Chapter 9 Value-Returning Functions and Mouse … 9 gaddis.pdf© 2010 Pearson Addison-Wesley. All...

Addison Wesley

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© 2010 Pearson Addison-Wesley. All rights reserved.

Chapter 9

Value-Returning Functions and Mouse

Input

Starting Out with

Games & Graphics in C++

Tony Gaddis

Copyright © 2010 Pearson Addison-Wesley

9.1 Writing a Value-Returning

Function

1-2

Concept:

A value-returning function is a function

that returns a value to the part of the

program that called it. The Dark GDK

provides various value-returning

functions that you have already used.

You can also write your own value-

returning functions.



Function

• A value-returning function returns a

value to the statement that called it

• Can be used like any other value

– Assigned to a variable

– Displayed on the screen

– Used in a mathematical

expression

– And so on

1-3

Figure 9-1 A statement that calls the dbRND function

Figure 9-2 The random function returns a value



Function

• You write a value-returning

function in the same way you write a void function,

with two exceptions:

– You must specify a data

type

– You must have a return

statement

1-4

Making the Most of the Return Statement

Writing Your Own Value-Returning Functions

Simplified version returns an expression



Function

• Boolean functions return the bool values true or false

• Used to test a condition

• Return either true or false to

indicate whether or not the

condition exists

1-5

Returning Boolean Values


9.2 Working with the Mouse

1-6

Concept:

The Dark GDK provides functions that

your program can use to interact with the

mouse.



• You can call the dbMouseX

and dbMouseY functions to

get the current position of the

mouse pointer

1-7

Getting the Mouse Coordinates

Figure 9-12 Example output of Program 9-6



• Mouse pointer is visible by default

• You can hide the mouse pointer with the dbHideMouse function

– Still active within the program’s window

– Can still get position with dbMouseX and dbMouseY functions

• You can show the mouse pointer with the dbShowMouse function

• You can position the mouse anywhere in the program’s window, at any time, by calling the dbPositionMouse function, passing the X

and Y coordinates as arguments

1-8

Showing, Hiding, and Positioning the Mouse



• You can use the dbMouseClick function to determine whether the user is

pressing a mouse button

• The dbMouseClick function can detect up to four buttons

• Returns an integer value that indicates which, if any, of the mouse buttons is

being pressed

– 0 if none of the mouse buttons are being pressed

– 1 if the left mouse button is being pressed

– 2 if the right mouse button is being pressed

– 4 if the third mouse button is being pressed

– 8 if the fourth mouse button is being pressed

1-9

Detecting Button Clicks



• In some situations you only

want to perform an action once

per mouse click

• This functionality is not

provided by the Dark GDK, so

we’ll have to write our own

function

– Determine whether or not

the mouse button is being

pressed

– If so, get the mouse

coordinates

– Perform a loop that makes

the program wait until the

mouse button is released

before continuing

1-10

Processing Full Mouse Clicks



• Clicking images with the

mouse is a common

technique used in games and

graphics programs

• If we know the XY

coordinates of the sprite’s

upper-left and lower-right

corners we can determine if

the mouse is inside the

sprite’s bounding rectangle

1-11

Clicking Sprites

Figure 9-15 A sprite’s upper-left and

lower-right corner coordinates



• Create a sprite

with the image you

want to use for the

mouse pointer

• Hide the regular

mouse pointer

• In the game loop

– Get mouse

pointer’s

location

– Move sprite

using mouse

pointer’s

coordinates1-12

Creating a Custom Mouse Pointer


9.3 The Bug Zapper Game

• The Bug Zapper game displays an animated sprite of a

bug

• The player zaps the bug by clicking it with the mouse

• After a bug is clicked, a new one appears at a random

location on the screen

• The game will run for 10 seconds, then end

• Zap as many bugs as possible before time runs out

• When the game is over, a screen displays how many

bugs were zapped

1-13


9.3 The Bug Zapper Game

1-14

Figure 9-18 Screens from the Bug Zapper game

Chapter 9 Value-Returning Functions and Mouse … 9 gaddis.pdf© 2010 Pearson Addison-Wesley. All...

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