Value-Returning Functions

	A call to a value-returning function is part of an expression.
	A value-returning function is used when there is only one result returned by a function and that result is to be used directly in an expression.
	A value-returning function returns one value, not through a parameter but by means of a return statement.

Value-Returning Function Example

	int Power (/* in */ int x,     // Base number
	/* in */ int n)    // Power to raise base to
	{
	int result;   // Holds intermediate powers of x
	result = 1;
	while (n > 0)
	{
	result = result * x;
	n--;
	}
	return result;
	}
	Usage
	y = Power(5, 3)

Another Value-Returning
Function Example

	#include <iostream>
	using namespace std;
	int Min (int val1, int val2, int val3);
	int main()
	{
	int num1;
	int num2;
	int num3;
	int min_num;

Another Value-Returning
Function Example

	cout << "Please enter an integer value for num1: "
	<< endl;
	cin >> num1;
	cout << "Please enter an integer value for num2: ”
	<< endl;
	cin >> num2;
	cout << "Please enter an integer value for num3: "
	<< endl;
	cin >> num3;
	min_num = Min(num1, num2, num3);
	cout << "The minimum number of the three entered is ”
	<< min_num << endl;
	}

Another Value-Returning
Function Example

	int Min (int val1, int val2, int val3)
	{
	int result;
	result = val1;
	if (val2 < result)
	result = val2;
	if (val3 < result)
	result = val3;
	return result;
	}

Revisiting the Power Function

	#include <iostream>
	#include <cmath>
	using namespace std;
	float power (int base, int exponent);
	int main()
	{
	float result;
	int base;
	int exponent;
	cout << "Please enter integer base value " << endl;
	cin >> base;

Revisiting the Power Function

	cout << "Please enter integer exponent value "
	<< endl;
	cin >> exponent;
	result = power(base, exponent);
	cout << base << " raised to the " << exponent
	<< " power is " << result << endl;
	}

Revisiting the Power Function

	float power (int base, int exponent)
	{
	int i =1;
	float result = 1.0;
	while (i <= abs(exponent))
	{
	result = result * base;
	i++;
	}
	if (exponent < 0)
	result = 1.0/result;
	return result;
	}

Problem-Solving Case Study
(Starship Weight and Balance)

	Problem: Write a program that determines the weight and center of gravity of a new plane, based on the number of crew members and passengers as well as the weight of the baggage, closet contents, and fuel.
	Input: Number of crew members, number of passengers, weight of closet contents, baggage weight, fuel in gallons.
	Output: Total weight, center of gravity.

Starship Weight and Balance (continued)

	Discussion: Sum all of the weights, using the standard average weight of a person of 170 pounds, and the fact that a gallon of fuel weighs 6.7 pounds.
		totalWeight = emptyWeight + (crew + passengers) * 170 + baggage + closet + fuel * 6.7
		centerofGravity = (emptyMoment + crewMoment + passengerMoment + cargoMoment + fuelMoment)/totalWeight

Main (Level 0)

	Get data
	Set totalWt = EMPTY_WEIGHT + (passengers + crew) * 170 + baggage + closet + fuel * 6.7
	Set centerof Gravity = (CrewMoment(crew) + PassengerMoment(passengers) + CargoMoment(closet, baggage) + FuelMoment(fuel) + EMPTY _MOMENT)/totalWt
	Print totalWt, centerof Gravity
	Print warning

Level 1 Functional Decomposition

	Get Data (Out: crew, passengers, closet, baggage, fuel)
	Prompt for numer of crew, number of passengers, weight in closet  and baggage compartments, and gallons of fuel
	Read crew, passengers, closet, baggage, fuel
	Echo print the input
	CrewMoment (In: crew) Out Function Value
	Return crew * 170 * 143
	Cargo Moment (In: closet, baggage) Out: Function value
	Return closet * 182 + baggage * 386

Level 1 Functional
Decomposition (continued)

	Passenger Moment (In: passengers)
	Set moment = 0.0
	IF passengers > 6
	Add (passengers – 6) * 170 * 341 to moment
	Set passengers = 6
	IF passengers > 4
	Add (passengers – 4) * 170 * 295 to moment
	Set passengers = 4
	IF passengers > 2
	Add (passengers – 2) * 170 * 219 to moment
	Set passengers = 2
	IF passengers > 0
	Add passengers * 170 * 265 to moment
	Return moment

Level 1 Functional
Decomposition (concluded)

	Fuel Moment (In: fuel) Out: Function value
	Set moment = 0.0
	Set fuelWt = fuel * 6.7
	IF fuel < 60
	Set fuelDistance = fuel * 314.6
	ELSE IF fuel < 361
	Set fuelDistance = 305.8 + (-0.01233 * (fuel – 60))
	ELSE IF fuel < 521
	Set fuelDistance = 303.0 + (0.12500 * (fuel – 361))
	ELSE IF fuel < 521
	Set fuelDistance = 323.0 + (-0.04444 * (fuel – 521))
	Return fuelDistance * fuelWt

Module Structure Chart

Case Study Implementation (abridged)

	const float PERSON_WT = 170.0;        // Average person weighs
	//   170 lbs.
	const float LBS_PER_GAL = 6.7;        // Jet-A weighs 6.7 lbs.
	//   per gal.
	const float EMPTY_WEIGHT = 9887.0;    // Standard empty weight
	const float EMPTY_MOMENT = 3153953.0; // Standard empty moment
	float CargoMoment( int, int );
	float CrewMoment( int );
	float FuelMoment( int );
	void  GetData( int&, int&, int&, int&, int& );
	float PassengerMoment( int );
	void  PrintWarning();

Case Study Implementation

	int main()
	{
	int   crew;          // Number of crew on board (1 or 2)
	int   passengers;    // Number of passengers (0 through 8)
	int   closet;        // Weight in closet (160 lbs. Max.)
	int   baggage;       // Weight of baggage (525 lbs. max.)
	int   fuel;          // Gallons of fuel (10 - 565 gals.)
	float totalWt;       // Total weight of loaded Starship
	float centerOfGravity; // Center of gravity of loaded Ship
	GetData(crew, passengers, closet, baggage, fuel);
	totalWt = EMPTY_WEIGHT + float(passengers + crew) * PERSON_WT + float(baggage + closet) + float(fuel) * LBS_PER_GAL;
	centerOfGravity = (CrewMoment(crew) + PassengerMoment (passengers) + CargoMoment(closet, baggage) + FuelMoment(fuel) + EMPTY_MOMENT) / totalWt;

Case Study Implementation

	void GetData( /*out*/ int& crew,
	/*out*/ int& passengers,
	/*out*/ int& closet,
	/*out*/ int& baggage,
	/*out*/ int& fuel       )
	{
	cout << "Enter the number of crew members." << endl;
	cin >> crew;
	cout << "Enter the number of passengers." << endl;
	cin >> passengers;
	cout << "Enter the closet cargo weight, in pounds."
	<< endl;
	cin >> closet;
	cout << "Enter the weight, in pounds, of cargo in
	the aft baggage compartment." << endl;
	cin >> baggage;
	cout << "Enter the number of U.S. gallons of fuel"
	<< endl << " loaded.“;
	cin >> fuel;
	}

Case Study Implementation

	float CrewMoment( /*in*/ int crew )  // Number of crew members
	// Computes the crew moment arm in inch-pounds from the number of
	// crew members.  Global constant PERSON_WT is used as the weight
	// of each crew member
	// Precondition:
	//    crew == 1  OR  crew == 2
	// Postcondition:
	//    Function value == Crew moment arm, based on the crew
	//    parameter
	{
	const float CREW_DISTANCE = 143.0;  // Distance to crew seats
	//   from front
	return float(crew) * PERSON_WT * CREW_DISTANCE;
	}

Case Study Implementation

	float PassengerMoment( /*in*/ int passengers )
	{
	const float ROW1_DIST = 219.0;  // Distance to row 1 seats
	const float ROW2_DIST = 265.0;  // Distance to row 2 seats
	const float ROW3_DIST = 295.0;  // Distance to row 3 seats
	const float ROW4_DIST = 341.0;  // Distance to row 4 seats
	float moment = 0.0;             // Running total of moment
	if (passengers > 6)             // For passengers 7 and 8
	{
	moment = moment + float (passengers - 6) *
	PERSON_WT * ROW4_DIST;
	passengers = 6;                    // 6 remain
	}
	if (passengers > 4)              // For passengers 5 and 6
	.
	.
	.
	return moment;
	}

Case Study Implementation

	float CargoMoment( /*in*/ int closet,     // Weight in closet
	/*in*/ int baggage )   // Weight of baggage
	// Computes the total moment arm for cargo loaded into the
	// front closet and aft baggage compartment
	// Precondition:
	//     0 <= closet <= 160  &&  0 <= baggage <= 525
	// Postcondition:
	//     Function value == Cargo moment arm, based on the closet
	//     and baggage parameters
	{
	const float CLOSET_DIST = 182.0;    // Distance to closet
	const float BAGGAGE_DIST = 386.0;   // Distance to baggage
	return float(closet) * CLOSET_DIST +
	float(baggage) * BAGGAGE_DIST;
	}

Case Study Implementation

	float FuelMoment( /* in */ int fuel )    // Fuel in gallons
	{
	float fuelWt;           // Weight of fuel in pounds
	float fuelDistance;     // Distance from front of plane
	fuelWt = float(fuel) * LBS_PER_GAL;
	if (fuel < 60)
	fuelDistance = float(fuel) * 314.6;
	else if (fuel < 361)
	fuelDistance = 305.8 + (-0.01233 * float(fuel - 60));
	else if (fuel < 521)
	fuelDistance = 303.0 + ( 0.12500 * float(fuel - 361));
	else
	fuelDistance = 323.0 + (-0.04444 * float(fuel - 521));
	return fuelDistance * fuelWt;
	}

Testing & Debugging – Stubs

	A stub is has the same name and interface as a function that actually would be called by the part of the program being tested, but it is usually much simpler
	Example
		void GetYear (/*inout*/ ifstream& dataIn,   // Input file
		/*out*/ string& year)  //Four digits of year
		// Stub for GetYear function in the ConvertDates program
		{
		cout << “Get Year was called here. Returning \”1949\”.”
		<< endl;
		year = “1948”;
		}

Testing & Debugging - Drivers

	A driver is a simple main function which cals a function being tested and permits direct control of testing.
	Example
		float FuelMoment (int);
		int main()
		{
		int testVal;
		cout << “Fuel moment for gallons from 10 – 565 in steps”
		<< “of 15:” << endl;
		testVal = 10;
		while (testVal <= 565)
		{
		cout << FuelMoment(testVal) << endl;
		testVal = testVal + 15;
		}
		}