COS 260 DAY 11 Tony Gauvin

Agenda Mini-quiz Five Today Friday the 13th Fixed Size Collections Questions Assignment 2 corrected Good Results Assignment 3 posted Due Oct 20 Assignment 4 will be posted soon Mini-quiz Five Today Friday the 13th Password “BreakingTheCode” Fixed Size Collections

Capstone Project Capstone Project Description Fall 2017.pdf October 2 Proposal Due E-mailed to Tony in Blackboard October 27 Progress Report E-mailed to Tony in Blackboard November 16 Progress Report E-mailed to Tony in Blackboard November 30 Progress Report E-mailed to Tony in Blackboard December 12@8AM All Deliverables & Presentation Due Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

Fixed-sized collections Objects First with Java Fixed-sized collections Introduction to arrays 6.0 © David J. Barnes and Michael Kölling

Features of arrays Fixed in length, unlike ArrayList, HashSet, HashMap, etc. Use a special syntax. For historical reasons. Objects with no methods. Methods are provided by other classes; e.g., java.util.Arrays. Methods that are static. https://docs.oracle.com/javase/specs/jls/se8/html/jls-10.html © 2017 Pearson Education, Inc. Hoboken, NJ. All rights reserved.

Fixed-size collections Objects First with Java Fixed-size collections Sometimes the maximum collection size can be pre-determined. A special fixed-size collection type is available: an array. Unlike the flexible List collections, arrays can store object references or primitive-type values (without autoboxing). © 2017 Pearson Education, Inc. Hoboken, NJ. All rights reserved. © David J. Barnes and Michael Kölling

The weblog-analyzer project Objects First with Java The weblog-analyzer project Web server records details of each access. Supports analysis tasks: Most popular pages. Busiest periods. How much data is being delivered. Broken references. Analyze accesses by hour – there is a fixed number of these in a day! © 2017 Pearson Education, Inc. Hoboken, NJ. All rights reserved. © David J. Barnes and Michael Kölling

Log file analyzer Parses entries from a log weblog.txt Year month day hour min 2015 06 01 03 31 Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

Creating an array object Objects First with Java Creating an array object Array type — does not contain size public class LogAnalyzer { private int[] hourCounts; private LogfileReader reader;   public LogAnalyzer() hourCounts = new int[24]; reader = new LogfileReader(); } ... Array object creation — specifies size © 2017 Pearson Education, Inc. Hoboken, NJ. All rights reserved. © David J. Barnes and Michael Kölling

Objects First with Java The hourCounts array © 2017 Pearson Education, Inc. Hoboken, NJ. All rights reserved. © David J. Barnes and Michael Kölling

Objects First with Java Using an array Square-bracket notation is used to access an array element: hourCounts[...] Elements are used like ordinary variables. The target of an assignment: hourCounts[hour] = ...; In an expression: hourCounts[hour]++; if(hourCounts[hour] > 0) ... © 2017 Pearson Education, Inc. Hoboken, NJ. All rights reserved. © David J. Barnes and Michael Kölling

Objects First with Java Standard array use private int[] hourCounts; private String[] names; ...   hourCounts = new int[24]; hourcounts[i] = 0; hourcounts[i]++; System.out.println(hourcounts[i]); declaration creation use © 2017 Pearson Education, Inc. Hoboken, NJ. All rights reserved. © David J. Barnes and Michael Kölling

Objects First with Java Array literals The size is inferred from the data. private int[] numbers = { 3, 15, 4, 5 }; declaration, creation and initialization Array literals in this form can only be used in declarations. Related uses require new: numbers = new int[] { 3, 15, 4, 5 }; © 2017 Pearson Education, Inc. Hoboken, NJ. All rights reserved. © David J. Barnes and Michael Kölling

Array Literals 3 15 4 5 1 private int[] numbers = { 3, 15, 4, 5 }; Declare numbers Create Initialize 3 15 4 5 1 Objects First with Java - A Practical Introduction using BlueJ, © David J. Barnes, Michael Kölling

Objects First with Java Array length private int[] numbers = { 3, 15, 4, 5 }; int n = numbers.length; not a method call! NB: length is a field rather than a method. It’s value cannot be changed – ‘fixed size’. © 2017 Pearson Education, Inc. Hoboken, NJ. All rights reserved. © David J. Barnes and Michael Kölling

Objects First with Java The for loop There are two variations of the for loop, for-each and for. The for loop is often used to iterate a fixed number of times. Often used with a variable that changes a fixed amount on each iteration. © 2017 Pearson Education, Inc. Hoboken, NJ. All rights reserved. © David J. Barnes and Michael Kölling

Objects First with Java For loop pseudo-code General form of the for loop for(initialization; condition; post-body action) { statements to be repeated } Equivalent while-loop version initialization; while(condition) { statements to be repeated post-body action } © 2017 Pearson Education, Inc. Hoboken, NJ. All rights reserved. © David J. Barnes and Michael Kölling

Objects First with Java Array iteration for loop version for(int hour = 0; hour < hourCounts.length; hour++) { System.out.println(hour + ": " + hourCounts[hour]); } while loop version int hour = 0; while(hour < hourCounts.length) { System.out.println(hour + ": " + hourCounts[hour]); hour++; } © 2017 Pearson Education, Inc. Hoboken, NJ. All rights reserved. © David J. Barnes and Michael Kölling

Array-related methods System has static arraycopy. java.util.Arrays contains static utility methods for processing arrays: binarySearch fill sort ArrayList has toArray. © 2017 Pearson Education, Inc. Hoboken, NJ. All rights reserved.

Objects First with Java Practice Given an array of numbers, print out all the numbers in the array, using a for loop. int[] numbers = { 4, 1, 22, 9, 14, 3, 9}; for ... © 2017 Pearson Education, Inc. Hoboken, NJ. All rights reserved. © David J. Barnes and Michael Kölling

Objects First with Java Practice Fill an array with the Fibonacci sequence. 0 1 1 2 3 5 8 13 21 34 ... int[] fib = new int[howMany]; fib[0] = 0; fib[1] = 1; for(...) ... © 2017 Pearson Education, Inc. Hoboken, NJ. All rights reserved. © David J. Barnes and Michael Kölling

for loop with bigger step Objects First with Java for loop with bigger step // Print multiples of 3 that are below 40. for(int num = 3; num < 40; num = num + 3) { System.out.println(num); } © 2017 Pearson Education, Inc. Hoboken, NJ. All rights reserved. © David J. Barnes and Michael Kölling

for loop and Iterator No post-body action required. for(Iterator<Track> it = tracks.iterator(); it.hasNext(); ) { Track track = it.next(); if(track.getArtist().equals(artist)) { it.remove(); } © 2017 Pearson Education, Inc. Hoboken, NJ. All rights reserved.

Objects First with Java Review Arrays are appropriate where a fixed-size collection is required. Arrays use a special syntax. For loops are used when an index variable is required. For loops offer an alternative to while loops when the number of repetitions is known. Used with a regular step size. © 2017 Pearson Education, Inc. Hoboken, NJ. All rights reserved. © David J. Barnes and Michael Kölling

The automaton project An array of ‘cells’. Each cell maintains a simple state. Usually a small numerical value. E.g., on/off or alive/dead. The states change according to simple rules. Changes affected by neighboring states. © 2017 Pearson Education, Inc. Hoboken, NJ. All rights reserved.

Cell states – blank cells are in state 0 A simple automaton nextState[i] = (state[i-1] + state[i] + state[i+1]) % 2; Step Cell states – blank cells are in state 0 + 1 2 3 4 5 6 © 2017 Pearson Education, Inc. Hoboken, NJ. All rights reserved.

The conditional operator Choose between two values: condition ? value1 : value1 for(int cellValue : state) { System.out.print(cellValue == 1 ? '+' : ' '); } System.out.println(); © 2017 Pearson Education, Inc. Hoboken, NJ. All rights reserved.

Further advanced material

Arrays of more than one dimension Array syntax supports multiple dimensions. E.g., 2D array to represent a game board, or a grid of cells. Can be thought of as an array of arrays. © 2017 Pearson Education, Inc. Hoboken, NJ. All rights reserved.

The brain project Cell[][] cells; ... cells = new Cell[numRows][numCols]; for(int row = 0; row < numRows; row++) { for(int col = 0; col < numCols; col++) { cells[row][col] = new Cell(); } © 2017 Pearson Education, Inc. Hoboken, NJ. All rights reserved.

Alternative iteration for(int row = 0; row < cells.length; row++) { Cell[] nextRow = cells[row]; for(int col = 0; col < nextRow.length; col++) { nextRow[col] = new Cell(); } ‘Array of array’ style. Requires no access to numRows and numCols. Works with irregular shape arrays, which are supported in Java. © 2017 Pearson Education, Inc. Hoboken, NJ. All rights reserved.

Arrays and Streams java.util.Arrays has several stream() methods that return a Stream based on an array. IntStream is a Stream of int values. Instream.range() creates a sequential IntStream. toArray returns an array from a Stream. © 2017 Pearson Education, Inc. Hoboken, NJ. All rights reserved.