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Void Functions

In the previous examples, all the functions returned a value/answer (output) as shown in the right side of the black box
However, not all functions return a value. A void function is a function that does not return a value

PrintRectangle() example

The following function prints a rectangle based on the given rows and cols

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

Calling the function:
```
PrintRectangle(3, 4);
```
```
****
****
****
```
Note that the function begins with void and does not have a return statement
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 = PrintRectangle(3, 4);
```

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

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

Calling the modified function:
```
PrintRectangle(2, 5, '?');
```
```
?????
?????
```

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...
}

int main() {
	// main's x and a are different than MyFunc's x and a
	int a;
	int x;
	
	// etc...
}
```
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
```
// PROBLEM: what does the function return if x >= 10?
int DoSomething(int x) {
	if (x < 10) {
		return 5;
	}
}
```

Structure charts

Programmers sometimes use hierarchical structure charts to show the relationship between multiple functions
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

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;
}

int main() {
	B();
	C(2, 3);
	return 0;
}

Here is the structure chart:

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