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BIT115: Introduction to Programming

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  1. BIT115: Introduction to Programming Instructor: Craig Duckett Lecture 17

  2. Lecture 17 Announcements Assignment 3 Revision DUE TONIGHTMonday, November 24th, by midnight Assignment 4 DUE Monday, December 1st(In ONE Week from Today) Assignment 4 Revision DUE Monday, December 8th(In TWO Weeks from Today) THANKSGIVING Holiday: SCHOOL CLOSED November 27, 28

  3. 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]

  4. 10 Programming Languages You Should Learn in 2014 http://mashable.com/2014/01/21/learn-programming-languages/?utm_cid=mash-com-fb-main-link

  5. And now ... The Quiz

  6. Introduction to Arrays What is an Array? • Primitive variables are designed to hold only one value at a time. • Arraysallow us to create a collection of like valuesthat are indexed. • An arraycan store any type of data but only one type of data at a time. • An arrayis a list of data elements. Let’s see what this all means...

  7. Introduction to Arrays What is an Array? So far, you have been working with variables that hold only onevalue. The integer variables you have set up have held only one number (and later in the quarter we will see how stringvariables 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:

  8. Introduction to Arrays 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 typeof 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.

  9. Introduction to Arrays 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 = newint[5]; // <-- New array object In between the square brackets you need the pre-definedsize 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 = newint[5]; // Done at same time

  10. Introduction to Arrays: Example importjava.util.*;public class Array_Demoextends 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! } }

  11. Introduction to Arrays When you declare an array with a given datatype, name and number, like grades = newint[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. The number inside the brackets is the array’s size declarator. 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 integer values. STEP 1: Declare Variable STEP 2: Allocate Memory STEP 3: Initialize Elements grades variable Element1 Element2 Element3 Element4 Element5

  12. Introduction to Arrays: Example importjava.util.*;public class Array_Demoextends 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! } }

  13. grades = newint[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  space reserved for data  value initialized in element  grades = newint[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 grades

  14. Introduction to Arrays: Example importjava.util.*;public class Array_Demoextends 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! } }

  15. Introduction to Arrays You can also declare the int separately and call it by its given name, like this: int testScores = 5; int[ ] grades = newint[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

  16. importjava.util.*;public class Array_Demoextends 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};inti;for(i = 0; i < grades.length; i++) {System.out.println("Grade " + (i + 1) + " is: " + grades[i]); } } }

  17. 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 = newint[6]; 0 0 0 0 0 0 index 0 index 1 index 2 index 3 index 4 index 5 Array element values are initialized to 0.Array indexes always start at 0.

  18. 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.

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

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

  21. 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

  22. 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 forloops, it is typical to use i, j, and k as counting variables • It might help to think of i as representing the word index

  23. “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 (inti = 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

  24. 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

  25. 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.

  26. 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

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

  28. Array Length • Arrays are objects and provide a public field named lengththat 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.

  29. The Enhanced for Loop • Simplified array processing (read only) • Always goes through all elements • General: for(datatypeelementVariable : array) statement; Example: int[] numbers = {3, 6, 9}; for(intval : 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 intval." { System.out.println("The next value is " + val); }

  30. 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.

  31. ICE: Arrays BIT115 > Lecture 17 > IN-CLASSES EXERCISE: Arrays