BIT115: Introduction to Programming Lecture 16 Instructor: Craig Duckett ARRAYS

Assignment 3 Revision Assignment 3 Revision (LECTURE 16) DUE TONIGHT The Fickle Finger of Fate

Assignment Dates (By Due Date) Assignment 1 (LECTURE 5) GRADED! Section 1: Monday, January 22nd Assignment 2 (LECTURE 8) GRADED! Section 1: Wednesday, January 31st Assignment 1 Revision (LECTURE 10) GRADED! Section 1: Wednesday, February 7th Assignment 2 Revision (LECTURE 12) GRADED! Section 1: Wednesday, February 14th Assignment 3 (LECTURE 13) GRADED! Section 1: Wednesday, February 21st Assignment 3 Revision (LECTURE 16) Section 1: Monday, March 5th Assignment 4 (LECTURE 18) NO REVISION AVAILABLE! Section 1: Monday, March 12th The Fickle Finger of Fate

Today’s Topics Introduction to Arrays Chapter 10.2: Creating Arrays Chapter 10.5: Arrays of Primitives Chapter 10.1.1-10.1.7: Arrays of Objects [If Time]

And now ... The Quiz

What is an Array? An array is a named collection of variables that share the same data type (for example, an int). It is like a list of items.

Introduction to Arrays What is an Array? Primitive variables are designed to hold only one value at a time. Arrays allow us to create a collection of like values that are indexed. An array can store any type of data but only one type of data at a time. An array is a list of data elements. Let’s see what this all means...

Introduction to Arrays What is an Array? So far, you have been working with variables that hold only one value. The integer variables you have set up have held only one number (and next week we will see how string variables will hold one long string of text). An array is a collection to hold more than one value of the same data type at a time. It's like a list of items—a list of integers, or a list of doubles, or a list of chars, or a list of strings, etc Think of an array as like the columns in a spreadsheet. You can have a spreadsheet with only one column, or several columns. The data held in a single-list (one-dimensional) array might look like this: index grades 100 1 89 2 96 3 4 98

Introduction to Arrays index grades 100 1 89 2 96 3 4 98 Now, the way we might have declared data like this up until now is to do something along these lines: int value1 = 100; int value2 = 89; int value3 = 96; int value4 = 100; int value5 = 98; However, if we knew before hand that we were going to be declaring five int integers (or ten, or fifteen, etc), we could accomplish the same type of declaration by using an array. To set up an array of numbers like that in the table above, you have to tell Java what type of data is going into the array, then how many positions the array has. You’d set it up like this: int[ ] grades; NOTE Arrays must be of the same data type, i.e., all integers (whole numbers) or all doubles (floating-point numbers) or all strings (text characters)—you cannot “mix-and-match” data types in an array.

Introduction to Arrays index grades 100 1 89 2 96 3 4 98 int[ ] grades; The only difference between setting up a primitive integer variable and an array is a pair of square brackets [ ] after the data type. The square brackets are enough to tell Java that you want to set up an array. The name of the declared array above is grades. Just like primitive variables, you can call them almost anything you like (except Java defined keywords). While the square brackets tells Java you want to set up an array, it doesn't say how many positions the array should hold. To do that, you have to set up a new array object: int[ ] grades; grades = new int[5]; // <-- New array object In between the square brackets you need the pre-defined size of the array. The size is how many slots (elements) the array should hold. If you prefer, you can put all that on one line: int[ ] grades = new int[5]; // Done at same time

How an Array is “Set Up” with the Code int [] grades; // This lets compiler know a // collection is coming, but not // its length grades = new int[5]; // This sets up the number // of slots (elements) based // on the length int [] grades = new int[5]; // This does both at same time

How an Array is “Set Up” with the Code int [] grades = new int[5]; grades 0 1 2 3 4 0 0 0 0 0 grades[0] = 100; grades[1] = 89; grades[2] = 96; grades[3] = 100; grades[4] = 98; grades 0 1 2 3 4 100 89 96 100 98

How an Array is “Set Up” with the Code // You can accomplish the same thing using an array literal int [] grades = {100, 89, 96, 100, 98}; grades 0 1 2 3 4 100 89 96 100 98

Introduction to Arrays: Example import java.util.*; public class Array_Demo extends Object { public static void main(String[] args) { int [] grades = new int[5]; grades[0] = 100; grades[1] = 89; grades[2] = 96; grades[3] = 100; grades[4] = 98; // <-- Something especially groovy happens here! } }

Introduction to Arrays When you declare an array with a given data type, name and number, like int [] grades = new int[5]; you are reserving a collection space in memory by that name, sized according to data type, and large enough to separately contain enough data for the declared size. index grades 100 1 89 2 96 3 4 98 The number inside the brackets is the array’s size declarator or length. It indicates the number of elements, or values, the array can hold. In the declaration above, grades references an array with enough memory being reserved for five (5) integer values. STEP 1: Declare Variable STEP 2: Allocate Memory STEP 3: Initialize Elements grades variable Element1 Element2 Element3 Element4 Element5

Introduction to Arrays: Example import java.util.*; public class Array_Demo extends Object { public static void main(String[] args) { int [] grades = new int[5]; grades[0] = 100; grades[1] = 89; grades[2] = 96; grades[3] = 100; grades[4] = 98; // <-- Something especially groovy happens here! } }

int [] grades = new int[5]; grades is a named reserved space set aside to hold exactly five [5] 32-bit elements all initializing to a value of zero 0. As we have learned about programming languages, the “index” always starts at 0, not 1, and procedes until the size of the array is reached. In our example, since we declared [5] the array element index starts with 0 and ends at 4. array element index  1 2 3 4 32-bits 32-bit space reserved for data  value initialized in element  index grades 100 1 89 2 96 3 4 98 int [] rades = new int[5]; // Steps 1 & 2 grades[0] = 100; // Steps 3 grades[1] = 89; grades[2] = 96; grades[3] = 100; grades[4] = 98; STEP 1: Declare Variable STEP 2: Allocate Memory STEP 3: Initialize Elements 5 1 2 3 4 100 89 96 98 grades

Introduction to Arrays: Example import java.util.*; public class Array_Demo extends Object { public static void main(String[] args) { int [] grades = new int[5]; grades[0] = 100; grades[1] = 89; grades[2] = 96; grades[3] = 100; grades[4] = 98; // <-- Something especially groovy happens here! } }

Introduction to Arrays index grades 100 1 89 2 96 3 4 98 You can also declare the int separately and call it by its given name, like this: int testScores = 5; int[ ]grades = new int[testScores]; (Whether you call the number inside the brackets or a named variable is up to your particular style of coding and preference.) So we are telling Java to set up an array with 5 positions in it. After this line is executed, Java will assign default values for the array. Because we've set up an integer array, the default values for all 5 positions will be zero ( 0 ). To assign values to the various positions in an array, you do it in the normal way: grades[0] = 100; grades[1] = 89; grades[2] = 96; grades[3] = 100; grades[4] = 98; If you know what values are going to go in the array, you can also set them up like this: int[ ] grades = { 100, 89, 96, 100, 98 }; // Java treats as new instance grades [0] = 100; grades [1] = 89; grades [2] = 96; grades [3] = 100; grades [4] = 98; { Length of the array is equal to the number of slots declared This is called the index

Length Index Subscripts int [] grades = new int[5]; grades[0] = 100; grades[1] = 89; grades[2] = 96; grades[3] = 100; grades[4] = 98; Length int [] grades = new int[5]; grades[0] = 100; grades[1] = 89; grades[2] = 96; grades[3] = 100; grades[4] = 98; int [] grades = new int[5]; grades[0] = 100; grades[1] = 89; grades[2] = 96; grades[3] = 100; grades[4] = 98; Index Subscripts

import java.util.*; public class Array_Demo extends Object { public static void main(String[] args) { // Setting up the integer 5-element array: int [] grades = new int[5]; grades[0] = 100; grades[1] = 89; grades[2] = 96; grades[3] = 100; grades[4] = 98; // Of course you could have done it this way: // int [] grades = {100, 89, 96, 100, 98}; int i; for(i = 0; i < grades.length; i++) { System.out.println("Grade " + (i + 1) + " is: " + grades[i]); } } }

Arrays of Primitives: Overview An array is an object so it needs an object reference. // Declare a reference to an array that will hold integers. int[] numbers; The next step creates the array and assigns its address to the numbers variable // Create a new array that will hold 6 integers. numbers = new int[6]; subscript 0 subscript 1 subscript 2 subscript 3 subscript 4 subscript 5 Array element values are initialized to 0 by default. Array indexes always start at 0. The first element in an array index is always subscript 0.

Arrays of Primitives: Overview It is possible to declare an array reference and create it in the same statement. int[] numbers = new int[6]; Arrays may be of any type. float[] temperatures = new float[100]; char[] letters = new char[41]; long[] units = new long[50]; double[] sizes = new double[1200]; long - 8 bytes signed. Ranges from -9,223,372,036,854,775,808 to +9,223,372,036,854,775,807. float - 4 bytes. Covers a range from 1.40129846432481707e-45 to 3.40282346638528860e+38 (positive or negative). double - 8 bytes. Covers a range from 4.94065645841246544e-324d to 1.79769313486231570e+308d (positive or negative). char - 2 bytes, unsigned, Unicode, 0 to 65,535. Chars are not the same as bytes, ints, shorts or Strings.

Arrays of Primitives: Overview The array length must be a non-negative number. It may be a literal value, a constant, or variable. int arraySize = 6; ( or final int ARRAY_SIZE = 6; ) int[] numbers = new int[arraySize]; Once created, an array length is fixed and cannot be changed.

numbers[0] = 20; //pronounced "numbers sub zero" Accessing the Elements of an Array numbers[0] numbers[1] numbers[2] numbers[3] numbers[4] numbers[5] 20 An array is accessed by: the reference name (e.g., numbers) a subscript that identifies which element in the array to access. numbers[0] = 20; //pronounced "numbers sub zero" Name Subscript

Inputting and Outputting Array Elements Array elements can be treated as any other variable. They are simply accessed by the same name and a subscript. See example: ArrayDemo1.java Array subscripts can be accessed using variables (such as for loop counters). See example: ArrayDemo2.java

Bounds Checking Array indexes always start at zero and continue to (array length - 1) int values = new int[10]; This array would have indexes 0 through 9 See example: InvalidSubscript.java In for loops, it is typical to use i, j, and k as counting variables It might help to think of i as representing the word index

Watch for “Off-by-One” Errors It is very easy to be “off-by-one” when accessing arrays // This code has an off-by-one error. int[] numbers = new int[100]; for (int i = 1; i <= 100; i++) // Would work with < only { numbers[i] = 99; } Here, the equal sign allows the loop to continue on to index 100, but 99 is the last index in the array This code would throw an ArrayIndexOutOfBoundsException

Array Initialization When relatively few items need to be initialized, an initialization list can be used to initialize the array int[]days = {31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31}; The numbers in the list are stored in the array in order: days[0] is assigned 31, days[1] is assigned 28, days[2] is assigned 31, days[3] is assigned 30, etc. See example: ArrayInitialization.java

Alternate Array Declaration Previously we showed arrays being declared: int[] numbers; However, the brackets can also go here: int numbers[]; These are equivalent but the first style is typical (and preferred by most developers/coders). Multiple arrays can be declared on the same line. int[] numbers, codes, scores; With the alternate notation each variable must have brackets. int numbers[], codes[], scores; The scores variable in this instance is simply an int variable.

Processing Array Contents Processing data in an array is the same as any other variable grossPay = hours[3] * payRate; Pre and post increment works the same: int[] score = {7, 8, 9, 10, 11}; ++score[2]; // Pre-increment operation score[4]++; // Post-increment operation See example: PayArray.java

Processing Array Contents Array elements can be used in relational operations if(cost[20] < cost[0]) { //statements } They can be used as loop conditions: while(value[count] != 0)

Array Length Arrays are objects and provide a public field named length that is a constant that can be tested double[] temperatures = new double[25]; The length of this array is 25. The length of an array can be obtained via its length constant int size = temperatures.length; The variable size will contain 25.

The Enhanced for Loop Simplified array processing (read only) Always goes through all elements General: for(datatype elementVariable : array) statement; Example: int[] numbers = {3, 6, 9}; for(int val : numbers) // <-- Only two parts. You can read the line as // "iterate on elements from the collection named numbers. The current // element will be referenced by the int val." { System.out.println("The next value is " + val); }

The Enhanced for Loop int[] numbers = {3, 6, 9}; for(int val : numbers) { System.out.println("The next value is " + val); } Java knows an Enhanced for loop when it sees one (the colon gives it away), so instead of using a counter as with the first part of a typical loop, it just looks for the length of the named array. As it loops it grabs the data from each subscript (part 1 of the enhanced for) that belongs to the named array (part 2 of the enhanced for) and walks (or auto-increments) the length of the array one element at a time.

Array Size The length constant can be used in a loop to provide automatic bounding. for(int i = 0; i < temperatures.length; i++) { System.out.println("Temperature " + i ": " + temperatures[i]); } Index subscripts start at 0 and end at one less than the array length.

ICE: Arrays