Arrays. Friday, November 9, Department of Computer Science Wellesley College

Transcription

1 Arrays Friday, November 9, CS Computer Programming Department of Computer Science Wellesley College Arrays: Motivation o o o Lists are great for representing a collection of values, but can only access elements in order from front to back. We often want an indexed collection where we can directly access an element at any index. Many languages (including Java) provide arrays for this purpose. An array is an indexed collection of variables, which we will call the slots of the array. The values in the slots are the elements of the array. ExecutionLand ObjectLand am Sam Arrays -

2 Anatomy of an Array Diagram ObjectLand an array object the type of the array object (T [ ] is pronounced array of T ) slot indices slots (variables) elements am Sam Arrays - Arrays Are Homogeneous Collections All the elements of a Java* array must have the same type. tests false boolean [] false false true am Sam line Location [] Location Location * We ll only talk about Java arrays in this lecture. Array details can differ from language to language. x y 5 x - y Arrays -

3 Arrays Have a Fixed Length The length of an array is the number of slots = the number of elements = one more than the largest index. E.g., the length of is. An array s length is fixed: once an array is created, its length cannot be changed. If a denotes an array, a.length stands for its length. E.g..length evaluates to. It s as if each array object has a length instance variable We usually do not explicitly show this variable. Its value cannot be changed. am Sam length am Sam Arrays -5 Length Examples Expression.length tests.length line.length single.length none.length Value tests line false boolean [] false false Location [] true Location Location single none x y 5 x y - Arrays -

4 Array Indexing If a denotes an array and i denotes an integer, the notation a[i] (pronounced a sub i ) refers to the ith slot of a. In most contexts, a[i] evaluates to the contents of the slot: [] // evaluates to am [] // evaluates to tests[+] // evaluates to true line[].x // evaluates to 5 tests line boolean [] false false false Location [] true Location x 5 Location x - am Sam y y Arrays - ArrayIndexOutOfBoundsException An ArrayIndexOutOfBoundsException occurs when an invalid index is used. Examples: [-] [] [] (gotcha!) [.length] (gotcha!) am Sam Arrays -

5 Array Slots are Mutable The assignment a[i] = val changes the contents of the ith slot of a to val. [] = ; [] = [] + []; []++; // means [] = [] + ; Arrays -9 Swapping Array Elements public static void swap ( strings, int i, int j) { am Sam swap(,,); Sam am swap(,,); Sam am Arrays - 5

6 Array Creation type of array elements = new int[5]; name of variable referring to array number of slots Slots are initially filled with the default value for element type: for int. for double false for boolean null for any Object Arrays - Initializing an Array Can change default slot values with assignments. = new int[5]; [] = ; [] = ; [] = ; [] = ; [] = ; The above code can be abbreviated as: int [ ] = new int [ ] {,,,, ; This form can be used wherever an array expression is expected. Arrays -

7 Array Elements can be Any Type = new {, am, Sam ; am Sam boolean [] tests = new boolean [] {false, false, false, true; tests false boolean [] false false true Location [] line = new Location [] {new Location(5,), new Location(-,); line Location [] Location Location x 5 x - y y Arrays - Pop Quiz! What Does This Code Print? Buggle [] bugs = new Buggle [] {new Buggle(), new Buggle(); bugs[].forward(); bugs[].left(); Buggle becky = new Buggle(); becky.setposition(bugs[].getposition()); becky.setheading(bugs[].getheading()); bugs[] = becky; bugs[] = bugs[]; bugs[].forward(); System.out.println(bugs[].getPosition()); Arrays -

8 Null Pointers The default array value for Objects is null (the null pointer), which represents no object. = new ; We depict variables holding the null pointer value as a box with a slash. [] = ; [] = am ; [] == null // evaluates to false [] == null // evaluates to true; am Arrays -5 NullPointerException am Any attempt to send a message to the null pointer value results in a NullPointerException. [].length() // evaluates to // (length of string am, not of array) [].length() // generates a NullPointerException NullPointerExceptions due to uninitialized variables are extremely common and will haunt you from now on. Arrays -

9 main() Arguments Explained! public class PrintArgs { public static void main (String [ ] args) { for (int i = ; i < args.length; i++) { System.out.println( args[ + i + ] = + args[i]); args > java PrintArgs CS rocks. Sam I am! args[] = CS args[] = args[] = rocks args[] =. args[] = Sam I am! CS rocks Sam I am!. Arrays - tostring() and fromstring() The default tostring() instance method for an array doesn t display its elements. Instead, it gives something unintelligible:.tostring() // evaluates to "[I@e5d" = + // evaluates to " = [I@e5d" The CS classes IntArray and StringArray provide tostring() class methods that are more sensible: IntArray.toString() // evaluates to {,,,, StringArray.toString() // evaluates to {I,am,Sam StringArray.toString() // evaluates to {I,am,null They also provide fromstring() class methods for creating arrays from strings written in this format: = IntArray.fromString( {,,,, ); = StringArray.fromString( {I,am,Sam ); Arrays - 9

10 split() The split() instance method of the String class is useful for creating string arrays: String [ ] seuss = I do not like green eggs and ham.split( ); StringArray.toString(seuss) // evaluates to {I,do,not,like,green,eggs,and,ham Arrays -9 Summing an Array Want IntArray.sum() Iteration Table i result 5 // Assume this is in the class IntArray public static int sum (int [ ] a) { Arrays -

11 Testing sum() Can test interactively in the Dr. Java Interaction Pane: > IntArray.sum(new {,,,,) Can include hardwired test arrays in a main() method: public static void main ( args) { String [ ] test = new int [ ] {,,,,; System.out.println( sum( + IntArray.toString(test)+ ) = + sum(test)); Can read test arrays from the command line: public static void main ( args) { String [ ] test = new int [args.length]; for (int i = ; i < args.length; i++) { test[i] = Integer.parseInt(args[i]); System.out.println( sum( + IntArray.toString(test) + ) = + sum(test)); Arrays - Summing from Top Down Want IntArray.sum() Iteration Table i result - // Assume this is in the class IntArray public static int sum (int [ ] a) { Arrays -

12 Doubling the Elements of an Array Before: IntArray.mapDouble(); After: // Assume this is in the class IntArray public static void mapdouble ( a) { Arrays - Destructive Mapping Idiom Before: IntArray.mapDouble(); After: mapdouble() illustrates the mapping idiom for arrays. It is destructive in the sense that it replaces the original values in the array slots. Arrays -

13 Nondestructive (Copying) Mapper Before: c = IntArray.mapDoubleCopy(); After: c // Assume this is in the class IntArray public static mapdoublecopy ( a) { Arrays -5 Destructive Array Reversal Before: IntArray.reverse(); After: // Assume this is in the class IntArray public static void reverse ( a) { Arrays -

14 A Broken Array Reversal The following destructive reverse method attempts to reverse an array using swap(). Unfortunately, it has a bug. Please fix the method so that it works. // Assume this is in the class IntArray public static void reverse ( a) { for (int i = ; i < a.length; i++) { IntArray.swap(a, i, a.length - i); Arrays - Nondestructive Array Reversal Before: c = IntArray.reverseCopy(); After: c // Assume this is in the class IntArray public static reversecopy ( a) { Arrays -

15 Array Filtering Before: evens = IntArray.filterEvensCopy(); After: evens evens // Assume this is in the class IntArray public static filterevenscopy ( a) { Arrays -9 Accessing Elements: Lists vs. Arrays Accessing the ith element of a list takes time proportional to i. list Ø Accessing the ith element of an array can be done directly. The time it takes is the same for any i. array Arrays - 5

16 Good news & bad news Find first element Find last element Add element to front Add element to end Find length Mutable Work for diff types Lists Arrays list Ø array Arrays - Java Arrays: Summary of Properties o Arrays are fixed-length indexed collections of elements. Once created, the size of an array cannot change. o Elements are -indexed (i.e., indices start at, not ). o Every element can be accessed cheaply, independent of index. o Arrays are homogeneous collections: all elements must have the same type. o There is a special concise syntax for creating, accessing and updating arrays. o Dynamic (run-time) index checking ensures that indices of all array operations are valid. o Arrays are objects that are not instances of any class. o Arrays are the answer to the following analogy question: Recursion : lists :: iteration :??? Arrays -