Arrays of Primitive Values, Schemes and Mind Maps of Computer Science

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Arrays of Primitive Values

15-110 Summer 2010

Margaret Reid-Miller

Arrays

• Arrays are objects that hold multiple values of the

same type.

• Each data value stored in an array is called an

element.

• Each element is accessed using an integer index or

subscript.

• As with strings, the first subscript is 0.
• Organizing data in an array allows programs to

access huge amount of data multiple times and in

any order!

Creating an Array

int[] counts = new int[23];!* counts[0] = 12;! counts[1] = 22;! counts[2] = 17;! counts[3] = 16;! counts[4] = 4;! counts[5] = 11;

[0] [1] [2] [3] [4] [5]

counts^12 22 17 16 4

bacteria colony count on

6 Petri dishes

no length length can be an int expression reference to the array

int sum = 0;! for (int i = 0; i < counts.length; i++) {! sum = sum + counts[i];! }! if (counts.length > 0){! System.out.print(“The average number of “ + !!! “bacteria colonies is “);! System.out.println(________________________);! }

Array Traversal

Constant : number of elements in the array Not a method: There is no () !!

Array Index

• The index of an array must be a literal, variable, or

expression of type int.

E.g.,

x[0] = 3; assign the first element 3;

x[num] = num; use an int variable as index

x[j-3] = x[j]; use an int expression as index

x[x[0]] = max; use an int array element as index

Bounds Checking

[0] [1] [2] [3] [4] [5]

index > 5 is out of bounds

• Out-of-bounds errors occur when you attempt to

access an array element that does not exist.

• If you write x[index] the compiler cannot

determine if index is out of bounds.

• When your run the program, however, Java checks

whether the index is out of bounds.

index < 0 is out of bounds

Initializer List

• You can declare, create, and initialize an array with a

list of literals.

! int[] counts = {12, 22, 17, 16, 4, 11};!

• You can use an initializer list only when the array is

first declared.

• Each value must match the type of the array.
• The values go into the array in the order given and

determine the length of the array.

Example

Find the minimum value stored in an array:

int min = _________ ;! for (int i = __; i < counts.length; i++) {! if (________________) {! min = counts[i];! }! }!

• What happens if there are two or more values in the

array that are the minimum?

• How do we modify the code to return the index of the

minimum bacteria count?!

counts[0] 1!

(counts[i] < min)!

! int[] sequence = new int[10];! ! fillAllSums(sequence);! ! …! }!!!!!!!! !! !!!!!!!! !! public static void fillAllSums(int[] seq) {! ! seq[0] = 0;! ! for (int i = 1; i < seq.length; i++) {! !! seq[i] = seq[i-1] + i;! ! }! }! sequence!

Arrays as Parameters

seq! 0 1 3 6 10 15 21 28 36 45 ae2f ae2f ae2f

! int[] sequence = new int[10];! ! fillAllSums(sequence);! ! …! }! ! public static void fillAllSums(int[] seq) {! ! seq[0] = 0;! ! for (int i = 1; i < seq.length; i++) {! !! seq[i] = seq[i-1] + i;! ! }! }!

Arrays as Parameters

0 1 3 6 10 15 21 28 36 45 Changes to an array in a method are visible outside the method! sequence! ae2f ae2f

The null reference

• An array variable holds the special value null if no

array is created (distinct from an array with 0 elements.)

int array[] counts;!!

public static int[] copy(int[] data){! if (data == null) return null;! int[] data2 = new int[data.length];! for (int i = 0; i < data.length; i++){! data2[i] = data[i];! }! return data2;! }! counts! null! Causes nullPointerException if data is null!