Void Functions

1. In the previous examples, all the functions returned a value/answer (output) as shown in the right side of the black box
   
   
2. However, not all functions return a value. Functions that do not return a value are called void functions
   
   
3. DrawRectangle example
   1. The following function displays a rectangle based on the given rows and cols

      void DrawRectangle(int rows, int cols)
      {
      	for (int r = 1; r <= rows; r++)
      	{
      		for (int c = 1; c <= cols; c++)
      			cout << "*";
      		cout << endl;
      	}
      }
      

   2. Calling the function:

      DrawRectangle(3, 4);
      

      ****
      ****
      ****
      

   3. Note that the function begins with void and does not have a return statement
   4. The call to the function does not result in an answer, so we do not call it like functions that do return an answer

      // This is WRONG... the function doesn't return back an answer!
      int ans = DrawRectangle(3, 4);
      

   5. Modify the function so it takes a third parameter: the character to use to display the box

      void DrawRectangle(int rows, int cols, char box)
      {
      	for (int r = 1; r <= rows; r++)
      	{
      		for (int c = 1; c <= cols; c++)
      			cout << box;
      		cout << endl;
      	}
      }
      

   6. Calling the modified function:

      DrawRectangle(2, 5, '?');
      

      ?????
      ?????
      

4. General notes on functions
   1. A program can have any number of functions
   2. A function can call any other functions
   3. A function can have any number of parameters
   4. Function parameters are just variables with values that come from the function call
   5. Function arguments must match function parameters: SNOT rule
      1. Same...
      2. Number - If a function has 3 parameters, there has to be 3 arguments
      3. Order - The 1st argument is copied into the 1st parameter, the 2nd argument is copied into the 2nd parameter, etc.
      4. Type - If the 1st parameter is a char and the 2nd is an int, the 1st argument should be a char and the 2nd argument an int
   6. Functions do not share variables

      void MyFunc(int x)
      {
      	int a;
      	
      	// etc...
      }
      
      void main()
      {
      	// main's x and a are different than MyFunc's x and a
      	int x, a;
      }
      

   7. void functions don't return an answer, they just do something like output to the console window; non-void functions always return a single answer
   8. void functions are called differently than non-void functions

      // Calling a void function
      DoSomething(5);
      
      // Calling a non-void function
      cout << GiveAnAnswer(5);
      

   9. void functions do not have a return statement but non-void functions do
   10. Every path through a non-void function must return a value

       int DoSomething(int x)
       {
       	if (x < 10)
       		return 5;
       		
       	// oh oh... what does the function return if x >= 10?
       }
       

5. Structure charts
   1. Programmers sometimes use hierarchical structure charts to show the relationship between multiple functions
   2. A structure chart shows who calls a function, but it doesn't reveal how many times a function is called or what is passed to a function
   3. Given the following program:

      #include <iostream> using namespace std; void A(int x) { cout << x; } void B() { A(5); } void C(int x, int y) { cout << x + y; } void main() { B(); C(2, 3); }

      Here is the structure chart:

   4. Note that function A was defined before function B which was necessary for it to compile properly. If they were switched, the compiler would complain when it reached this line:

      A(5);			
      

      because the compiler would not yet know about function A. Therefore, functions need to be defined bottom-up from the ordering in the structure chart