Print “The recommended activity is”

Print activity IF temperature > 85

Get Temperature     Print “swimming.”

Prompt user for temperature ELSE IF temperature > 70

Read temperature       Print “tennis.”

Echo print temperature ELSE IF temperature > 32

      Print “golf.”

ELSE IF temperature > 0

      Print “skiing.”

ELSE

        Print “dancing.”

Activity Program

//*****************************************************

// Activity program

// This program outputs an appropriate activity

// for a given temperature

//*****************************************************

#include <iostream>

using namespace std;

void GetTemp( int& ); // Function prototypes

void PrintActivity( int );

Activity Program

int main()

{

    int temperature;    // The outside temperature

    GetTemp(temperature);              // Function call

    PrintActivity(temperature);        // Function call

    return 0;

}

Activity Program (Continued)

void GetTemp( int& temp )        // Reference parameter

// This function prompts for a temperature to be

// entered, reads the input value into temp and

// echo-prints it

{

    cout << "Enter the outside temperature:" << endl;

    cin >> temp;

    cout << "The current temperature is " << temp

         << endl;

}

Activity Program (Concluded)

void PrintActivity( int temp )       // Value parameter

// Given the value of temp, this function prints a

// message indicating an appropriate activity

{   cout << "The recommended activity is ";

    if (temp > 85)

        cout << "swimming." << endl;

    else if (temp > 70)

        cout << "tennis." << endl;

    else if (temp > 32)

        cout << "golf." << endl;

    else if (temp > 0)

        cout << "skiing." << endl;

    else

        cout << "dancing." << endl;

}

Matching Arguments
with Parameters

Only a variable can be passed as an argument to a reference parameter because a function can assign a new value to the argument.

In contrast, an arbitrarily complicated expression can be passed to a value parameter.

Consider the following function heading:

void DoThis (float val, //Value

int& count) //Reference

Then, the following function calls are valid

DoThis(someFloat, someInt)

DoThis(9.83, intCounter)

DoThis(4.9 * sqrt(y), myInt);

More About Reference Parameters

With reference parameters, the argument and the parameter become synonyms for the same memory location, but only during a particular call to the function.

With value parameters, implicit type coercion occurs if the matched items have different data types. In contrast, with reference parameters, the matched items must have exactly the same type.

Designing Functions

The specification of what a function does and how it is invoked defines its interface.

By hiding a module implementation, we can make changes to it without changing the main function, as long as the interface remains the same.

Designing a function should be divided into two tasks:

Designing the interface

Designing the implementation

Designing a Function Interface

Identify the communication that occurs with the function, list

Incoming values that the function receives from the caller (value)

Outgoing values that the function produces and returns to the caller (reference)

Incoming/outgoing values – values the caller has that the function receives and returns (reference)

Function Interface Examples

void PrintAverage( /* in */ float sum,

                   /* in */ int count)

// Precondition:

//    sum is assigned && count > 0

// Postcondition:

//    The average sum/count has been output on one line

void Calc( /* in */ int alpha,

           /* inout */ int& beta )

// Precondition:

//    alpha and beta are assigned

// Postcondition:

//    beta == beta@entry * 7 - alpha

Problem-Solving Case Study –
Comparison of Furniture-Store Sales

Problem: The regional sales manager wants monthly, department-by-department comparisons, in the form of bar graphs, of two Chippendale stores in town.

Input: Two data files, each containing the following values for each department:

Department ID numer (int)

Number of business days (int)

Daily sales values (several float values)

Output: A bar chart showing total sales for each department.

Problem-Solving Case Study (Continued)

Assumptions: Each file is in order by department ID. Both stores have the same departments.

Functions:

OpenForInput

PrintHeading

GetData

PrintData

Main

Open data files for input

IF either file could not be opened

Terminate program

Print heading

Get data for a Store 1 department

Get data for a Store 2 department

WHILE NOT EOF on file store1 AND NOT EOF on file store2

Print data for the Store 1 department

Print data for the Store 2 department

Get data for a Store 1 department

Get data for a Store 2 department

Print Data (In: deptID,
storeNum, deptSales)

Print deptID

Print storeNum

WHILE deptSales > 250.0

Print a ‘*’

Subtract 500.00 from deptSales

Terminate current output line

Case Study: Implementation

//********************************************************

// Graph program

// This program generates bar graphs of monthly sales

// by department for two Chippendale furniture stores,

// permitting department-by-department comparison of sales

//********************************************************

#include <iostream>

#include <iomanip>    // For setw()

#include <fstream>    // For file I/O

#include <string>     // For string type

using namespace std;

Case Study: Implementation

void GetData( ifstream&, int&, float& );

void OpenForInput( ifstream& );

void PrintData( int, int, float );

void PrintHeading();

int main()

{

    int      deptID1;      // Department ID for Store 1

    int      deptID2;      // Department ID for Store 2

    float    sales1;       // Department sales for Store 1

    float    sales2;       // Department sales for Store 2

    ifstream store1;       // Accounting file for Store 1

    ifstream store2;       // Accounting file for Store 2

Case Study: Implementation

    cout << "For Store 1," << endl;

    OpenForInput(store1);

    cout << "For Store 2," << endl;

    OpenForInput(store2);

    if ( !store1 || !store2 )        // Make sure files

        return 1;                    //   were opened

    PrintHeading();

    GetData(store1, deptID1, sales1);   // Priming reads

    GetData(store2, deptID2, sales2);

Case Study: Implementation

   while (store1 && store2)         // While not EOF...

    {

        cout << endl;

        PrintData(deptID1, 1, sales1); // Process

                                        // Store 1

        PrintData(deptID2, 2, sales2); // Process

                                        // Store 2

        GetData(store1, deptID1, sales1);

        GetData(store2, deptID2, sales2);

    }

    return 0;

}

Case Study: Implementation

void OpenForInput( /* inout */ ifstream& someFile )

// Prompts the user for the name of an input file

// and attempts to open the file

// Postcondition:

//     The user has been prompted for a file name

// && IF the file could not be opened

//         An error message has been printed

// Note:

//     Upon return from this function, the caller must test

//     the stream state to see if the file was successfully

//     opened

Case Study: Implementation

{

    string fileName;    // User-specified file name

    cout << "Input file name: ";

    cin >> fileName;

    someFile.open(fileName.c_str());

    if ( !someFile )

        cout << "** Can't open " << fileName << " **"

             << endl;

}

Case Study: Implementation

void PrintHeading()

// Prints the title for the bar chart, a heading, and the

// numeric scale for the chart. The scale uses one mark

// per $500.

// Postcondition:

//     The heading for the bar chart has been printed

{

   cout << "Bar Graph Comparing Departments of Store #1

            and Store #2" << endl << endl << "Store

            Sales in 1,000s of dollars" << endl

    << "#0         5        10        15        20        25"

    << endl

    << “ |.........|.........|.........|.........|.........|"

    << endl;

}

Case Study: Implementation

void GetData( /* inout */ ifstream& dataFile,

              /* out */   int&      deptID,

              /* out */   float&    deptSales )

// Takes an input accounting file as a parameter, reads

// the department ID number and number of days of sales

// from that file, then reads one sales figure for each

// of those days, computing a total sales figure for

// the month. This figure is returned in deptSales.

// (If input of the department ID fails due to

// end-of-file, deptID and deptSales are undefined.)

// Precondition:

//     dataFile has been successfully opened

// && For each department, the file contains a

//     department ID, number of days, and one sales

//     figure for each day

Case Study: Implementation

// Postcondition:

//     IF input of deptID failed due to end-of-file

//          deptID and deptSales are undefined

//     ELSE

//          The data file reading marker has advanced

//          past one department's data

//       && deptID == department ID number as read from

//          the file

//       && deptSales == sum of the sales values for

//          the department

{

    int   numDays; // Number of business days in the month

    int   day;      // Loop control variable for daily sales

    float sale;     // One day's sales for the department

Case Study: Implementation

     dataFile >> deptID;

    if ( !dataFile )         // Check for EOF

        return 1;            // If so, exit the function

    dataFile >> numDays;

    deptSales = 0.0;

    day = 1;          // Initialize loop control variable

    while (day <= numDays)

    {

        dataFile >> sale;

        deptSales = deptSales + sale;

        day++;            // Update loop control variable

    }

}

Case Study: Implementation

void PrintData( /* in */ int   deptID,

                /* in */ int   storeNum,

                /* in */ float deptSales )

// Prints the department ID number, the store number, and

// a bar graph of the sales for the department. The bar

// graph is printed at a scale of one mark per $500.

// Precondition:

//     deptID contains a valid department number

// && storeNum contains a valid store number

// && 0.0 <= deptSales <= 25000.0

// Postcondition:

//     A line of the bar chart has been printed with one

//     * for each $500 in sales, with remainders over

//     $250 rounded up

// && No stars have been printed for sales <= $250

Case Study Implementation (Concluded)

   {

    cout << setw(12) << "Dept " << deptID << endl;

    cout << setw(3) << storeNum << "     ";

    while (deptSales > 250.0)

    {

        cout << '*' ;        // Print '*' for each $500

        // Update loop control variable

        deptSales = deptSales - 500.0;

    }

    cout << endl;

}


	When a function is executed, it uses the arguments given to it in the function call.
	C++ supports two kinds of parameters
		Value parameters - the function receives a copy of the argument’s value
		Reference parameters (& after data type) - the function receives the memory address of the caller’s argument
	Examples:
	void Example (int& param1, // reference
	int param2, // value
	float param3 ) // value


	Because value parameters are passed copies of their arguments, anything that has a value may be passed to a value parameter, including constants, variables and complicated expressions.
	Examples
		PrintLines(3);
		PrintLines(lineCount);
		PrintLines(2 * abs(10 – someInt));
	There must be the same number of arguments in a function call as there are parameters in the function heading. Also, each argument should have the same data type as the parameter in the same position (if not, they are coerced).


	When a function returns, the contents of its value parameters are destroyed, along with the contents of its local variables.
	The difference between value parameters and local variables is that the values of local variables are undefined when a function starts to execute, whereas value parameters are automatically initialized to the values of the corresponding arguments.


	A function is allowed to inspect and modify the value of a reference parameter.
	For a reference parameter, there is only one copy of the information which is used by both the caller and the callee function.
	In essence, the same memory location is known by two different names.


Main Print Activity
	Get temperature Print “The recommended activity is”
	Print activity IF temperature > 85
Get Temperature Print “swimming.”
	Prompt user for temperature ELSE IF temperature > 70
	Read temperature Print “tennis.”
	Echo print temperature ELSE IF temperature > 32
		Print “golf.”
	ELSE IF temperature > 0
		Print “skiing.”
	ELSE
		Print “dancing.”


	//*****************************************************
	// Activity program
	// This program outputs an appropriate activity
	// for a given temperature
	//*****************************************************

	#include <iostream>
	using namespace std;

	void GetTemp( int& ); // Function prototypes
	void PrintActivity( int );


	int main()
	{
	int temperature; // The outside temperature

	GetTemp(temperature); // Function call
	PrintActivity(temperature); // Function call
	return 0;
	}



	void GetTemp( int& temp ) // Reference parameter

	// This function prompts for a temperature to be
	// entered, reads the input value into temp and
	// echo-prints it

	{
	cout << "Enter the outside temperature:" << endl;
	cin >> temp;
	cout << "The current temperature is " << temp
	<< endl;
	}


	void PrintActivity( int temp ) // Value parameter
	// Given the value of temp, this function prints a
	// message indicating an appropriate activity
	{ cout << "The recommended activity is ";
	if (temp > 85)
	cout << "swimming." << endl;
	else if (temp > 70)
	cout << "tennis." << endl;
	else if (temp > 32)
	cout << "golf." << endl;
	else if (temp > 0)
	cout << "skiing." << endl;
	else
	cout << "dancing." << endl;
	}


Only a variable can be passed as an argument to a reference parameter because a function can assign a new value to the argument.
In contrast, an arbitrarily complicated expression can be passed to a value parameter.
Consider the following function heading:
void DoThis (float val, //Value
		int& count) //Reference
Then, the following function calls are valid
	DoThis(someFloat, someInt)
	DoThis(9.83, intCounter)
	DoThis(4.9 * sqrt(y), myInt);


	With reference parameters, the argument and the parameter become synonyms for the same memory location, but only during a particular call to the function.
	With value parameters, implicit type coercion occurs if the matched items have different data types. In contrast, with reference parameters, the matched items must have exactly the same type.


	The specification of what a function does and how it is invoked defines its interface.
	By hiding a module implementation, we can make changes to it without changing the main function, as long as the interface remains the same.
	Designing a function should be divided into two tasks:
		Designing the interface
		Designing the implementation


	Identify the communication that occurs with the function, list
		Incoming values that the function receives from the caller (value)
		Outgoing values that the function produces and returns to the caller (reference)
		Incoming/outgoing values – values the caller has that the function receives and returns (reference)


	void PrintAverage( /* in */ float sum,
	/* in */ int count)
	// Precondition:
	// sum is assigned && count > 0
	// Postcondition:
	// The average sum/count has been output on one line

	void Calc( /* in */ int alpha,
	/* inout */ int& beta )
	// Precondition:
	// alpha and beta are assigned
	// Postcondition:
	// beta == beta@entry * 7 - alpha


	Problem: The regional sales manager wants monthly, department-by-department comparisons, in the form of bar graphs, of two Chippendale stores in town.
	Input: Two data files, each containing the following values for each department:
		Department ID numer (int)
		Number of business days (int)
		Daily sales values (several float values)
	Output: A bar chart showing total sales for each department.


	Assumptions: Each file is in order by department ID. Both stores have the same departments.
	Functions:
		OpenForInput
		PrintHeading
		GetData
		PrintData


	Open data files for input
	IF either file could not be opened
	Terminate program
	Print heading
	Get data for a Store 1 department
	Get data for a Store 2 department
	WHILE NOT EOF on file store1 AND NOT EOF on file store2
	Print data for the Store 1 department
	Print data for the Store 2 department
	Get data for a Store 1 department
	Get data for a Store 2 department


	Print deptID
	Print storeNum
	WHILE deptSales > 250.0
	Print a ‘*’
	Subtract 500.00 from deptSales
	Terminate current output line


	//********************************************************
	// Graph program
	// This program generates bar graphs of monthly sales
	// by department for two Chippendale furniture stores,
	// permitting department-by-department comparison of sales
	//********************************************************

	#include <iostream>
	#include <iomanip> // For setw()
	#include <fstream> // For file I/O
	#include <string> // For string type

	using namespace std;



	void GetData( ifstream&, int&, float& );
	void OpenForInput( ifstream& );
	void PrintData( int, int, float );
	void PrintHeading();

	int main()
	{
	int deptID1; // Department ID for Store 1
	int deptID2; // Department ID for Store 2
	float sales1; // Department sales for Store 1
	float sales2; // Department sales for Store 2
	ifstream store1; // Accounting file for Store 1
	ifstream store2; // Accounting file for Store 2



	cout << "For Store 1," << endl;
	OpenForInput(store1);
	cout << "For Store 2," << endl;
	OpenForInput(store2);

	if ( !store1 \|\| !store2 ) // Make sure files
	return 1; // were opened
	PrintHeading();

	GetData(store1, deptID1, sales1); // Priming reads
	GetData(store2, deptID2, sales2);


	while (store1 && store2) // While not EOF...
	{
	cout << endl;
	PrintData(deptID1, 1, sales1); // Process
	// Store 1
	PrintData(deptID2, 2, sales2); // Process
	// Store 2
	GetData(store1, deptID1, sales1);
	GetData(store2, deptID2, sales2);
	}
	return 0;
	}


	void OpenForInput( /* inout */ ifstream& someFile )

	// Prompts the user for the name of an input file
	// and attempts to open the file
	// Postcondition:
	// The user has been prompted for a file name
	// && IF the file could not be opened
	// An error message has been printed
	// Note:
	// Upon return from this function, the caller must test
	// the stream state to see if the file was successfully
	// opened


	{
	string fileName; // User-specified file name

	cout << "Input file name: ";
	cin >> fileName;

	someFile.open(fileName.c_str());

	if ( !someFile )
	cout << " Can't open " << fileName << " "
	<< endl;
	}


	void PrintHeading()

	// Prints the title for the bar chart, a heading, and the
	// numeric scale for the chart. The scale uses one mark
	// per $500.
	// Postcondition:
	// The heading for the bar chart has been printed

	{
	cout << "Bar Graph Comparing Departments of Store #1
	and Store #2" << endl << endl << "Store
	Sales in 1,000s of dollars" << endl
	<< "#0 5 10 15 20 25"
	<< endl
	<< “ \|.........\|.........\|.........\|.........\|.........\|"
	<< endl;
	}


	void GetData( /* inout */ ifstream& dataFile,
	/* out */ int& deptID,
	/* out */ float& deptSales )

	// Takes an input accounting file as a parameter, reads
	// the department ID number and number of days of sales
	// from that file, then reads one sales figure for each
	// of those days, computing a total sales figure for
	// the month. This figure is returned in deptSales.
	// (If input of the department ID fails due to
	// end-of-file, deptID and deptSales are undefined.)
	// Precondition:
	// dataFile has been successfully opened
	// && For each department, the file contains a
	// department ID, number of days, and one sales
	// figure for each day


	// Postcondition:
	// IF input of deptID failed due to end-of-file
	// deptID and deptSales are undefined
	// ELSE
	// The data file reading marker has advanced
	// past one department's data
	// && deptID == department ID number as read from
	// the file
	// && deptSales == sum of the sales values for
	// the department

	{
	int numDays; // Number of business days in the month
	int day; // Loop control variable for daily sales
	float sale; // One day's sales for the department


	dataFile >> deptID;
	if ( !dataFile ) // Check for EOF
	return 1; // If so, exit the function

	dataFile >> numDays;
	deptSales = 0.0;
	day = 1; // Initialize loop control variable
	while (day <= numDays)
	{
	dataFile >> sale;
	deptSales = deptSales + sale;
	day++; // Update loop control variable
	}
	}


	void PrintData( /* in */ int deptID,
	/* in */ int storeNum,
	/* in */ float deptSales )

	// Prints the department ID number, the store number, and
	// a bar graph of the sales for the department. The bar
	// graph is printed at a scale of one mark per $500.
	// Precondition:
	// deptID contains a valid department number
	// && storeNum contains a valid store number
	// && 0.0 <= deptSales <= 25000.0
	// Postcondition:
	// A line of the bar chart has been printed with one
	// * for each $500 in sales, with remainders over
	// $250 rounded up
	// && No stars have been printed for sales <= $250


	{
	cout << setw(12) << "Dept " << deptID << endl;
	cout << setw(3) << storeNum << " ";

	while (deptSales > 250.0)
	{
	cout << '' ; // Print '' for each $500

	// Update loop control variable
	deptSales = deptSales - 500.0;

	}
	cout << endl;
	}