1. CSC 211 Java I
for loops and arrays

2. Today’s plan Midterm Primitive data types vs. object variables
for Loops
Arrays

3. Midterm Statistics High: 99%, Low: 43%, Average: 81%
100 – 90: 6
89 – 80: 6
79 – 70: 2
<=69:
Course grading review:
Labs: 20%
OWL: 10%
Programs: 25%
Midterm: 45% (both exams together)

4. Today’s plan Midterm Primitive data types vs. object variables
for Loops
Arrays

5. The Key:
When it comes to objects, you must distinguish between the object’s reference and the object’s state (data).
This is vital! While a bit confusing at the moment, we will discuss it in class.
Review online and/or with your textbook
Practice and become familiar with this principle

6. Overview:
When it comes to primitive data types, the information stored by the variable is the value.
When it comes to objects, the information stored by the variable is the memory address of the object.

7. ** Primitive types vs objects
“I love Java” x 4
love
Memory location x033622x3544
int x;
x=4;
String love;
love=new String(“I love Java”);
x holds a value
love holds a reference (to an object) -- actually the reference to the memory
location where the object is stored

8. Assignment of primitive data typex y 4 5
int x = 4, y = 5;
Recall: Since x and y are primitive data types (i.e. int), they store values. So when you say y = x, you are assigning the value of x to y.
However, things work differently with objects….
With objects, if you say objB = objA, you are not assigning the value of one to the other. Instead, you are assigning the memory address.
y = x;

9. Assignment of strings s f s f s f t t = = s; s = = f; s.equals(t);
“soup”
“fish” s f
String s =new String(“soup”), f =new String(“fish”);
String t = new String(“soup”); s f
“soup”
“fish” s f
“soup”
“fish” t
“soup”
f = s;
t = = s;
s = = f;
s.equals(t);

10. public boolean equals (String yourString)
Comparing Strings
public boolean equals (String yourString)
boolean check1, check2;
String love=new String(“I love Java”);
check1 = love.equals(“I LOVE JAVA”);
Value of check1 is?
check2 = love.equalsIgnoreCase(“I LOVE JAVA”);
Value of check2 is?

11. Today’s plan Midterm Primitive data types vs. object variables
for Loops
Arrays

12. Remember the while loop:
If condition is never true the while_true statements are never executed
Remember the while loop:
condition
while (condition)
while_true_statements { }
while_true_statements
true
Back to the program
Remaining program statements
false
If condition is always true the while_true statements are executed forever

13. Are you with me?
Write a piece of code that prints out the even numbers between 2 and 10.

14. Can we do it differently?
In this last example, we knew exactly how many iterations of the loop are needed (i.e. we stopped at 10).
Sometimes, we don’t know ahead of time when we’re going to stop. E.g. Situations where at the end of each loop we ask the user if they want to continue)
However, for situations where we do know when we’re going to stop, we have a built in counter for the loop
Java has another ‘shortcut’ form of loop syntax that handles these cases very well

15. The for loop
The for loop is coordinated by a counter for which we need to set:
An initial value
A test condition (as long as the condition is true the loop will keep going)
An way of updating the counter

16. Remaining program statements
The for loop
Initialize counter
condition
Update counter
for (initialize; test; update)
for_statement
true
for_statement { }
Remaining program statements
false
Back to the program
for(int count = 1 ; ) {
//statements to be repeated (looped) }
count <= 10 ;
count ++

17. Let’s get evens
Write a piece of code that prints out the even number between 2 and 10?
for( {
System.out.println(count) }
int count = 2;
count <= 10;
count += 2 )
int count = 2;
while (count <= 10) {
System.out.println(count);
count += 2; }
output 2 4 6 8 10

18. The for loop Semantically equivalent to a while loop
You can always do with a while loop what you can do with a for loop
Sometimes a for loop is very awkward
E.g. When no initialization needs to be done
Use it when some kind of counter of the iterations is natural

19. Exercise Do Parts 1.1 - 1.3 of the lab
All even numbers from 2 to max
Sum of squares of all even numbers from 2 to max
Tracing the program for max = 4
Not sure what is happening inside your loop?
***** During the debugging phase, insert print/println statements that display the current value of the counter and any other critical variable(s)

20. Exercise Do Part 1.4 of the lab Ask user to enter his/her name
Display the characters of the name to the console window
In all caps
In reverse order
Confused how to do this?
Plan!! (Use JEnglish)

21. Today’s plan Midterm Primitive data type vs object variables for Loops
Arrays

22. Lots of similar things …
Create a program that asks the user to enter 5 assignment scores
Print out the scores in decreasing order
Calculate the average score
Now… Imagine you have 40 assignment scores
… or 400 assignment scores

23. Are you with me? Give a rough skeleton of the program
Only main ingredients: type of loop, # of variables, etc.
What changes for 5 vs. 400 students?

24. … One big container
Instead of declaring 400 double variables, (firstScore, secondScore, thirdScore …) one per student
Declare one Score variable with 400 designated spots. We need an array!

25. Pills anyone?
Sun
Mon
Tue
Wed
Thu
Fri
Sat

26. Each value has a numeric index
Arrays
An array is an ordered list of data
scores
The entire array
has a single name
Each value has a numeric index
An array of size N is indexed from zero to N-1
This array holds 10 values that are indexed from 0 to 9

27. Arrays
A particular value in an array is referenced using the array name followed by the index in brackets 94
scores[2]
The expression scores[2] refers to the number 94, the third element in the array

28. The 411 on Arrays
An array stores multiple values. All values in the array must be of the same data-type
These can be primitive types or objects
We can create
An array of double
An array of char
An array of String objects
An array of Triangle objects
etc…
In Java, the array itself is an object
Therefore the name of the array is a object reference variable, and the array itself is instantiated separately

29. Declaring Arrays double[] scores = new double[10];
An array of doubles is an object of the class double[] which identifies an array that can contain doubles
The name I choose for my array is scores
Instantiate an object of the double[] class
double[] scores = new double[10];
My array will contain 10 elements. It has index from 0 to 9

30. Filling an array double[] scores = new double[3]; scores[0] = 70;
38.5 90

31. length of an array
Each array object has a public constant called length that stores the size of the array
E.g. scores.length
length holds the number of elements
In an array of 13 elements (indexed from 0 to 12),
System.out.println(scores.length);
would output: 13

32. Exercise Do Parts 2.1- 2.4 of the lab
Declare and instantiate the scores array
Populate the scores array using input from the user
Print all scores to the console separated by commas
Print the first and last score entered

33. Bounds checking
Once an array is created, it has a fixed size (for example N).
An index used in an array reference must specify a valid element: the index value must be in bounds (0 to N-1)
The Java interpreter will complain if an array index is out of bounds.
This is called automatic bounds checking
Note: As mentioned above, the compiler will NOT complain.(ie: your program will compile. The problems is that your program may “break” crash!
In many ways this is worse!! It’s much better to catch things when compiling. At least then you have a chance to fix it.

34. More array declarations
boolean[] flags;
flags = new boolean[100];
OR you could also write:
boolean[] flags = new boolean[100];
String[] roster = new String[25];

35. Array declaration revisited
The brackets of the array type can be associated with the element type or with the name of the array
double[] prices;
double prices[];
Preferred format

36. Initializer list
An initializer list can be used to instantiate and initialize an array in one step
The values are delimited by braces and separated by commas
Examples:
double[] units = {147, 323.2, 89.0, 933, ,
269, 97.18, 114, 298.3, 476};
char[] letterGrades = {'A', 'B', 'C', 'D', 'F'};

37. Initializer list Note that when an initializer list is used:
The new operator is not used
No size value is specified
The size of the array is determined by the number of items in the initializer list
An initializer list can only be used at the time that you declrae the array

38. Exercise Do Parts 2.5 - 2.7 of the lab
Calculate and display the average score
Find and display highest grade and position in the array
Find the lowest grade and compute a new average with the lowest grade dropped

39. Exercise Do Parts 2.8 - 2.9 of the lab Create a copy of the array
Use the equality operator to test equality of the two arrays as objects