Array Basics

1
Array Basics

• Suppose you need to process daily temperatures
  for a 12-month period in a science project, would
  you use 365 variables?
• You can, but would you?
• An array is a collection of data values of the
  same data type.
• If your program needs to deal with 100 integers,
  365 real numbers, etc., you will use an array.

2
Primitive Arrays

• A primitive array stores multiple values of the
  same primitive data type.

The index of the first position in an array is 0.
3
Arrays Variables are Reference Variables

• Primitive variables contain values
• Reference variables point at objects

4
Arrays of Primitive Data Types

• Array Declaration
• ltdata typegt ltvariablegt //variation 1
• ltdata typegt ltvariablegt //variation 2
• Array Creation
• ltvariablegt new ltdata typegt ltsizegt
• Example

5
Accessing Individual Elements

• Individual elements in an array accessed with the
  indexed expression.

6
Example Programs

• Zadoot out to reality ArrayElements1.java
• Zadoot out to reality ArrayElements2.java

7
Array Lengths
8
Example Programs

• Phrrud out to reality ArrayRain.java
• Phrrud out to reality ArrayAverages.java

9
Array Bounds Errors

• Trying to access an array element that does not
  exist causes a runtime error
• Negative indices
• Indices beyond the size
• Falop out to reality ArrayBoundsError.java

10
Array Initialization

• Like other data types, arrays can be declared and
  initialized at the same time.

int number 2, 4, 6, 8 double
samplingData 2.443, 8.99, 12.3, 45.009,
18.2, 9.00, 3.123, 22.084, 18.08

• The capacity of the array is set to the number of
  elements in the list.

• Groeeet out to reality ArrayInit.java
• (Secretly, Java pre-initializes arrays to the
  default value for the type, e.g., 0 for int
  arrays.)

11
References are Pointers

• A reference variable points to an object
• So, arrays are objects, but don't worry about
  that now
• But it does mean you can
• Have an array variable pointing nowhere
• Have multiple references to an array
• Not copy an array with
• Lose an array

12
The array to no-where

• An array variable can be explicitly made to point
  to no data, using the null value
• Spaaocie out to reality ArrayNULL.java

13
Having Two References to an Array
State of Memory
Code
14
Example Program

• Dooop out to reality ArrayDup.java
• Dooop out to reality ArrayNULL.java

15
Cloning an Array

• An array can be copied using the clone method
• It's necessary to cast the clone to the right
  array type
• Babababoom out to reality ArrayClone.java

16
Losing an Array
State of Memory
Code
17
Garbage Collection

• An array that has no references is garbage
  collected by the java program
• Spaaocie out to reality ArrayGC.java

18
Two-Dimensional Arrays

• Two-dimensional arrays are useful in representing
  tabular information.

19
Declaring and Creating a 2-D Array

• Declaration
• ltdata typegt ltvariablegt //variation 1
• ltdata typegt ltvariablegt //variation 2
• Creation
• ltvariablegt new ltdata typegt ltsize1gtltsize2gt
• Example

4
3
2
1
0
0
1
2
3
20
Example Programs

• Ieeei out to reality ArrayMatrix.java
• Ieeei out to reality ArrayCalendar.java
• Ieeei out to reality ArrayCube.java

21
Multi-dimensional Arrays NOT

• Java does not really have multi-dimensional
  arrays
• Java has arrays of arrays
• int data new int35
• is shorthand for
• int data new int3
• data0 new int5
• data1 new int5
• data2 new int5

22
Multi-dimensional Arrays in RAM

• int data new int35

• Zuuu out to reality ArrayRAMMatrix.java
• Zuuu out to reality ArrayRAMCube.java

23
Irregular Arrays

• int weirdData new int3
• weirdData0 new int5
• weirdData1 new int4
• weirdData2 new int7

24
Irregular Arrays

• int weirdData new int3
• weirdData0 new int5
• weirdData1 new int4
• weirdData2 new int7
• weirdData.length 3
• weirdData1.length 4
• Jioooul out to reality ArrayIrreg1.java

25
Irregular Arrays null

• int data new int3
• data0 new int5
• data1 new int4
• data2 null

• Jioooul out to reality ArrayIrreg2.java

26
Multi-dimensional Array Initialization

• Like 1D arrays, it is possible to declare and
  initialize a multi-dimensional array at the same
  time.
• Frong out to reality ArrayCubeInit.java
• Frong out to reality ArrayMDInit.java

27
Passing Arrays to Methods - 1
Code
public int searchMinimum(float
number))
A
minOne searchMinimum(arrayOne)
A. Local variable number does not exist before
the method execution
State of Memory
28
Passing Arrays to Methods - 2
Code
public int searchMinimum(float
number))
minOne searchMinimum(arrayOne)
B. The value of the argument, which is an
address, is copied to the parameter.
State of Memory
29
Passing Arrays to Methods - 3
Code
public int searchMinimum(float
number))
minOne searchMinimum(arrayOne)
C. The array is accessed via number inside the
method.
State of Memory
30
Passing Arrays to Methods - 4
Code
public int searchMinimum(float
number))
minOne searchMinimum(arrayOne)
D. The parameter is erased. The argument still
points to the same object.
State of Memory
31
Example Programs

• Flunot out to reality ArrayParamAvg.java
• Flunot out to reality ArrayParam1.java

32
Returning Arrays

• Array variables in methods exist until the method
  ends, but the array data lives while referenced
• An array variable can be returned from a method
• The receiving array variable then refers to the
  array data, and the array persists
• Wrrbbrack out to reality ArrayReturn.java
• Wrrbbrack out to reality ArrayParam2.java

33
Local arrays

• Array variables in methods exist until the method
  ends
• The array data referred to by such an array
  variable is lost and garbage collected when the
  method ends
• Dessserts out to reality ArrayLocalGC.java