Intro to Computer Science Class

1
Intro to Computer Science Class 4If
Statements, While Loops and For Loops

• Instructor Ms. Catherine Stocker
• Teaching Assistants Alex, Katie, Siraaj,
  Isaiah, Allison, Thibault
• University of Pennsylvania
• 13 February 2008

2
Todays Agenda

• Discussion/Demo (Reaction paper, BetterBot and
  Extras)
• .hist files
• Comments and Javadocs
• Review
• If Statements
• Boolean Expressions
• While Loops
• For Loops
• LoopingBot

3
Discussion/Demo

• Lets talk about the reaction paper and demo
  BetterBot and BetterBot Extras.

4
What We Covered in the Demo .hist files

• A convenient way to test your code -- interaction
  history files.
• Two ways to make a .hist file
• 1) After running through the interactions once,
  Tools -gt Save Interactions History as Script
• 2) Create a new file in DrJava (New button in top
  left).
• Save file as name.hist (and change to Files of
  Type All Files).
• Put //DrJava saved history v2 at the top of the
  file and //End of Interaction// at the end of
  each statement you type.
• To run a .hist file
• Tools-gtLoad Interactions History As Script.
• Four buttons will appear on the right side of the
  Interactions Pane.
• Hit the Next and Execute buttons one after the
  other.
• (check out resources page)

5
What We Covered in the Demo Comments and Javadocs

• Comments make code more readable to humans
• They are ignored by the computer/compiler/Java
• // A single line comment starts with two forward
  slashes
• / A multi-line comment starts with a forward
  slash and star.
• People usually put a star at the beginning of
• other lines like this. The comment ends with
  star slash.
• /
• / You can also do a single line comment like
  this. /
• / This is a javadoc comment. Note the extra
  star.
• These comments are used to generate a separate
• users manual on a web page (called the
  javadoc).
• /

6
Reminder What Are These and How Are They Used?

• class Athlete
• String name
• String sport
• int salary
• int daysInjured
• String popularity
• Athlete(String n, String s)
• name n
• sport s
• salary 0
• daysInjured 0
• popularity average
• void negotiateContract(int newSalary)
• salary newSalary

7
Reminder What Are These and How Are They Used?

• class Athlete
• String name
• String sport
• int salary
• int daysInjured
• String popularity
• Athlete(String n, String s)
• name n
• sport s
• salary 0
• daysInjured 0
• popularity average
• void negotiateContract(int newSalary)
• salary newSalary

class (object blueprint)
instance variables (hold the state)

• constructor.
• Creates the object
• Sets the starting state

• methods
• modify the instance variables (behaviors changing
  the state)
• return the instance variables (tell us about the
  state)

8
ReminderVariables
x
5
y
10

• Purpose? Store and retrieve a value in/from
  memory
• Must be declared once before being used, why?
• Indicates how much space in memory is needed to
  hold it and which operations are allowed to be
  performed on it.
• How do we declare? type_of_variable
  variable_name
• How do we assign? variable_name data_value
• 2 types of data (type_of_variable)
• Primitive int, double char, boolean
• Acted on by symbolic operators (,-,/,-,,gt,lt,
  etc).
• Non-Primitive String and any class we or Java
  create.
• Acted on by methods variable_name.method_name(par
  ameters)
• Data must be created first to be assigned
  variable_name new constructor(parameter_list)
• Rules for naming variables
• Should be descriptive!
• Cant use keywords
• Case-sensitive (hello ? Hello)

9
ReminderVariables
x
5
y
10

• Purpose? Store and retrieve a value in/from
  memory
• Must be declared once before being used, why?
• Indicates how much space in memory is needed to
  hold it and which operations are allowed to be
  performed on it.
• How do we declare? type_of_variable
  variable_name
• How do we assign? variable_name data_value
• 2 types of data (type_of_variable)
• Primitive int, double char, boolean
• Acted on by symbolic operators (,-,/,-,,gt,lt,
  etc).
• Non-Primitive String and any class we or Java
  create.
• Acted on by methods variable_name.method_name(par
  ameters)
• Data must be created first to be assigned
  variable_name new constructor(parameter_list)
• Rules for naming variables
• Should be descriptive!
• Cant use keywords
• Case-sensitive (hello ? Hello)

10
ReminderThree Ways to Use Variables

• class Athlete
• String name
• String sport
• int salary
• int daysInjured
• String popularity
• Athlete(String n, String s)
• name n
• sport s
• salary 0
• daysInjured 0
• popularity average
• void negotiateContract(int newSalary)
• int bargain 2
• salary newSalary/bargain

11
ReminderThree Ways to Use Variables

• class Athlete
• String name
• String sport
• int salary
• int daysInjured
• String popularity
• Athlete(String n, String s)
• name n
• sport s
• salary 0
• daysInjured 0
• popularity average
• void negotiateContract(int newSalary)
• int bargain 2
• salary newSalary/bargain

• Instance variables
• Stores state of each object
• Declared at top of class
• Scope Accessible to entire object.

• Parameters
• Values passed into methods
• Scope Accessible to entire method

• Local variables
• Stores temporary state of a method
• Declared at top of method
• Scope Accessible to entire method

12
Reminder Methods

• modifier return_type method_name(parameter_list)
  
• method_body
• modifier
• What can see it, for now, assume public.
• return_type
• What values can this take? Any primitive or
  non-primitive type void.
• Whats it specifying? Data type of the value
  returned by the method, void if nothing.
• method_name
• What are the rules? Same as for variable names
  generally contains a verb
• parameter_list
• What does it do? Pass data to the method -- Can
  be empty or contain a list.
• method_body
• Includes? Declaration of local variables,
  manipulation of variables (instance, local or
  parameters), return statement (returns data of
  type return_type).

public void party() daysInjured
daysInjured 10
13
Reminder Methods

• modifier return_type method_name(parameter_list)
  
• method_body
• modifier
• What can see it, for now, assume public.
• return_type
• What values can this take? Any primitive or
  non-primitive type void.
• Whats it specifying? Data type of the value
  returned by the method, void if nothing.
• method_name
• What are the rules? Same as for variable names
  generally contains a verb
• parameter_list
• What does it do? Pass data to the method -- Can
  be empty or contain a list.
• method_body
• Includes? Declaration of local variables,
  manipulation of variables (instance, local or
  parameters), return statement (returns data of
  type return_type).

public void party() daysInjured
daysInjured 10
14
Reminder Snapshot of a modified Athlete class

• public class Athlete
• int salary
• .
• public void negotiateContract ( int newSalary )
  
• int bargain 2
• salary newSalary/bargain
• public int getSalary ( )
• return salary
• ...//the rest of the methods

15
Reminder Snapshot of a modified Athlete class

• public class Athlete
• int salary
• .
• public void negotiateContract ( int newSalary )
  
• int bargain 2
• salary newSalary/bargain
• public int getSalary ( )
• return salary
• ...//the rest of the methods

return_type (void so no return statement)
method_name
parameter_list
modifier
method_body
parameter_list (empty)
return_type
method_name
modifier

method_body (returns an int)
16
Reminder The Constructor
public Athlete(String n, String s) name
n sport s salary 0 daysInjured
0 popularity average

• modifier class_name(parameter_list)
• constructor_body
• Creates the object from the class blueprint.
• Looks like a method, with some differences
• No return_type.
• Its name is always the same as the class.
• Sets up the initial state.
• When is it used?
• Variable declaration and assignment for
  non-primitive types type_of_variable
  variable_name new constructor(parameter_list)
• ex. Athlete a1 new Athlete(Ryan, Baseball
  Player)
• Any class you create becomes a non-primitive type

17
Reminder The Constructor
public Athlete(String n, String s) name
n sport s salary 0 daysInjured
0 popularity average

• modifier class_name(parameter_list)
• constructor_body
• Creates the object from the class blueprint.
• Looks like a method, with some differences
• No return_type.
• Its name is always the same as the class.
• Sets up the initial state.
• When is it used?
• Variable declaration and assignment for
  non-primitive types type_of_variable
  variable_name new constructor(parameter_list)
• ex. Athlete a1 new Athlete(Ryan, Baseball
  Player)
• Any class you create becomes a non-primitive type

18
Reminder Introduction to Inheritance

• modifier class subclass_name extends
  superclass_name
• class_body
• What is the purpose of inheritance?
• Subclass inherits all the functionality of the
  superclass while keeping it intact.
• Maybe cant get to the code to modify it, like
  with the Bot class
• Copy and Pase is a waste of time and space
  (memory).
• Constructor of superclass not inherited, so must
  call super(parameters) within the subclass
  constructor.
• Most common way this is done is Object class
• All classes extend the object class

public class BetterBot extends Bot public
BetterBot(BotWorld world) super(world)
//additional methods
19
Reminder Introduction to Inheritance

• modifier class subclass_name extends
  superclass_name
• class_body
• What is the purpose of inheritance?
• Subclass inherits all the functionality of the
  superclass while keeping it intact.
• Maybe cant get to the code to modify it, like
  with the Bot class
• Copy and Pase is a waste of time and space
  (memory).
• Constructor of superclass not inherited, so must
  call super(parameters) within the subclass
  constructor.
• Most common way this is done is Object class
• All classes extend the object class

public class BetterBot extends Bot public
BetterBot(BotWorld world) super(world)
//additional methods
20
Control Flow

• What happens to our eatAllDots method if there
  was a different dot configuration?
• Right now our programs always execute the
  statements in a method in the order that we write
  (from top to bottom). We need a way to let our
  programs make a decision.
• if statements, while loops, for loops
• These all allow the program to decide whether
  or not to execute sections of code.

21
if Statements

• Tells your program to only execute a certain
  section of code if a boolean expression evaluates
  to true.
• If the first boolean expression is false, then
  check the second boolean expression and execute
  the 2nd section of code if that expression
  evaluates to true.
• You can have 0-unlimited number of else if
  expressions.
• If none of the boolean expressions evaluate to
  true, then execute the body of the else.
• You can have 0 or 1 else
• The if statement must always start with an if,
  can have an unlimited number of else ifs in the
  middle, and if there is an else it must come at
  the end.

if ( boolean_expression )
body_of_statement
if ( boolean_expression1 )
body_of_statement1 else if ( boolean_expression2
) body_of_statement2
if ( boolean_expression1 )
body_of_statement1 else if ( boolean_expression2
) body_of_statement2 else
body_of_statement3
22
Boolean Expressions

• true, false
• -- or
• if a or b is true, then the expression is true.
  If not, the expression is false.
• -- and
• If a and b are true, then the expression is true.
  If not, the expression is false.
• ! not
• If a is true then the expression is false. If
  not, the expression is true.
• -- is equal to
• ! -- is not equal to
• lt -- less than
• lt -- less than or equal
• gt -- greater than
• gt -- greater than or equal
• Methods that return booleans

23
Boolean Expressions

• true, false
• -- or
• if a or b is true, then the expression is true.
  If not, the expression is false.
• -- and
• If a and b are true, then the expression is true.
  If not, the expression is false.
• ! not
• If a is true then the expression is false. If
  not, the expression is true.
• -- is equal to
• ! -- is not equal to
• lt -- less than
• lt -- less than or equal
• gt -- greater than
• gt -- greater than or equal
• Methods that return booleans

24
Boolean Expressions

• What would these evaluate to?
• boolean isTiredtrue
• boolean isHungryfalse
• int ageOfBob 15
• double heightOfBob 60.5
• isTired isHungry
• isTired isHungry
• ageOfBob heightOfBob
• ageOfBob ! heightOfBob
• ageOfBob lt heightOfBob
• !( (heightOfBob gt ageOfBob) (isHungry) )

25
Boolean Expressions

• What would these evaluate to?
• boolean isTiredtrue
• boolean isHungryfalse
• int ageOfBob 15
• double heightOfBob 60.5
• isTired isHungry true
• isTired isHungry false
• ageOfBob heightOfBob false
• ageOfBob ! heightOfBob true
• ageOfBob lt heightOfBob true
• !( (heightOfBob gt ageOfBob) (isHungry)
  ) false
• Very important to put parentheses around your
  expressions to make sure the computer evaluates
  them the way you want them evaluated

26
Back to if statements

• What are these if statements deciding and when do
  you execute each body of the statements
• if(likesTV isSaturday)
• watchTV()
• else
• goToSchool()
• if(myAge gt yourAge)
• return true
• else if (myAge yourAge)
• return calculateExactAgeDifference()
• else
• return false

27
while Loops

• while (boolean_expression)
• body_of_loop
• Similar to if statement, except, instead of
  executing the body of the loop once and
  continuing, it loops through the body until the
  boolean expression becomes false.
• Examples
• boolean wantsToRideCoaster true
• while (wantsToRideCoaster)
• //rideRollerCoaster() returns true if, after
  the ride, she wants to ride again.
• wantsToRideCoaster rideRollerCoaster()
• int numLoops 0
• while (numLoopslt10)
• doSomething()
• numLoops numLoops1
• You will do something like this a lot (run a loop
  exactly n times), so we need a more efficient way
  to do that (while loops are still good for the
  1st example though).

28
for Loops

• for (initialization boolean_expression
  increment)
• body_of_loop
• initialization executed once, before loop
  begins, initializes variable being checked.
• Scope Variables declared during this step is
  anywhere in the for loops. So any code outside
  of the for loop cannot use it.
• boolean expression Like while loop tested
  before each loop iteration and, if false, stops
  executing loop body.
• increment changes value of variable after each
  iteration through the loop.
• i means i i1
• Doesnt have to increment (just what it normally
  is). Could decrement (i-- means ii-1) or any
  mathematical operation.
• Examples
• Remember int numLoops 0
• while (numLoopslt10)
• doSomething()
• numLoops numLoops1

Now for (int i0 ilt10 i)
doSomething()
29
Now

• Write down something you were confused about from
  class and a short explanation about what confused
  you (1-3 sentences). (Submit using the submission
  link).
• LoopingBot
• https//www.seas.upenn.edu/eas285/forSLAStudents
  /lectures/lecture4_IfStatementsLoopsComments/modif
  iedloopingbot/assignment

30
Later

• Finish LoopingBot. Comment your methods in
  proper javadoc style.
• Ask questions on the bulletin board.
• Submit using the link on the webpage