1. Chapter 3 Variables, Calculations, and Colors
Starting Out with
Games & Graphics in C++
Tony Gaddis

2. 3.1 Introduction Programs almost always work with data of some type.
For example, we have seen programs that use:
XY coordinate data to draw shapes on the screen
Strings of characters to display text on the screen
In this chapter, we will:
Take a closer look at how numerical data can be used in a program
Discuss how a program can:
Store data in memory
Perform calculations
Retrieve data from functions in the Dark GDK library
Go beyond the simple world of black and white and discuss how to draw in color

3. 3.2 Literal Data
Concept:
A literal is an item of data that is typed into a program’s code.

4. 3.2 Literal Data Literal Data Numeric Literals String Literals
Simplest form of data
Typed into the program’s code
Used to represent known values
Numeric Literals
For example:
dbCircle(319, 239, 150);
String Literals
dbPrint(“Hello World!”);

5. 3.3 Variables
Concept:
A variable is a storage location in memory that is represented by a name.

6. 3.3 Variables Variable Storage location in memory
Represented by a name
For example:
Variable named score used to hold a player’s score in memory
In C++, variable must be declared with
Data type
name

7. 3.3 Variables Data Types int float double DWORD Whole numbers
For example, 6, -56, 9, and 45
float
Floating-point numbers
For example, 8.9, , and 3.0
double
Twice the precision of float
DWORD
Unsigned, 32 bits, used for Dark GDK colors

8. 3.3 Variables Variable Names Must be First character must be
One word
No spaces
First character must be
Letter a – z, A – Z, or underscore (_)
After first character
Letters a – z, A – Z, numbers 0 – 9, or underscore(_)
Uppercase and lowercase characters are distinct
LineLength is not the same as linelength

9. 3.3 Variables Declaring a Variable Specify data type and name
For example:
int centerX;
float distance;
Assigning Values
centerX = 319;
distance = 159.9;
319 = centerX; // ERROR!
Initializing
For example
int centerX = 319;
int centerX = 319, centerY = 239, radius = 150;

10. 3.3 Variables Uninitialized Variables
A variable that has been declared but not assigned a value
If the variable is used without initializing first, unpredictable results occur, because “garbage” is stored in this variable (whatever was previously stored in that memory location)

11. 3.3 Variables Floating-Point Truncation
If you store a floating-point variable into an integer variable, the decimal places will be dropped
For example:
int size;
size = 12.2;
However, it is safe to store an integer into a floating-point or double variable
float grade;
grade = 90;

12. 3.4 Calculations Concept:
You can use math operators to perform simple calculations. Math expressions can be written using math operators and parentheses as grouping symbols. The result of a math expression can be assigned to a variable.

13. 3.4 Calculations Math Operators Addition Subtraction Multiplication
Adds two numbers
(+)
Subtraction
Subtracts one number from another
(-)
Multiplication
Multiplies one number by another
(*)
Division
Divides one number by another and give the quotient
(/)
Modulus
Divides on integer by another and gives the remainder
(%)

14. 3.4 Calculations Order of Operations Parentheses (always first)
From left to right
Multiplications, divisions, or modulus operations(1st)
Additions or subtractions(2nd)

15. 3.4 Calculations Integer Division
When you divide an integer by an integer in C++
The result is always given as an integer
If the result has a fractional part, it will be truncated

16. 3.4 Calculations Integer Division
The last statement divides the value of length by 2 and assigns the result to half
The result of the division will be truncated, giving the value 37

17. 3.4 Calculations Combined Assignment Operators
Do not require the programmer to type the variable name twice
Give a clear indication of what is happing in the statement

18. 3.5 Getting Values from Functions
Concept:
A value-returning function returns a value back to the statement that called it.

19. 3.5 Getting Values from Functions
Value-returning functions
Performs some operation
Returns a value
Can be:
Assigned to a variable
Used in mathematical expressions
Figure 3-15 The dbScreenWidth function returns a value

20. 3.5 Getting Values from Functions
Getting a Random Number
The Dark GDK library provides a function named dbRND that generates random numbers
Random numbers are
Commonly used in games
For example
To represent the values of dice
To represent the face values of cards
Useful in simulation programs to determine various actions and events that take place in the program
Useful in statistical programs that must randomly select data for analysis
Commonly used in computer security to encrypt sensitive data

21. 3.5 Getting Values from Functions
Getting a Random Number
The dbRND function
Returns a random integer number
From 0 to an upper limit that you specify as an argument
For example, the following statement stores a random number from 0 to 100 in the integer variable number
int number = dbRND(100);

22. 3.5 Getting Values from Functions
Seeding the Random Number Generator
Numbers returned from the dbRND function are not truly random, but pseudorandom numbers
Pseudorandom numbers are generated by a formula that must be initialized with a starting value, called a seed value
The same seed value will always produce the same sequence of random numbers

23. 3.5 Getting Values from Functions
Seeding the Random Number Generator
Providing a different seed value will change the sequence of random numbers
You can call the dbRandomize function to change the seed value that is used by the dbRND function
The dbRandomize function accepts an integer argument which is used as the new seed value for dbRND

24. 3.5 Getting Values from Functions
Seeding the Random Number Generator
A common practice for getting unique seed values is to call the dbTimer function
The dbTimer function returns the computer’s internal system time in milliseconds
You can pass the value of the dbTimer function as an argument to the dbRandomize function

25. 3.5 Getting Values from Functions
Nesting Function Calls
Take a look at the following code:
This code performs two steps
(1) it gets the internal system time
(2) it sends that value to the dbRandomize function
A seasoned programmer would look at this code and realize it could all be done in one step

26. 3.5 Getting Values from Functions
Nesting Function Calls
Take a look at the following code:
This is known as a nested function call
The statement above
Calls the dbRandomize function
Passes the return value of the dbTimer function as an argument
Eliminates the seed variable
Accomplishes in one line what previously took two lines

27. 3.5 Getting Values from Functions
Math Functions
C++ and the Dark GDK provide numerous functions for performing mathematical operations
For example, C++ provides a function named pow that raises a number to a power

28. 3.5 Getting Values from Functions
Math Functions
Here is the pow function’s general format:
pow(Base, Exponent);
When this function executes
It returns the value of Base raised to the power of Exponent

29. 3.5 Getting Values from Functions
Math Functions
Here is an example of how the pow function works:
The function returns the value of base raised to the power of exponent
The value that is returned from the function is stored in the result variable

30. 3.5 Getting Values from Functions
Math Functions

31. 3.6 Reading Numeric Input from the Keyboard
Concept:
Programs commonly need the user to enter data at the keyboard. We will use the Dark GDK library’s dbInput function to do this.

32. 3.6 Reading Numeric Input from the Keyboard
The Dark GDK library provides a function named dbInput that:
Waits for the user to type something on the keyboard and press the Enter key
Returns the data that the user types as a string
We can convert the string returned by the dbInput function
to an int with the atoi function
to a double with the atof function

33. 3.6 Reading Numeric Input from the Keyboard
Example of converting user input to an integer:
Example of converting user input to a floating-point number:

34. 3.6 Reading Numeric Input from the Keyboard
A complete program that gets data from the user and uses that data to draw a circle:

35. 3.6 Reading Numeric Input from the Keyboard
Figure 3-19 Example output of Program 3-12

36. 3.6 Reading Numeric Input from the Keyboard
How atoi and atof Handle Invalid Characters
The atoi and atof functions
Ignore any spaces that might appear at the beginning of the string
Perform the conversion process until an invalid character is encountered
Return 0, if the string
Is empty
Cannot be converted to a number

37. 3.6 Reading Numeric Input from the Keyboard
Converting Numeric Values to Strings
Numeric values need to be converted to strings before they can be displayed with
dbPrint
dbText
dbCenterText
The dbStr function can be used to convert numeric values to strings
For example:
int score = 550;
dbPrint( dbStr(score) );

38. 3.7 Colors
Concept:
The Dark GDK uses the RGB color system to generate colors. In the RGB system, you define a color by specifying values for its red, green, and blue components.

39. 3.7 Colors RGB Color System 3 color channels: Stored as DWORD
Green
Blue
value from 0 to 255
0 is minimum brightness
255 is maximum brightness
Stored as DWORD
Figure 3-21 Red, green, and blue channels
Figure 3-22 Memory format for storing an RGB color

40. dbClear(red, green, blue);
3.7 Colors
The dbClear function clears the Dark GDK window and fills it with a specified color
Here is the general format of how you call the dbClear function:
dbClear(red, green, blue);
For example
dbClear(255, 0, 127);
Clears the screen to purple
dbClear(255, 0, 0);
Clears the screen to red
dbClear(255, 255, 255);
Clears the screen to white

41. dbRGB(red, green, blue);
3.7 Colors
Storing RGB Colors in Memory
The DWORD data type is typically used to store RGB colors
The Dark GDK library provides a function named dbRGB that returns the DWORD value of an RGB color.
Here is the general format of how you call the dbRGB function:
dbRGB(red, green, blue);

42. 3.7 Colors
Storing RGB Colors in Memory
The dbRGB function can be used to store an RGB color value in a DWORD variable
For example, the following code sample shows how to use the dbRGB function to store an RGB color in a DWORD variable:

43. dbInk(foreground, background);
3.7 Colors
Drawing in Color
The Dark GDK library provides a function named dbInk, which changes the current drawing colors
Here is the general format of how you call the dbInk function:
dbInk(foreground, background);

44. 3.7 Colors Drawing in Color
The foreground argument is a DWORD value specifying the foreground color
All subsequent shapes and text will be drawn in the foreground color
The background argument is a DWORD value specifying the background color
Only applies to text
after calling the dbSetTextOpaque function
Has no effect on shapes
For drawing shapes, you can pass any color you like for the background

45. 3.7 Colors The following code segment shows how to draw a blue circle:
Drawing in Color
The following code segment shows how to draw a blue circle:

46. 3.7 Colors
More About Clearing the Window
The Dark GDK library provides two functions for clearing the window
dbClear
Clears the window to a specific background color
Accepts arguments for the background color’s red, green, and blue components
dbCLS
Clears the window to black if called without an argument
Clears the window to a specific color when you pass a DWORD argument

47. 3.7 Colors More About Clearing the Window
The dbCLS function can be called without arguments to clear the screen to black, as shown here:
The dbCLS function can be called with a DWORD argument to clear the screen to a specific color, as shown here:

48. 3.8 Named Constants Concept:
A named constant represents a value that cannot change while the program is running. You can use the C++ key word const in a variable declaration to create a named constant.

49. 3.8 Named Constants Named Constant Value cannot change
While program is running
Declare with keyword const
For example:
const DWORD myBlue = dbRGB(0, 0, 75);
const int UPPER_LEFT; // ERROR!
const int UPPER_LEFT = 0; // Must initialize

50. 3.9 Changing the Size of the Program Window
Concept:
You can use the dbSetDisplayMode function to set the size of the program’s window.

51. 3.9 Changing the Size of the Program Window
dbSetDisplayMode
Changes size of program’s window
Width
The width, in pixels, of the program’s window
Height
The height, in pixels, of the program’s window
Color depth
Integer specifying the number of bits used to store the color displayed in the window
For example
dbSetDisplayMode(150, 100, 32);

52. Chapter 3 Variables, Calculations, and Colors
QUESTIONS ?