Objective #1: Be able to build structured programs that are divided into functions.

• Every C++ program must have a main function. However, it is wise to develop an algorithm for any given project that divides large tasks into smaller ones. Each of the smaller tasks (let's say, subtasks) can be coded as C++ functions that go together with the main function to make up a structured program.
• Using functions makes it easier to code programs.
• Using functions also makes it easier to debug large programs.
• Using functions makes it easier to maintain and upgrade a program after the first version has been finished.
• Since functions are meant to perform single and well-defined tasks, they may be reused in other programs or, even, by other programmers
• Functions are given names, just like variables. However, a function name must be followed by parentheses. C++ programmers generally add functions to the bottom of a C++ program, that is after the main function. While this is not a requirement of the compiler, it makes your code easier to read and follow by fellow programmers. If a function's body is placed after the main function, then its function prototype (also called the function declaration) must be placed at the top of the program, above the main function. The function prototype "warns" the compiler about the eventual use of the prototyped function. An error will definitely result if you call a function from within the main function that has not been prototyped.

  Example: The following program includes a function prototype which "declares" to the program the existence of the function printMyName which is detailed below the main function. The main function "calls" the printMyName and sends the parameter userInput which has a value of 3. printMyName receives the value and stores it in the parameter numOfTimes. The function then prints out the string "Mr. Minich" 3 times using a for loop.

  

Objective #2: Understand what is meant by the phrase "scope of variables."

• Variables are either global or local in nature. Global variables are ones that are declared outside and above the main function. They can be used in any function throughout the program. It is not wise to use global variables any more than you have to. One small error (for example, miscalculating a value) could lead to multiple, hard-to-find errors in large programs. Local variables are ones that are declared inside of a function, including main. They cannot be used or referred to in other functions.
• In order to strengthen a function's reusability, you should avoid the use of global variables and try to use local variables instead.
• The scope of a variable is the area in which it can be legally referenced. A global variable's scope is the whole program. A local variable's scope is the function in which it is declared.
• In the example program above (with the function printMyName), the variable userInput is a local variable which belongs to the main function. The variable i is a local variable to the function printMyName. The parameter numOfTimes isn't really considered to be a variable. It is a parameter technically. See the notes below for more details on parameters.

Objective #3: Be able to use functions in a C++ program.

• A function must be given a valid identifier as a name. A good name for a function that computes sales tax would be computeSalesTax. Like variables, I prefer that you name functions by capitalizing multiple words except for the first letter of the identifier.
• The first line of a function is called the function header. Before the name of the function you must specify a "return type." The return type is the data type of the value that is returned by the function to the calling function (usually the main function in this course).
• After the name of the function in the function header, you must include a parameter list. The parameter list is a pair of parentheses with 0 or more parameters listed. Each parameter is passed to the function from the calling function. Immediately preceding each parameter, you must identify the data type of that parameter. If the function does not return any value, you must type the word void as the return type. An example of a function header is:
  
  double computeTax(double myPrice, double myTaxRate)
  
  where the name of the function is computeTax, the return type is double, and the passed parameters are myPrice and myTaxRate.
• Following the function header comes the function body, which is enclosed in a pair of curly braces. If the function is not a void function, there must be a return statement at the end of the function. The return statement returns a computed value to the calling function. Example:
  
  double computeTax(double myPrice, double myTaxRate)
  {
     return (myPrice * myTaxRate);
  }// end of computeTax
  
  In the example the expression (myPrice * myTaxRate) is first computed and the resulting value is returned to the calling function. Note that the parentheses in around the expression in the return statement are optional. However, you could also use a local variable to first store the computed amount of tax and then return the same value as in:
  
  double computeTax(double myPrice, double myTaxRate)
  {
     double result = 0.0;
  
     result = myPrice * myTaxRate;
  
     return result;
  }// end of computeTax
  
  Some would argue that this function is slightly more readable however it does require an additional 8 bytes of memory for the local variable result. Since result is declared within the function, it cannot be used or referenced in any other function of the C++ program.
• You should avoid using cin or cout statements within functions. Unless the whole purpose of a function is to obtain (and or validate) a user's input, you should not use a cin statement within a function (besides main). Unless the whole purpose of a function is to display something such as a menu, you should avoid using a cout statement within a function (besides main). If you did create a function with the sole purpose of displaying a menu, it would likely be a void function.
  

Objective #4: Understand how data is passed to functions.

• Data is passed to functions as parameters (sometimes referred to as actual parameters or arguments). When a function is "called" by the main function (or another function), one or more parameters may be passed to the function. On the receiving end, the function accepts these parameters (technically, here they are called formal parameters). The variable names of the parameters (that is, actual parameters) from the "calling" function do not have to be the same as the names of the formal parameters in the "called" function. But, the datatypes of the arguments and the parameters should match exactly. An error could occur if the data types do not match.
  
  
• We type return 0; as the last statement in the main functions of all the programs that we write in this C++ course. This returns a message (of "0") to the operating system indicating that our C++ program ran successfully. A returned nonzero value is often used by programmers to indicate that an error occurred during the program's execution.
  
  
• Often, though, you want your function to return a computed value to the calling function. This value is used in the exact statement that the function was called. For example,

  fahrenheit = celsiusToFahrenheit(celsius);

  sends the argument, celsius, to the function, celsiusToFahrenheit, where it is used to determine the equivalent Fahrenheit temperature. Then, the final value is returned from within the function to the main function where it is assigned to the fahrenheit variable.
  

• However, in this call statement, instead of storing the value returned by the function in a variable, the returned value is being displayed using cout
  
  cout << celsiusToFahrenheit(celsius) << endl;
  
  
• "Passing by value" is the preferred method of passing data to a function. You simply use a variable name, an actual numeric literal, or an expression in the parentheses of the call statement. This method is the safest because it does not actually change the value of the argument in the calling function.
  
  In the example below, the argument named price is passed by value to the function addTax.
  
  #include <iostream>
  using namespace std;
  
  double addTax(double);
  
  int main()
  {
     double price = 10.00;
     double finalPrice = 0.0;
  
     finalPrice = addTax(price);
  
     cout << "The final price with tax is " << finalPrice << endl;
     return 0;
  }// end of main
  
  double addTax(double initialPrice)
  {
     return (initialPrice + initialPrice * 0.06);
  }// end of addTax
  

Objective #5: Be able to use void functions when appropriate.

• A void function is simply a function that does not return a value. The function header and the function prototype for a void function begins with the word void.
• Often a void function simply consists of cout statements.
• An example of a void function might be one that is used to print a menu such as:
  
  void printMenu()
  {
     cout << "1. apples" << endl;
     cout << "2. bananas" << endl;
     cout << "3. carrots" << endl;
     cout << "4. exit" << endl;
  }// end of printMenu
  
  Notice that a void function does not have a return statement. Therefore the keyword void is typed at the beginning of the function header as the return type of the function. It would be efficient to use this function in a program that must display a menu many times. By calling the function over and over again, you are cutting down on the amount of code (i.e. cout statements) that must be placed in the main function. To call a void function you would use the call statement that is found in the short program below
  
  #include <iostream>
  using namespace std;
  
  int main()
  {
     int userChoice = 0;
  
     while (userChoice != 4)
     {
        printMenu();
        cout << "Enter choice: ";
        cin >> userChoice;
     }
  
     return 0;
  }// end of main

Objective #6: Learn how to use the library functions that are included with the compiler.

• There are many functions available to C++ programmers which were written by other programmers. These functions are known as library functions (and sometimes called predefined functions.) You can use the #include compiler directive at the top of your program to take advantage of these functions. The source code for those functions does not need to be included within your program.
  
• There are many library functions in C++. You should be familiar with the library functions sqrt, pow, abs, ceil, floor , and rand. You must add #include <cmath> to the top of your program if you use the functions sqrt, pow, ceil, & floor. You should add #include <cstdlib> to the top of your program if you use the functions abs and rand.
  
  • The sqrt function can be used to compute a square root. The following statement displays the square root of 13:
    
    cout << sqrt(13) << endl;
    
  • The pow function can be used to calculate a number to a power. The statement
    
    cout << pow(3, 2) << endl;
    
    displays the value 9.
    
  • The abs function is used to calculate the absolute value of an integer. The statement
    
    cout << abs(-9) << endl;
    
    displays the positive value 9.
    
  • The ceil function returns the next integer greater than it's parameter. The statement
    
    cout << ceil(9.2) << endl;
    
    displays the value 10. While
    
    cout << ceil(-3.9) << endl;
    
    displays the value -3.
    
    
  • The floor function returns the next smallest integer than it's parameter. The statement
    
    cout << floor(9.9) << endl;
    
    displays the value 9. While
    
    cout << floor(-3.9) << endl;
    
    displays the value -4.
    
    
  • The rand function is used to generate a random value as in
    
    cout << "A random number is " << rand() << endl;

We will not be studying the following material in this course.

Objective #7: Understand how to pass by reference.

• "Passing by reference" is to be used when you want the function to actually and permanently change the values of one or more variables. You must use an ampersand (&) before the formal parameter names in the function header to denote passing by reference. We will not be studying passing by reference in this course.

• "Passing by reference" is to be used when you want the function to actually and permanently change the values of one or more variables. This method is dangerous but beneficial sometimes since it allows you to affect the values of arguments in your calling function. You would purposefully pass by reference if you want to return 2 or more values to the calling function. Since it is not possible in C++ to execute two return statements within a function and since it is not possible to return two values in the same return statement (i.e. return myResult, myOtherResult is illegal), passing by reference can allow you to modify two or more variables in the calling function (i.e. main). Also, if you need to pass a really large variable or object, it saves memory to pass by reference. Since a copy of the variable is not being made when you pass by reference, less memory overhead is required. For int, double, and char variables, passing by reference to save memory is not really necessary. However, if you were to pass a large "object" such as a piece of clipart or a whole file, you would intentionally pass by reference.
  
  You must use an ampersand (&) before the formal parameter names in the function header to denote passing by reference.
  
  In the example below, the argument named price is passed by reference to the function addTax.
  
  #include <iostream>
  using namespace std;
  
  void addTax(double & incomingPrice);
  
  int main()
  {
     double price = 10.00;
  
     addTax(price);
     cout << "The final price with tax is " << price << endl;
  
     return 0;
  }// end of main
  
  void addTax(double & incomingPrice)
  {
     incomingPrice = incomingPrice + incomingPrice * 0.06;
  }// end of addTax