Chapter 9 - Arrays

Question 1

What is a class?

Answer 1

It stores a group of related data, and it stores the methods that operate on that data.

Question 2

What is an Array?

Answer 2

An Array is a limited version of a class.

• Similar to a class it stores related data, but it does not store methods.
• Also, unlike a class and Arrays data must all be the same type.

Question 3

What are Arrays composed of?

Answer 3

Each part of an Array is called an element.

Question 4

How does a class access one of its members?

Answer 4

Using dot notation

Question 5

How does an Array access one of its elements?

Answer 5

Using square brackets around an index

I.E. [x]

Question 6

How are index values determined in an Array?

Answer 6

The first index number starts at 0 and the final number is the number of elements in an array minus 1.

Question 7

What command would you type to change the value of an array element?

Answer 7

The name of the Array = new value

I.E: phonelist[0] = 555-55-5555

(Name of the array and element number) = new Phone number

Question 8

What command would you type to print out a phone number?

Answer 8

System.out.println(Arrayname and element number);

I.E: System.out.println(phoneList[1]);

Question 9

What is an Array considered?

Answer 9

A Variable

Question 10

How do you declare and Array as a variable?

Answer 10

“element-type”[] “array-variable”;

Examples:

int[] ids;

double[] workHours;

String[] names;

Question 11

What are Arrays in Java?

Answer 11

Objects

Question 12

After declaring an Array as a variable, what must be done to create the Array as an object?

Answer 12

You must use the “new” operator.

“array-variable” = new “element-type”[array-size];

Examples:

long[] phoneList; (this establishes the array variable)

phoneList = new long[10]; (this creates the array)

Question 13

How do you combine and arrays declaration as a variable and creation?

Answer 13

“element-type”[] “array-name” = new “element-type”[length of array];

Example: long[] phoneList = new long[10];

Question 14

How would you write a statement that declares, creates and assigns a 100-element array that stores book titles?

Answer 14

long[] bookTitles = new long[100];

Question 15

What is an array initializer?

Answer 15

A single statement made up of an array declaration, creation and {} assignment.

Example: “element-type”[] “array-name” = {“element-values-list”};

String[] students = {“Hamoud”, “Lee”, “Brandon”};

*Note that the array is created without the “new” operator.

Question 16

If an Array initializer is used, what is the size of the array?

Answer 16

The number of elements in the initialization list.

For example: {“John”, “Michael”, “Tom”}; would be an array size of 3

Question 17

What does it mean when an array is instantiated?

Answer 17

It means the elements are assigned default values according to the array data type.

Basically, if an array is not given an assigned value instead of giving a complier error it will just assign default values to the array.

Question 18

What are the default values when an array is instantiated? (They’re also the default values for instance variables and class variables)

Answer 18

Example:

float[] gpas = new float[1000];

String[] states = new String[50];

Default values:

This would return 0.0 1000 times

This would return null 50 times

Question 19

How would you print and array that’s been initialized like this:

String[] colors = {“blue”, “gray”, “lime”, “teal”, “yellow”}; ?

Answer 19

for (int i = 0; i < colors.length; i++) {

System.out.println(colors[i]); }

(note that colors.length is the name of the array, and calls the length of the array since it’s assigned a set number of elements, in this case 5)

Question 20

How do you obtain an array’s length?

Answer 20

Specify the array name, dot, and then “length”

Example:

bookTitles.length

Question 21

“length” has two uses, what are they?

Answer 21

• a String method
• an array property

only use parentheses with Strings.

ANSY = arrays no, strings yes

Question 22

What is a partially filled array?

Answer 22

When you use some of an array’s elements, but not all.

Question 23

What do you have to do if you have a partially filled array?

Answer 23

Keep track of the number of filled elements in the array so you can process the filled elements differently from the non-filled elements.

Typically done through a variable that keeps track of the filled elements.

Question 24

What happens if you assign one array reference variable to another array reference variable?

Answer 24

Both array reference variables then point to the same single array object.

This is always the case with objects and their associated reference variables.

Question 25

Why would you want to copy an array?

Answer 25

To copy the original and point to different array objects. To do it, you need to assign array elements one at a time.

Example attached: The arrays elements are assigned one at a time, with February copying January. Then the code changes the “1” element, to a different price, meaing the second element in the array. Then prints them out.

Question 26

What is a histogram?

Answer 26

A graph that displays quantities for a set of categories. It indicates the category quantities with bars.

Shorter bars = smaller quantities

Longer bars = larger quantities

Example attached

Question 27

When using a histogram, what type of array can you use to track outcomes?

Answer 27

You can use a frequency array

For example: if coding histogram to track the number of heads from 3 coins being flipped 100 times

frequency[0] = number of time no heads show up

frequency[1] = number of times 1 heads shows up

frequency[2] = number of times 2 heads shows up

frequency[3] = number of times 3 heads shows up

Question 28

What are the elements in a frequency array called?

Answer 28

A bin

Question 29

What happens to a bin in a frequency array after each simulation?

Answer 29

1 is added to the appropriate frequency bin

Question 30

How do you go about searching an array for a particular value?

Answer 30

In Pseudocode:

i <- 0

while i < number of filled elements(the [number])

if “array-name”[i] equals the searched-for value

then do something and stop the loop

increment i

Question 31

What is a simple sorting algorithm?

Answer 31

The Selection Sort algorithm.

Example:

for (i<–0; i < list’s_length; i++)

find the smallest value in list from list[i]

down to the bottom of the list

swap the found value with list[i]

This will find the smallest value in the list from list[i] down to the bottom of the list and swap the found value with list[i]

Visual attached

Question 32

What should you use to sort an Array if you have multiple arrays to sort?

Answer 32

A generic sorting method.

This method recieves and array as a parameter (any array in the code) and sorts it.

Question 33

What has to be done to make the sorting method generic so that it can be used by multiple classes?

Answer 33

Put the sort method in a utility class.

Question 34

What is a utility class?

Answer 34

A class with general-purpose methods that other classes can easily use.

To make the methods easy to use, use class methods (as opposed to instance methods).

Question 35

What happens if you make the sort method an instance method?

Answer 35

You’re required to instantiate the sort method’s enclosing class prior to call the sort method.

Example: assuming the sort method’s enclosing class is named Sort:

Sort s = new Sort();

s.sort(studentIds);

Question 36

What happens if you make the sort method a class method?

Answer 36

You are not required to instantiate the sort method’s enclosing class prior to calling the sort method.

Instead, you simply prefix the sort method call with the class name and then dot.

Example: assuming the sort method’s enclosing class is named Sort:

Sort.sort(studentIds);

Question 37

What should you use if you have a group of related data that’s organized in a table format?

Answer 37

Two-dimensional array.

Question 38

How do two-dimensional arrays differ from regular arrays?

Answer 38

Same syntax except they use a second pair of []’s.

For example: int[][] x = {{1, 2, 3}, {4, 5, 6}};

The first index identifies the row and the second index identifies the column position within a row.

Question 39

What are the two ways to assign values into a two-dimensional array’s elements?

Answer 39

• An array initialzier (only useable if you know the values)
• Assignment statements

Examples:

Initializer = int [][] x = {{8, -2, 4}, {1, 0, 5}};

Assignment = attached

Question 40

What command allows you to loop through the rows in a two-dimensional array?

Answer 40

“array-name”.length

Question 41

What command allows you to loop through the elements within a particular row in a two-dimensional array?

Answer 41

“array-name”[0].length

Example: attached

Question 42

What must you ensure in order to create and array of objects?

Answer 42

• Instantiate the array with the new operator
• Instantiate each object that’s stored in the array with individual new operators

Question 43

What is a for-each loop?

Answer 43

Alternative to the traditional for loop. Used when you want to iterate through all array elements and don’t know or care which particular elements are loaded.

Example:

for (“element-type” “element-reference” : “array-reference”)

Spoken sounds like: For each “element-reference” in “array reference”

Visualized in attachement

Question 44

How does a for-each loop reduce code clutter?

Answer 44

• It avoids index initialization, testing and incrementing.
• Avoding indexing simplifies element method calls, instead of calling primes[i] you just call p

Question 45

What does a for-each loop not use when looping through its elements?

Answer 45

In index variable