Chapter 5

Functions For All Subtasks

Void functions

• Do not return a value.• Keyword void is used as the return type in the

function prototype to show it returns nothing.• Syntax:

void Function_Name (Parameter_List);

When to use void functions• Output

void show_results (double fDegree, double cDegree){

cout.setf(ios::fixed)cout.setf(ios::showpoint);cout.precision(1);cout <<fDegree<<“ degrees Fahrenheit is equivalent to \n” <<cDegree<<“ degrees Celcius.\n”;return; //optional

}• instructions to user(which has no input variables)

void instruct (){

cout<< “This program computes the volume of a “ <<”right circular cone\n”;

return; // This is optional}

• when we want to change more than 1 variable (call-by reference)

return Statements in void Functions

• Syntax:return;

• Is optional• Usually used when need to break out of the

function.void UnitPrice (double total, int units){

if (!units) // units == 0return;

cout << “The unit price is ” << total/units << endl;}

Call-by-Reference

• When we want to make changes to variables in the caller program inside a function. – Call-by-value sends in a copy of a value stored. It

cannot change the value of variable in the caller program.

• If we need to make changes to more than 1 value.– A function can only return one value. – We need to be able to go to the memory location and

change the value there for a permanent change.

Implementing Call-by-reference• To change the stored value, we need to pass in the address. • Use & to designate “the address of” • & is shown only in function prototype/definition, not in

function call.• Must pass in variables so computer knows address and can

change. Cannot pass in constants.• Formal Parameters : Placeholders in the function code.

These are not declared and don’t represent any memory location.

• Arguments: Used in the function call. An actual value or location in memory where a value is stored. This information is passed into the formal parameters in the function code and used in the function computations.

Call-by-reference Example#include <iostream>using namespace std;void swap (int&, int&); // Function prototypeint main (){

int x, y; // Declare two variables to hold integerscout << "Enter two integers .. seperated by a space: "; // User promptcin >> x >> y;cout << "The initial values of two numbers: x=" << x << “, y=" << y << “.\n“;

swap (x, y); // Function callcout << "The current values of two numbers: x=" << x << “, y=" << y << “.\n“;

return 0; // This program requires a return integer value}

void swap_values (int& first_num, int& second_num) // header{ // body

first_num = second_num;second_num = first_num;

}

The initial values of two numbers: x=3, y=4

The current values of two numbers: x=4, y=4

??

?

?

xy

firstsecond

3

4

2000

1000

1000

2000

Call-by-reference Example#include <iostream>using namespace std;void swap (int&, int&); // Function prototypesint main (){

int x, y; // Declare two variables to hold integerscout << "Enter two integers .. seperated by a space: "; // User promptcin >> x >> y;cout << "The initial values of two numbers: x=" << x << “ y=" << y << “.\n“;

swap (x, y); // Function callcout << "The current values of two numbers: x=" << x << “ y=" << y << “.\n“;

return 0; // This program requires a return integer value}

void swap_values (int& first_num, int& second_num) // header{ //body

int temp = first_num ;first_num = second_num;second_num = temp;

}

Mixed parameter lists

• We can have both call-by-reference and call-by-value within a function.

void SalaryCalculate(float rate, float& salary, int years);

• You need to decide whether the function will be changing the data. If it is changing the data, the data MUST be call-by-reference.

Call-by-reference vs. Call-by-value

• Call-by-value (IN)– variable information is to be used, not changed– variable is to be temporarily changed– Ex: double SalesTax (double price, double rate) ;

• Call-by-reference (OUT)– Variable has no meaningful value goes in and valid value comes out.– Ex: void InputChoice (double& ans);

• Call-by-reference (IN/OUT)– Variable has meaningful value but it is to be altered inside function.– Ex: void SalaryCalculate(float rate, float& salary, int years);

Pre and Post conditions

• Precondition: What must be true of data before a function is called (requirements). Should be validated before the function is called to avoid erroneous results.

• Postcondition: What is true of data after a function is called.

Pre and Post conditions (examples)double ConvertCoins (int number, double coinValue);// Pre-Cond: Get number of coins of 1 type and their value in dollar.// Post-Cond: The total amount of money converted to dollars and returned.int ConvertCoins (int number, int coinValue);// Pre-Cond: Get number of coins of 1 type and their value in cents.// Post-Cond: The total amount of money converted to cents and returned.

void ConvertCoins (int number, double coinValue, double& total);// Pre-Cond: Get number of coins of 1 type and their value in dollar.// Post-Cond: The total amount of money converted to dollars.void ConvertCoins (int number, int coinValue, int& total);// Pre-Cond: Get number of coins of 1 type and their value in cents.// Post-Cond: The total amount of money converted to cents.

Testing and Debugging Functions

• Create the Structure Chart with data flow to define function interfaces

• Drivers: The caller functions calling other functions.

• Stubs: Dummy functions. They usually consist of a message saying the function was called. They can return a constant value (if needed).

Structure Chart • Structure Chart represents the Driver program.• Each subtask is a function • Data flow tells what the parameters for the function should be• Data flow tells whether the parameters are passed by value or reference.

PaymentCalculation

PayStubDisplay

Main

Get InitialSalary

salary

salaryPayOutsalary

salaryPayOut

Structure Chart • Structure Chart represents the Driver program.• Each subtask is a function • Data flow tells what the parameters for the function should be• Data flow tells whether the parameters are passed by value or reference.

PaymentCalculation

PayStubDisplay

Main

Get InitialSalary

salary

salary

salary

salary

PayOut

Stub Examplesvoid main () // This is real code{

double salary, pay;salary = GetSalary ();PaymentCalculate(salary, pay);

}double GetSalary () // This is a stub{

cout << “Inside GetSalary() \n”;return 1000.0;

}void PaymentCalculate (double salary, double& payout){ // This is a stub

cout << “Inside PaymentCalculate() \n”;payout = 750.0;PayStubDisplay(salary, payout);

}void PayStubDisplay (double salary, double payout){ // This is a stub

cout << “Inside PayStubDisplay() \n”;}

Output:

Inside GetSalary()Inside PaymentCalculate()Inside PayStubDisplay()

Driver Examplesvoid main () // This is a driver{

double pay;PaymentCalculate(500, pay);PaymentCalculate(1000, pay);PaymentCalculate(1500, pay);

}void PaymentCalculate (double salary, double& payout) // This is real code{

if (salary < 1000.0)payout = salary * 0.9;

else if (salary >= 1000.0 && salary < 1500.0 )payout = salary * 0.8;

elsepayout = salary * 0.7;

PayStubDisplay (salary, payout);}void PayStubDisplay (double salary, double payout) // This is a stub{

cout << “Inside PayStubDisplay() with salary=$” << salary << “ and payout=$” << payout \n”;}

Output:

Inside PayStubDisplay() with salary=$500 and payout=$450.00Inside PayStubDisplay() with salary=$1000 and payout=$800.00Inside PayStubDisplay() with salary=$1500 and payout=$1050.00

General Debugging Techniques• Keep an Open Mind:

Don’t assume code is correct/incorrect.

• Change only code that you believe to be incorrect.Don’t randomly change things without cause

• Show your program to another person to help you debug.Fresh mind has new looks to your program.

• Check Common Errors:uninitialized variablesOff-by-one-errorsExceed data boundaryAutomatic type conversion/Integer divide rather than double divide. Using = instead of ==

• Localize the error:Use cout statements or debugger to trace code behavior.