Understanding the Use of Arrays

Understanding the Use of Arrays
with Examples in Java

Prof. David Bernstein
James Madison University

Computer Science Department

bernstdh@jmu.edu

Review: Opinions?

What do you think of the following fragment?

total = 0.0; total = total + income1; total = total + income2; total = total + income3; total = total + income4; total = total + income5; total = total + income6; total = total + income7; mean = total / 7.0;

Review: An Example of an Array

Arrays offer the same kinds of benefits of subscripts in mathematical notation -- this fragment is more compact, more readable, and more flexible.

total = 0.0; for (int i=0; i<n; i++) { total = total + income[i]; } mean = total / income.length;

Review: An Array in Memory

Review: A Physical Analogue

The Entities:
- An element of an array is like an apartment
- An array is like an apartment building
The Physics:
- Apartments can't be moved, created, or destroyed
- Each apartment can be referred to by number
- Can change the contents of each apartment but not the apartments themselves

Review: Arrays in Java

Declaration using the [] Modifier:
- Syntax: base_type[] array_name;
- Example: int[] income;
Memory Allocation using the [] Length Specifier:
- Syntax: array_name = new base_type[length];
- Example: income = new int[20];
Accessing Elements using the [] Operator:
- Syntax: array_name[index]
- Example: income[5] = 250000;

Elements of Arrays as Parameters

Actual Parameters:
- Syntax: array_name[index]
- Example: x = payment(income[0]);
Formal Parameters:
- Can't be array elements

Arrays as Parameters

Formal Parameters:
- Syntax: (base_type[] array_name)
- Example: payment(int[] income)
- Example: main(String[] args)
Actual Parameters:
- Syntax: (array_name)
- Example: y = meanPayment(taxableIncome);

Returning an Array

Syntax:
- base_type[] method_name([formal_parameter]...)
Example:
- public int[] schedule(){...}

Attributes and Methods Owned by Arrays

Attributes:
- length contains the number of elements (and is initialized when memory is allocated)
Methods:
- clone() makes a (shallow) copy

An Example of the length Attribute

int i; int[] strikeouts; strikeouts = new int[9]; for (i=0; i<strikeouts.length; i++) { strikeouts[i] = 0; }

An Example of the length Attribute (cont.)

int i; int[] strikeouts; strikeouts = new int[9]; . . . // This will not work because the length attribute // is declared to be final (which means it cannot be // changed after it is initialized) // strikeouts.length = 5;

An Example of the clone() Method

int[] copy, original; original = new int[3]; original[0] = 2001; original[1] = 1995; original[2] = 2001; copy = original.clone();

Common Faults and Symptoms

Common Faults:
- Failing to allocate any memory
- Failing to allocate enough memory
- Accessing element 0
- Accessing element length
Obvious Symptoms:
- NullPointerException when using method/attribute
- IndexOutOfBoundsException when accessing an element
Less Obvious Symptoms:
- Calculations that omit the 0th or last element (or both)

Arrays Utility Class

The Arrays class (in the java.util package) class contains several static methods that are very convenient when working with arrays.

import java.util.Arrays; int target; int[] a; a = new int[100]; Arrays.fill(a, 0); // Fill a with zeroes Arrays.fill(a, 1, 10, 99) // Fill a[10] thru a[99] with ones ... Arrays.sort(a); // Sort a in ascending order // Searches for 5 in a and returns 5 if found // The array must be sorted // target = Arrays.binarySearch(a,5);

There's Always More to Learn