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Arrays

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  1. Arrays Dr. Ramzi Saifan Slides adapted from Prof. Steven Roehrig

  2. Arrays • An array is collection of variables all of the same type. • Java doesn't allow access to outside the limits of an array. • Unlike C++, the number of elements is not specified in array's brackets. • int a[10]; // is syntax error • a.length() returns the length of the array. • a[5] refers to 6th element of the array.

  3. Arrays • An array is a “first-class object,” unlike a C/C++ array, which is just a bunch of contiguous memory locations. • As for all objects, the array name is a reference to the actual object. • This object has space sufficient for • the number of references you specify, if your array holds objects, or • the number of primitive values you specify, if your array holds primitive types, • plus space to hold the array length.

  4. 1-D Arrays • int [] i; // Array Declaration (one dimensional array) • i=new int [size]; // Array construction and initialization • You can do the above in a line: int [] i= new int [size]; //same as objects • Index of any array starts from 0 • Possible declaration, construction, and initialization: • inti[]=new int [3]; • int size =5; int []i=new int [size]; • int []j={1,2,4,7,8}; • int []j=new int [] {1,2,4,7,8};

  5. Arrays of Primitives int[] a = new int[3]; // compiler does initialization 0 0 0 3 This is schematic only, e.g., length may not be first in memory. a.length a[0] a[1] a[2] int[] a = {41, 32, 19}; // compiler does initialization 32 41 19 3 a.length a[0] a[1] a[2]

  6. Arrays of Primitives (cont.) • Here are two common (compile) errors: • But this is OK: int[] a; a[2] = 5; // a uninitialized int[] b; int len = b.length; // b uninitialized int[] a = {41, 32, 19}; int[] b; b = a; int len = b.length;

  7. Arrays of Objects class Stock { int sharesHeld = 0; double currentPrice = 100.00; } Stock[] a = new Stock[3]; // initialization to     3 The array components a[0], a[1] and a[2] are null () references. a a.length a[0] a[1] a[2]

  8. Arrays of Objects for (int i = 0; i < 3; i++) a[i] = new Stock(); f12 f0 f24 3 Stock Stock Stock a a.length a[0] a[1] a[2]

  9. 2-D Arrays • int []m[] =new int [rows][cols]; (declaration, construction, and initialization) (two dimensional array) • int [][]m1=new int [3][5]; • int m2[][]={{1,2,3}, {-4,5,7,8}, new int [3]}; • int m2[][] = new int [][]{{1,2,3}, {-4,5,7,8}, new int [3]}; • int [] h, h1 [], h2 [], h3; // in this case, h1 and h2 are two-dimensional arrays, h and h3 are one-dimensional arrays • int rows=3; int m3[][] = new int [rows][]; m3[0]= new int []{1,2,3,4}; m3[1]=new int [5]; m3[2]= new int []{1,2};

  10. Passing Arrays to Methods public class Investor { private Stock[] portfolio; private Advisor ohTrustedOne; public double value; Investor() {portfolio = new Stock[0];} Investor(Stock[] p, Advisor a) { portfolio = p; ohTrustedOne = a; value = ohTrustedOne.findValue(p); } } public class Stock { public intsharesHeld = 0; public double currentPrice = 100.00; public String symbol; Stock() {} public Stock(int s, double p, String name) { sharesHeld = s; currentPrice = p; symbol = name; } } public class Advisor { double findValue(Stock[] p) { double value = 0; for (inti = 0; i < p.length; i++) value += p[i].currentPrice * p[i].sharesHeld; return value; } }

  11. Test Passing Arrays public class TestInvestments { public static void main(String[] args) { Stock[] p = new Stock[] {new Stock(1000, 53.45, "GnMotr"), new Stock(100, 29.05, "GenElec"), new Stock(220, 44.08, "GenMills")}; Advisor Jack= new Advisor(); Investor sucker = new Investor(p, Jack); System.out.println(sucker.value); } }

  12. Dangers in Passing Arrays • The called method gets a reference to the actual array, and can modify it. • If you are cautious, you can make a copy and send that.

  13. Copies of Arrays • Shallowest: make a copy of the array reference • Shallow: make a new array of references of the same type and size, and copy into it the existing array of references • Deep: the above, plus make copies of all the objects themselves

  14. System.arraycopy() • The method System.arraycopy() does a shallow copy • System.arraycopy(source_array, starting index, Dest_array, starting_index, Number_of_elements_to_be_copied). • This static method is very helpful to control the copy of an array.

  15. Deep Copy of an Array Stock[] copyPortfolio() { // illustrates returning an array Stock[] copy = new Stock[portfolio.length]; for (int i = 0; i < portfolio.length; i++) { Stock s = new Stock(); s.currentPrice = portfolio[i].currentPrice; s.sharesHeld = portfolio[i].sharesHeld; s.symbol = new String(portfolio[i].symbol); copy[i] = s; } return copy; }

  16. Useful methods for arrays • To access the elements using the for loop • for (int i = 0; i < a.length; i++)       sum += a[i]; • for (int element : a) • System.out.println(element); • int[] a = new int[10000]; • Arrays.toString(a); //For one dimentional arrays • Arrays.sort(a); • Arrays.fill(type[] a, type v) ; //sets all elements of the array to v. • To visit all elements of a two-dimensional array a, nest two loops, like this: • for (double[] row : a) • for (double value : row) • do something with value • System.out.println(Arrays.deepToString(a)); //for two dimentional arrays • The output is formatted like this: • [[16, 3, 2, 13], [5, 10, 11, 8], [9, 6, 7, 12], [4, 15, 14, 1]]

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