Lab 3
Functions and Types
Pre-requisites
Read Chapter Chapter 3 and Chapter 4
Vocabulary
Function
Scope
Global
Local
return value
Array
Multidimensional array
Type
Field
Instance
Post-lab Questions
Write your answers after completing the lab, but read them carefully now and keep them
in mind during the lab.
1) Why do some functions have a return statement while others do not?
2) Suppose you have two variables of the same name, one is local scope and the other is
global. How does the computer distinguish between the two?
3) Types are very useful for modeling real things in a computer. Suppose you wanted to
represent an animal at a zoo. Come up with a type that would represent an animal. The
type should contain all the necessary information the zoo needs to keep a particular
animal healthy. For example, the food it eats, how often it eats, and its sleep cycle—
nocturnal or diurnal, plus whatever else comes to your mind. Then declare two instances,
one for a lion, the other for a monkey.
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Discussion and Procedure
A FUNCTION is defined using the 'Function' keyword:
Function {funcname}{typetag}( {params} )
{statements}
End Function
{funcname} is any valid identifier.
{typetag} is the type of value returned by the function. If {typetag} is omitted, the function returns an integer
value by default.
{params} is a comma separated list of variables which is passed to the function when it is called, each
parameter may be given an optional type tag. Parameters are always local.
A function may use the 'Return' statement to return a result. Return may optionally be followed by an
expression.
If there is no Return statement, or a Return without any expression is used, the function returns a default
value of 0 for numeric functions, an empty string ("") for string functions, or a 'Null' object for custom type
functions.
Part 1. Built in Functions
abstraction: make larger units of code to do things we often need accomplished
help>command reference list of various built in functions and what they do
labels $, #, % are used to indicate return values, (% or integer is default)
Some Functions
Input$(prompt$)
Print
MilliSecs( )
Mouse Functions
GetMouse( )
MouseX( )
MouseY( )
Math Functions
Sqr#(float)
Rnd (start#,end#)
SeedRnd seed
Get input from the user.
prompt$ = any valid string (optional)
Print a string to the screen.
Return the system's timer value in milliseconds.
Checks to see if a mouse button has been clicked - returns the number
of the button or 0 if no button is clicked.
Returns the mouse's X screen coordinate.
Returns the mouse's Y screen coordinate.
Returns the square root of a given value.
start# = Lowest value to generate
end# = Highest value to generate
Start the random number generator off at seed (seed usually is Millisecs( ) )
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Floor#(float)
Abs(number)
Rounds a decimal floating variable down to the nearest whole number.
float=floating point number
Returns the absolute (positive) value of a number.
number = any valid number or numeric variable
; GetMouse Example
While Not KeyHit(1)
button=GetMouse()
If button <> 0 Then
Print "You pressed mouse button #" + button
End If
Wend
Notes
A) Experiment with mouseX and mouseY to display mouse coordinates
B) Modify to display "GOT ME!" if the mouse X value is the larger than 300
C) Modify to display "Ticklish!" if the mouse X value is the less than 30 AND the
mouse Y value is <30
D) modify to display a random number between 50 and 75 whenever the mouse is
clicked
E) add the SeedRnd command to properly seed the random number generator (before
looping)
Part 2. User Defined Functions that return a value
;demo03-06.bb - Converts Fahrenheit to Celsius
;MAIN PROGRAM
Print "Welcome to our FtoC converter"
;get fahrenheit and put it in fvalue
fvalue = Input$("What Fahrenheit value do you wish to convert?")
;Convert fvalue to Celcius
cvalue = ConvertFtoC(fvalue)
;print results
Print fvalue + " Fahrenheit = " + cvalue + " Celsius."
;Wait for user to press a key
WaitKey
;END OF MAIN PROGRAM
Function ConvertFtoC(fvalue)
;convert value and return it
Return 5.0/9.0 * (fvalue - 32)
End Function
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Notes
A) change parameter fvalue to f
B) make a C to F converter using the formula F = 9/5 C + 32
C) make a distance function: takes 2 points (x1,y1,x2,y2) and uses the formula
d = square root of ( (x2-x1)2 + (y2-y1)2 )
Test: distance(100, 150, 103, 154) should give answer 5
D) make a collision detection function: takes 2 points (x1,y1,x2,y2) , returns 1 (true)
if distance is smaller than 10 pixels (use distance function from B), returns 0
otherwise
Test: collision (100, 150, 103, 154) should give answer 1
Test: collision (10, 15, 103, 154) should give answer 0
Part 3. User Defined Functions that don't return a value
;demo03-07.bb - CallMe()
Global x
;call CallMe() function
CallMe()
;concatenate "x is equal to" and the variable x
Print "x is equal to " + x
WaitKey
;FUNCTION CALLME()
;sets x to 314
Function CallMe()
x = 314
End Function
Notes
A) change "Global x" to "Global x=5" and see what happens. Then just "x=5" –
Global vs local variables
;demo StarWars0
Const TIE$="<-o->"
Const XWING$=">o<"
;call Text function
Text 100, 200, TIE$
Text 200, 150, XWING$
WaitKey
Notes
A) draw another TIE fighter at 190, 155
B) keeping track of x, y and hitpoints for each ship can be tedious. Solution:
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Part 4. Types
;StarWars1 using TYPES Ship is now a multi-field variable
Type Ship
Field x,y ;the position of the ship
Field hitpoints ;how many hits left
Field shipstring$ ;what the ship looks like
End Type
Const TIE$="<-o->"
Const XWING$=">o<"
Global player.ship = New ship
player\x = 100
player\y = 200
player\hitpoints = 3
player\shipstring = XWING$
;call Text function
Text player\x, player\y, player\shipstring$
;
X
WaitKey
Notes
A) Make another ship variable "enemy", use TIE$ for it's image, and make it appear
at 200, 150, as in the last example
B) Add a function to display a ship at bottom of StarWars1.bb file:
Function Display(aShip.ship)
Text aShip\x, aShip\y, aShip\shipstring$
End Function
C) Use the function by adding a function call, replace line ; X with
Display(player)
D) Now modify the Text command that shows the enemy by using another function
call to Display(enemy)
Part 5. Crude Animation
In this section, we will make our spaceship start to move. Make sure you save your
program as StarWars2 first.
Notes
A) Save your program as StarWars2.bb
B) Add the game loop shown below to your program. The spaceships will move!
---already existing lines that initialize player and enemy----While Not KeyHit(1)
Add after initializing player and enemy
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Display(player)
Display(enemy)
Delay 100
Cls
; erase the screen
player\x=player\x+1
Wend
 end of added code
Waitkey
C) Add the following if statement after the line player\x=player\x+1
;If player presses up, move him up.
If KeyHit(UPKEY)
player\y = player\y - 3
If player\y <= 0
player\y = 10
EndIf
EndIf
D) Add a new function at the bottom of your program called DrawHUD( ),
containing these lines:
Function DrawHUD()
Text 200, 10, "mouseX "
Text 200, 20, "player\x
Text 300, 10, "mouseY "
Text 300, 20, "player\y
End Function
+
"
+
"
MouseX()
+ player\x
MouseY()
+ player\y
E) Add a function call to DrawHUD( ) right before the Wend statement, you should
see the position of your mouse and spaceship
F) Make the game stop if you hit the ship with the mouse. Put this if statement in the
while loop. Then see if you can get it to stop the game by moving the mouse over
ship:
If MouseX()=player\x And MouseY()=player\y Then
Print "crash"
Exit
EndIf
G) CHALLENGE: Use your collision and distance functions from part 2 to make it
easier to hit the ship.
H) CHALLENGE: Add other arrow handlers: Copy the TestInput( ) function from
Demo03-11 to bottom of your program, and put a function call to it in place of
your use it in place of your If KeyHit(UPKEY) …etc… code
I) CHALLENGE: modify the program to check if the player hits the enemy
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