1. Addison Wesley is an imprint of © 2010 Pearson Addison-Wesley. All rights reserved. Chapter 3 Variables, Calculations, and Colors Starting Out with Games & Graphics in C++ Tony Gaddis

2. Copyright © 2010 Pearson Addison-Wesley 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 1-2

3. Copyright © 2010 Pearson Addison-Wesley 3.2 Literal Data 1-3 Concept: A literal is an item of data that is typed into a program’s code.

4. Copyright © 2010 Pearson Addison-Wesley 3.2 Literal Data Literal Data –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 –For example: dbPrint(“Hello World!”); 1-4

5. Copyright © 2010 Pearson Addison-Wesley 3.3 Variables 1-5 Concept: A variable is a storage location in memory that is represented by a name.

6. Copyright © 2010 Pearson Addison-Wesley 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 1-6

7. Copyright © 2010 Pearson Addison-Wesley 3.3 Variables 1-7 Data Types –int Whole numbers –For example, 6, -56, 9, and 45 –float Floating-point numbers –For example, 8.9, 96.948, and 3.0 –double Twice the precision of float –DWORD Unsigned, 32 bits, used for Dark GDK colors

8. Copyright © 2010 Pearson Addison-Wesley 3.3 Variables Variable Names –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 1-8

9. Copyright © 2010 Pearson Addison-Wesley 3.3 Variables Declaring a Variable – Specify data type and name For example: –int centerX; –float distance; –Assigning Values For example: –centerX = 319; –distance = 159.9; »319 = centerX; // ERROR! –Initializing For example –int centerX = 319; –int centerX = 319, centerY = 239, radius = 150; 1-9

10. Copyright © 2010 Pearson Addison-Wesley 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) 1-10

11. Copyright © 2010 Pearson Addison-Wesley 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 For example: –float grade; –grade = 90; 1-11

12. Copyright © 2010 Pearson Addison-Wesley 3.4 Calculations 1-12 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. Copyright © 2010 Pearson Addison-Wesley 3.4 Calculations Math Operators –Addition 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 (%) 1-13

14. Copyright © 2010 Pearson Addison-Wesley 3.4 Calculations Order of Operations –Parentheses (always first) From left to right –Multiplications, divisions, or modulus operations(1 st ) –Additions or subtractions(2 nd ) 1-14

15. Copyright © 2010 Pearson Addison-Wesley 3.4 Calculations 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 1-15 Integer Division

16. Copyright © 2010 Pearson Addison-Wesley 3.4 Calculations 1-16 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. Copyright © 2010 Pearson Addison-Wesley 3.4 Calculations 1-17 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. Copyright © 2010 Pearson Addison-Wesley 3.5 Getting Values from Functions 1-18 Concept: A value-returning function returns a value back to the statement that called it.

19. Copyright © 2010 Pearson Addison-Wesley 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 1-19 Figure 3-15 The dbScreenWidth function returns a value

20. Copyright © 2010 Pearson Addison-Wesley 3.5 Getting Values from Functions 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 1-20 Getting a Random Number

21. Copyright © 2010 Pearson Addison-Wesley 3.5 Getting Values from Functions 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 1-21 Getting a Random Number int number = dbRND(100);

22. Copyright © 2010 Pearson Addison-Wesley 3.5 Getting Values from Functions 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 1-22 Seeding the Random Number Generator

23. Copyright © 2010 Pearson Addison-Wesley 3.5 Getting Values from Functions 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 1-23 Seeding the Random Number Generator

24. Copyright © 2010 Pearson Addison-Wesley 3.5 Getting Values from Functions 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 1-24 Seeding the Random Number Generator

25. Copyright © 2010 Pearson Addison-Wesley 3.5 Getting Values from Functions Take a look at the following code: 1-25 Nesting Function Calls 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. Copyright © 2010 Pearson Addison-Wesley 3.5 Getting Values from Functions Take a look at the following code: 1-26 Nesting Function Calls 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. Copyright © 2010 Pearson Addison-Wesley 3.5 Getting Values from 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 1-27 Math Functions

28. Copyright © 2010 Pearson Addison-Wesley 3.5 Getting Values from Functions Here is the pow function’s general format: 1-28 Math Functions pow(Base, Exponent); When this function executes It returns the value of Base raised to the power of Exponent

29. Copyright © 2010 Pearson Addison-Wesley 3.5 Getting Values from Functions Here is an example of how the pow function works: 1-29 Math Functions 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. Copyright © 2010 Pearson Addison-Wesley 3.5 Getting Values from Functions 1-30 Math Functions

31. Copyright © 2010 Pearson Addison-Wesley 3.6 Reading Numeric Input from the Keyboard 1-31 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. Copyright © 2010 Pearson Addison-Wesley 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 1-32

33. Copyright © 2010 Pearson Addison-Wesley 3.6 Reading Numeric Input from the Keyboard Example of converting user input to an integer: 1-33 Example of converting user input to a floating-point number:

34. Copyright © 2010 Pearson Addison-Wesley 3.6 Reading Numeric Input from the Keyboard 1-34 A complete program that gets data from the user and uses that data to draw a circle:

35. Copyright © 2010 Pearson Addison-Wesley 3.6 Reading Numeric Input from the Keyboard 1-35 Figure 3-19 Example output of Program 3-12

36. Copyright © 2010 Pearson Addison-Wesley 3.6 Reading Numeric Input from the Keyboard 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 1-36 How atoi and atof Handle Invalid Characters

37. Copyright © 2010 Pearson Addison-Wesley 3.6 Reading Numeric Input from the Keyboard 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) ); 1-37 Converting Numeric Values to Strings

38. Copyright © 2010 Pearson Addison-Wesley 3.7 Colors 1-38 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. Copyright © 2010 Pearson Addison-Wesley 3.7 Colors RGB Color System –3 color channels: Red Green Blue value from 0 to 255 –0 is minimum brightness –255 is maximum brightness –Stored as DWORD 1-39 Figure 3-22 Memory format for storing an RGB color Figure 3-21 Red, green, and blue channels

40. Copyright © 2010 Pearson Addison-Wesley 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: 1-40 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. Copyright © 2010 Pearson Addison-Wesley 3.7 Colors 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: 1-41 Storing RGB Colors in Memory dbRGB(red, green, blue);

42. Copyright © 2010 Pearson Addison-Wesley 3.7 Colors 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: 1-42 Storing RGB Colors in Memory

43. Copyright © 2010 Pearson Addison-Wesley 3.7 Colors 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: 1-43 Drawing in Color dbInk(foreground, background);

44. Copyright © 2010 Pearson Addison-Wesley 3.7 Colors 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 1-44 Drawing in Color

45. Copyright © 2010 Pearson Addison-Wesley 3.7 Colors The following code segment shows how to draw a blue circle: 1-45 Drawing in Color

46. Copyright © 2010 Pearson Addison-Wesley 3.7 Colors 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 1-46 More About Clearing the Window

47. Copyright © 2010 Pearson Addison-Wesley 3.7 Colors The dbCLS function can be called without arguments to clear the screen to black, as shown here: 1-47 More About Clearing the Window The dbCLS function can be called with a DWORD argument to clear the screen to a specific color, as shown here:

48. Copyright © 2010 Pearson Addison-Wesley 3.8 Named Constants 1-48 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. Copyright © 2010 Pearson Addison-Wesley 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 1-49

50. Copyright © 2010 Pearson Addison-Wesley 3.9 Changing the Size of the Program Window 1-50 Concept: You can use the dbSetDisplayMode function to set the size of the program’s window.

51. Copyright © 2010 Pearson Addison-Wesley 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); 1-51

52. Addison Wesley is an imprint of © 2010 Pearson Addison-Wesley. All rights reserved. QUESTIONS ? Chapter 3 Variables, Calculations, and Colors