Arrays and Strings CSCI 392 Week Three Declaring

Arrays and Strings CSCI 392 Week Three

Declaring an Array Variable Here are two version of the same declaration int[] intlist; intlist[]; Declaring a two-dimensional array int[][] twodimarray; int twodimarray[][];

Creating Space for an Array Given the declaration int[] myarray; You must also tell the VM to create space myarray = new int[100]; You can do both steps in one line int[] myarray = new int[100]; You can also wait until runtime to decide how much space to use int[] myarray = new int[compute. Size()];

Initializing Array Values Version One int[] one_to_four = new int[] {1, 2, 3, 4}; Version Two int[] one_to_four = {1, 2, 3, 4}; An Array of Strings String[] names = {"Bob", "Joe", "Sue", "Fred"}; Computing Initial Values at Runtime int[] examscores = {grade. Exam(1), grade. Exam(2)};

Using Arrays n Array Variables are Objects. So, they have their own class methods and instance properties. For example: n n myarray. length Arrays. sort(myarray) n Array indexes must be of type int. n Using an index out of bounds throws Array. Index. Out. Of. Bounds. Exception

Copying Arrays n Shallow Copy int[] original = {1, 2, 3, 4} int[] copy = (int[]) original. clone(); n Better copy method System. arraycopy (original, 0, copy, 0, original. length);

class array_test_1 { static void main(String[] args) { char[] msg = {'B', 'o', 'b'}; char[] copy 1 = msg; char[] copy 2 = (char[]) msg. clone(); copy 1[0] = 'b'; copy 2[2] = 'p'; System. out. print ("msg = "); System. out. println(msg); System. out. print ("copy 1 = "); System. out. println(copy 1); System. out. print ("copy 2 = "); System. out. println(copy 2); } } msg = bob copy 1 = bob copy 2 = Bop

class array_test_2 { static void main(String[] args) { char[] msg = {'B', 'o', 'b'}; char[] copy 1 = msg; char[] copy 2 = (char[]) msg. clone(); if (msg == copy 1) System. out. println ("msg equals copy 1"); else System. out. println ("msg not equals copy 1"); if (copy 1 == copy 2) System. out. println ("copy 1 equals copy 2"); else System. out. println ("copy 1 not equals copy 2"); } } msg equals copy 1 not equals copy 2

Array Methods import java. util. Arrays; // compare the contents of two arrays if (Arrays. equals (array 1, array 2)) System. out. println ("contents are the same"); // quick sort an array Arrays. sort (myarray); // initialize the first ten positions with a 1 Arrays. fill (myarray, 0, 9, 1); // fill entire array with value of 1 Arrays. fill (myarray, 1);

char[] != String The "+" operator is overloaded for the String class, so this code does not run correctly. class string_test_1 { static void main(String[] args) { char[] msg = {'B', 'o', 'b'}; System. out. println ("msg = " + msg); } } Correct Version System. out. println("msg = " + String. valueof(msg);

char[] != String This code yields a compiler error. class string_test_2 { static void main(String[] args) { String myvar = "bob"; myvar[0] = 'B'; // error: myvar is not an array } }

String Methods n Compare two strings String name 1, name 2; . . . if ( name 1. compare. To(name 2) < 0 ) // like C++ n Convert an integer into a string String mystring = String. value. Of (myint); n Convert a string into an integer int myint = Integer. parse. Int (mystring); n Append name += " "; name = first + last;

More String Methods n Extract a Substring String s 1 = name. substring(0, 4); n Remove spaces at the end name = name. trim(); n Does the string contain a number? String str 1; . . . char c = str 1. char. At (0); // get first char if (Character. is. Digit(c)) // test that char System. out. print ("str 1 starts with a number");

Files n See course web page for example code