Peter Andreae Computer Science Victoria University of Wellington Copyright: Peter Andreae, Victoria University of Wellington Arrays with meaningful indices COMP 102 # T1

© Peter Andreae COMP :2 Menu Limitations on the for each loop Arrays with meaningful indices Assignment 8 Administrivia:

© Peter Andreae COMP :3 Can’t use “foreach” everywhere Can’t use to change the values in the array for (int i = 0 ; i < data.length ; i = i+1) { data[ i ] = data[i] * 2; } for (int value : data) { value = value * 2; } If array contains objects, you can modify the objects: (but not change them!) for (Shape sh : shapes) { sh.setColor(Color.white); } data: value: shapes: sh: c: red x: y: c: blue x: y:

© Peter Andreae COMP :4 Arrays with Meaningful Indices Arrays to store a collection: index doesn't mean anything Sometimes, the index represents meaningful information array of guest names for hotel rooms (numbered 1 to MaxRoom) house info for each house on street count of occurrences of each number image p1p1 ?? count: 7 p5p5 p3p3 p8p8 p2p2 p6p6 p4p4 data: n1n1 n5n5 n8n8 n 10 n7n7 n3n3 n6n6 n4n s1s1 null s5s5 s3s3 s8s8 s2s2 s6s6 s9s9 names:

© Peter Andreae COMP :5 Hotel Register initialise empty private String[ ] guests = new String[MaxRoom+1]; // gives indexes from 0 to MaxRoom, ignore index 0 assign to room public void assign(String name; int room){ this.guests[room] = name; } look up to see if empty public boolean isEmpty(int room){ return (this.guests[room]==null); } n1n1 n5n5 n8n8 n 10 n7n7 n3n3 n6n6 n4n4 guests: Don ’ t need to search

© Peter Andreae COMP :6 Hotel Register: remove Checkout guest from room /* returns true if name was checked out of room successfully, false otherwise */ public boolean checkout(String name, int room){ if ( this.guests[room].equals(name) ){ this.guests[room] = null; return true; } return false; } if ( this.guests[room] != null && this.guests[room].equals(name) ) … if ( name.equals(this.guests[room]) ) … What ’ s the problem? How do we fix it? n1n1 n5n5 n8n8 n 10 n7n7 n3n3 n6n6 n4n4 guests:

© Peter Andreae COMP :7 Alternative Checkout guest: search and remove: /* returns true if name was checked out successfully, false otherwise */ public boolean checkout(String name){ for ( int r=1 ; r<this.guests.length ; r++ ){ // or <=MaxRoom if (this.guests[r] != null ){ if (this.guests[r].equals(name) ){ this.guests[r] = null; return true; } return false; } n1n1 n5n5 n8n8 n 10 n7n7 n3n3 n6n6 n4n4 guests:

© Peter Andreae COMP :8 Find an empty room Find the index of an empty room public int findEmpty(int room){ for (int r=1; r<this.guests.length; r++){// or <=MaxRoom if (this.guests[r]==null) { return r; } } return -1; } Check a guest into an empty room (return room number) public void checkIn(String name){ this.guests[ this.findEmpty() ] = name; } public boolean checkIn(String name){ int r = this.findEmpty(); if (r < 0) { return false; } this.guests[r] = name; return true; } What ’ s the problem? How do we fix it? n1n1 n5n5 n8n8 n 10 n7n7 n3n3 n6n6 n4n4 guests:

© Peter Andreae COMP :9 Arrays of Counts to keep track of counts of numbers we have seen, Scanner sc = new Scanner(new File( … )); int[ ] counts = new int[10000]; int otherCount = 0; while ( sc.hasNextInt() ){ int num = sc.nextInt(); if ( num>=0 && num<10000 ) { counts[num]++; } else { otherCount++; } Initialised with 0 counts: … …

© Peter Andreae COMP :10 Arrays of Booleans looking for duplicates: read file, check and set Issue: keep track of numbers we have seen, efficiently Scanner sc = new Scanner(new File( … ))); boolean[] numbersSeen = new boolean[100000]; while ( sc.hasNext() ){ if ( sc.hasNextInt() ){ int num = sc.nextInt(); if ( num>=0 && num< ){ if (numbersSeen[num] ) { System.out.println(num + "is a duplicate"); } } numbersSeen[num] = true; } else{ String junk = sc.next(); } } Initialised with false numbersSeen: fffffffffff … fffff … Could we do this with words?

© Peter Andreae COMP :11 Assignment 8 BalloonGame Array of Balloon objects Scoring

© Peter Andreae COMP :12 Assignment 8 Genealogy File of data Array of Person db : ID: 6 name: Angel dob: 1805 motherID: -1 fatherID: -1 ID: 0 name: Devin dob: 1810 motherID: -1 fatherID: -1 ID: 5 name: Addison dob: 1907 motherID: 1 fatherID: 7 ID: 1 name: Alex dob: 1840 motherID: 6 fatherID: Angel Addison Devin Alex Jessie Jordan Alexis Kasey Kelly Dominique CurrentID: