LECTURE NOTES
CHAPTER 8

Skip: 8.2, autoboxing, 8.4

ARRAYS

- manipulate a collection of related values
- impractical to use a sequence of variables
- array
  - special kind of object
  - sequence of values of the same type

- construction:
  new <type>[<length>];
  - length: number of elements in array

Example:

  new int[7]; // Store daily low temps for this week

- declare variable to hold array reference
  <type>[] <name>
  - type can be a primitive or an object
  - think of <type>[] as an "array type"

  - what's an example that we've seen?
  - main method actually takes a list of String arguments
  - user can supply these when executing your program from a command line

Example:

  int[] lowTemps;

- default values:
  - 0: numbers
  - false: booleans
  - null: objects

- index
  - number used to reference a particular element
  - indices start at 0
  - index of last element is one less than array length
  - access an element of the array
  <identifier>[<index>]

Example:

  int lowSunday = temp[0];

  ask them --> int lowSaturday = temp[6];
               int lowSaturday = temp[7];           

- bounds errors
  - access a nonexistant element
  - error handling: exception and termination

- length field returns length:
  <array variable>.length
  - anomoly that not a method
  - final: cannot be changed
- length is fixed
- remember:
  - String: a.length()
  - array: a.length

Example:

  // Call doSomething on every element of array
  for (int i = 0; i < array.length; i++) {
      doSomething(array[i]);
  }

- initialization
  - remember to initialize an array before trying to access it
  - initialize elements on creation
    - created array will have length of the number of elements specified
    - syntax:
      <type>[] <identifier> = {<list of values>}
      new <type>[] {<list of values>}

Example:

  1) Suppose weekly lows are: 27, 28, 32, 17, 25, 13, 19.
     Make an array weekLows with these values:

     int[] weekLows = {27, 28, 32, 17, 25, 13, 19};

  2) A static Weather method takes an array of ints and calculates the average.
     Call this method with weekLows, without storing the values in a variable.

     Weather.average(new int[] {27, 28, 32, 17, 25, 13, 19});

WRAPPERS

- wrapper class: turn a primitive into an object
- one for each primitive type:
  byte    Byte
  boolean Boolean
  char    Character
  double  Double
  float   Float
  int     Integer
  long    Long
  short   Short
  - upper case letters
  - Character and Integer differ from primitive names
- each object contains a value of the corresponding primitive type
- used anywhere objects are required instead of primitives
- create with a constructor: provide primitive as argument
- assign object to primitive: use <primitive>Value() method
- storing wrapped numbers is quite inefficient

TWO-DIMENSIONAL ARRAYS

- arrays store linear sequences
- collection with 2D layout

- declare an array where each element is another array

- declaration syntax
  <type>[][] <identifier>
- creation syntax
  new <type>[<rows>][<cols>]
- access syntax
  <identifier>[<row>][<col>]

- use nested loops to access all elements

Example:

int dim = ...
double[][] prod, mul1, mul2;
prod = new double[dim][dim];
mul1 = new double[dim][dim];
mul2 = new double[dim][dim];

// populate mul1 and mul2

// matrix multiply
for (int i = 0; i < dim; i++) {
    for (j = 0; j < dim; j++) {
        prod[i][j] = 0;
        for (k = 0; k < dim; k++) {
            prod[i][j] += mul1[i][k] * mul2[k][j];
        }
    }
}

Picture

- arrays with variable row lengths

  - allocation

    - allocate an array to hold all of the rows
      - don't specify their size
      - syntax:
        <type>[][] <identifier> = new <type>[<rows>][];

    - allocate each row separately
      <identifier>[<row>] = new <type>[<cols>];

  - access
    - <identifier>[<row>][<col>]
    - be especially careful that col is a valid index

  - uncommon

Example:

int[][] march = new int[5][];
march[0] = new int[5];
march[1] = new int[7];
march[2] = new int[7];
march[3] = new int[7];
march[4] = new int[5];

for (int i = 0; i < march.length; i++) {
    for (int j = 0; j < march[i].length; j++) {
        // do something
    }
}

Picture

MULTIDIMENSIONAL ARRAYS

  - arrays with more than two dimensions

  - cube array definition
    <type>[][][] <identifier> = new <type>[<d1>][<d2>][<d3>];

  - especially rare in OO programming

Example:

Molecule[][][] = new Molecule[][][];

COPYING ARRAYS

- an array variable holds a reference to the actual array
- copying the variable yields another reference to the same array

Picture

- make a true copy
  - clone method
  - cast return type from Object to array type

Picture

- copy elements from one array into another
  - static System.arraycopy(from, fromStart, to, toStart, count)
  - add or remove elements in the middle of an array
    - add a new element at position i
      - move all elements from i onword one position up
        System.arraycopy(data, i, data, i + 1, data.length - i - 1);
      - insert new value
        data[i] = x;
      - last element is lost

Picture

    - remove the element at i
      - copy all elements above downward
        System.arraycopy(data, i + 1, data, i, data.length - i - 1);

    - grow an array that has run out of space
      - create a new, larger array
        double[] newData = new double[2 * data.length];
      - copy all elements into the new array
        System.arraycopy(data, 0, newData, 0, data.length);
      - store the reference to the new array in the array variable
        data = newData;

Picture

SIMPLE ARRAY ALGORITHMS

- ArrayAlgorithms.java
- counting matches
- linear search
- finding the maximum
- finding the minimum