Lecture 07, Fall 2018 Wednesday September 19

Transcription

1 Arrays Oliver W. Layton CS231: Data Structures and Algorithms Lecture 07, Fall 2018 Wednesday September 19

2 Follow-up (1/2) Overriding a "built-in" method (e.g. equals()) with same parameter numbers and types, but different return type is NOT ALLOWED (Java will not compile your program). The following will NOT work (neither together in the same program nor just one of them): public boolean equals(object o) { //... } public int equals(object o) { //... }

3 Follow-up (2/2) Postfix/prefix ++/--. Let's look at example code. Java events example code. Let's take a look at a same GUI program with budons (WinButton.java).

4 Recall from Friday... args in public static void main(string[] args). args isn't a String, it's a String array. Arrays are a data structure that: have fixed length/size only hold a single data type (e.g. String) The variable args holds all the arguments a user passes in the command line, like sys.argv in Python.

5 Number of arguments (length field) Unlike Python args[0] holds the first argument passed in as a String. args[1] holds the second argument passed in as a String, etc. Check number of arguments via args.length Let's work out a quick example together.

6 What is an array? A con$guous chunk (block) of memory that only holds a single type of data (String, int, double, etc...). Type can be primi>ve (unlike what...?) or any object. Java guarantees that the items will be posi>oned next to each other in memory. How? We tell Java up-front the maximum number of elements that we will put into the array. It cannot change! Bigger arrays require that Java finds a bigger block of memory. Java stores an extra public int field (array.length) with each array that indicates the size/length/number of elements it holds.

7 Crea%ng an array int[] myintarray = new int[50]; // one line int[] otherintarray; otherintarray = new int[50]; // two lines A#er instan,a,on, So we can fill it with elements for myintarray[0] through myintarray[49].

8 Accessing elements Square brackets like Python lists. For example, myintarray[30].

9 Default values int: System.out.println(myIntArray[30]); // 0 objects: Die[] mydiearray = new Die[50]; System.out.println(myDieArray[30]); // null

10 Filling arrays with values 1. First instan+ate (via new): int[] myintarray = new int[50]; 2. O3en we loop to fill array with values: System.out.println(myIntArray[30]); // prints 0 for (int i = 0; i < myintarray.length; i++) { myintarray[i] = i; // set i-th element to the loop control variable i } System.out.println(myIntArray[30]); // prints 30 But we can assign values directly: myintarray[0] = 100; or myintarray[1] = 200;

11 Out of bounds You CANNOT access/add elements beyond the amount you allocate space for. The following would FAIL: System.out.println(myIntArray[51]); //CRASH with ArrayIndexOutOfBoundsException

12 Declare, instan-ate, and fill all at once int[] intvals = {1,2,3,4,5,6,7,8,9,0}; Be careful with objects like Die. Unless you're doing something out of the ordinary, you'll probably want to allocate A NEW OBJECT for each slot in the array: System.out.println(myDieArray[30]); // prints null for (int i = 0; i < mydiearray.length; i++) { } mydiearray[i] = new Die(1); // allocate memory for a new Die object System.out.println(myDieArray[30]); // prints Face value: 1

13 Common error: copying reference to single object in each array slot (1/3) Let's look at some code that highlights a common error when making arrays of objects.

14 Common error: copying reference to single object in each array slot (2/3) Die[] mydiearray = new Die[50]; Die thedie = new Die(); // make object once for (int i = 0; i < mydiearray.length; i++) { mydiearray[i] = thedie; // set it to all positions of array } System.out.println(myDieArray[30]); // Face value: 1 System.out.println(myDieArray[20]); // Face value: 1 mydiearray[20].setvalue(5); // (only?) change Die #20 System.out.println(myDieArray[30]); // Face value: 5 <-- WHOOPS!!!!! System.out.println(myDieArray[20]); // Face value: 5 What's the problem?

15 Common error: copying reference to single object in each array slot (3/3) What's going on is that we are assigning THE SAME object reference to ALL posi<ons in the array...it all points to the SAME single object!!! We need to allocate new memory for EACH object at EACH posi<on --- in other words, we need to use the keyword new every <me we assign a value to the array in the for loop.

16 Arrays vs. ArrayList Arrays and ArrayList are both list data structures: natural ways to store collec6ons of ordered data. ArrayList uses an array under-the-hood to represent the elements that you insert! ArrayList can be slower to add elements to than an array because of poten6al resizing. Because elements are stored next to each other in memory, Java doesn't have to search around memory and so accessing elments via array[i] or arrlist.get(i) is very fast. Arrays have minimal memory overhead. ArrayLists take up more resources. Arrays are built into the core Java language (no import statements!), ArrayList are a supplemental class.

17 Resizing arrays You fill up an array and need more space. What do you do? Die[] newarray = new Die[largerSize]; for (int i = 0; i < originalarray.length; i++) { newarray[i] = originalarray[i] }

18 Two dimensional (2D) arrays Double array with 3 rows and 3 columns: double[][] mydoublear = new double[3][3]; 2D array filled with values at instan:a:on: int[][] matrix = {{0, -1, 0}, {-1, -2, -1}, {-2, 16, -2}, {-1, -2, -1}}; What does this matrix look like? Accessing elements (like Python): matrix[0][1] We typically populate 2D arrays using nested for loops (outer and inner loops).

19 Common 2D array opera-ons How we can print 2D arrays? How would we go about assigning all of the elements in the last column to 99? How would we go about assigning all the elements in the first row to -999? How would we create a new matrix with 5 rows and 6 columns and make each element the sum of the row and the column number?

20 Jagged dimensions Just like Python, we don't need the same number of columns in each row --- the array can be jagged: int[][] tri = new int[4][]; tri[0] = new int[] {0}; tri[1] = new int[] {0, 1}; tri[2] = new int[] {0, 1, 2}; tri[3] = new int[] {0, 1, 2, 3}; 1. Because there are a variable number of columns, we only NEED to specify the # rows when instan=a=ng the 2D array. Java DOES NOT allow you to leave the number of rows blank. For example, int[][] tri = new int[][4]; WOULD NOT work. 2. Instan=a=ng the individual rows highlights how Java represents 2D arrays: It's really a container that holds references to 1D arrays (4 in this case). Let's draw this on the board.