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Chapter 11

Chapter 11. Arrays and Vectors. Print all prices, mark the lowest. 19.95 23.95 24.95 18.95 <- lowest price 29.95 19.95 20.00 22.99 24.95 19.95. Variable number of values. Can't just have variables data1, data2, ..., data10 Code would be tedious What if we have 1000 numbers?

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Chapter 11

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  1. Chapter 11 Arrays and Vectors

  2. Print all prices, mark the lowest • 19.9523.9524.9518.95 <- lowest price29.9519.9520.0022.9924.9519.95

  3. Variable number of values • Can't just have variables data1, data2, ..., data10 • Code would be tedious • What if we have 1000 numbers? • Array = collection of values of the same type • double[] data = new double[10];

  4. Figure 1 An Array Reference and an Array

  5. Accessing array elements • Index (or subscript) operator data[4] = 29.95;System.out.println(data[4]); • Unpleasant detail: data[4] is the fifth value in the array • data[0] = the first valuedata[1] = the second value. . .data[9] = the tenth value

  6. Figure 2 Filling an Array Element

  7. Common Errors • Bounds errorint i = 20;System.out.println(data[i]); • Most common form: double[] data=new double[20];data[20] = 19.95; • Forgot to initialize:double[] data;data[0] = 19.95;

  8. Array length • Public instance variable length yields number of elements in array, e.g. a.length • Read-only. Can't assign to a.length • Most common use: for (int i = 0; i < a.length; i++) do something with a[i] • Asymmetric bounds: 0 £i < a.lengthDon't use for (int i = 0; i <= a.length - 1; i++)

  9. Copying Arrays • Array variables are references:double[] data=new double[10];double[] prices = data;Now both variables point to the same array • data[0] = 19.95;Now both data[0] and prices[0] are 19.95

  10. Figure 3 Copying an Array Reference

  11. Copying all values of an array • Explicit loop:double[] prices = new double[data.length];for (int i = 0; i < data.length; i++) prices[i] = data[i]; • Use System.arraycopySystem.arraycopy(data, 0, prices, 0, data.length);

  12. Figure 4 The System.arrayCopy Method

  13. Partially Filled Arrays • Problem: Don't know how many data values are in the input set • Remedy: Make the array larger than the largest data set, remember how much is filled • final int DATA_LENGTH = 1000;double[] data = new double[DATA_LENGTH];int dataSize = 0; • Fill in values and increment size data[dataSize] = new value;dataSize++;

  14. Figure 5 Size of a Partially Filled Array

  15. Partially Filled Arrays • Use the _LENGTH, Size naming convention! • What if the array fills up? Can refuse additional entries:if (dataSize >= DATA_LENGTH) System.out.println("Sorry"); • Can grow array:double[] newData=new double[2*data.length];System.arrayCopy(data,0,newData,0, data.length);data = newData;

  16. Figure 6 Growing a Dynamic Array

  17. Program BestPrice.java public class BestPrice { public static void main(String[] args) { final int DATA_LENGTH = 1000; double[] data = new double[DATA_LENGTH]; int dataSize = 0; // read data ConsoleReader console = new ConsoleReader(System.in); boolean done = false; while (!done) { System.out.println("Enter price, 0 to quit:"); double price = console.readDouble(); if (price ++ 0) // end of input done = true;

  18. else if (dataSize < data.length) { // add price to data array data[dataSize] = price; dataSize++; } else // array is full { System.out.println("Sorry, the array is full."); done = true; } } // compute lowest price if (dataSize == 0) return; // no data double lowest = data[0]; for (int i = 1; i < dataSize; i++) if (data[i] < lowest) lowest = data[i]; // print out prices, marking the lowest one

  19. for (int i = 0; i < dataSize; i++) { System.out.print(data[i]); if (data[i] == lowest) System.out.print(” <--lowest price"); System.out.println(); } } }

  20. Array Parameters • static double average(double[] data){ if (data.length == 0) return 0; double sum = 0; for (int i = 0; i < data.length; i++) sum = sum + data[i]; return sum / n;} • Call asdouble[] prices = new double[20];. . .double avg = average(prices);

  21. Figure 7 Passing an Array to a Method

  22. Returning an Array • Construct an array with random test datastatic int[] randomData(int length, int n){ Random generator = new Random(); int[] data = new int[length]; for (int i = 0; i < data.length; i++) data[i] = generator.nextInt(n); return data;} • Call asint[] data = randomData(length, n);

  23. Common algorithms: find value • double[] prices = . . .;double targetPrice = 1000;int i = 0;boolean found = false;while (i < prices.length && !found){ if (prices [i] <= targetPrice) found = true; else i++;}if (found) System.out.println("Item " + i + " has a price of " + prices[i]);

  24. Common algorithms: count values • double[] prices = . . .;double targetPrice = 1000;int count = 0;for(i = 0; i < prices.length; i++){ if (prices [i] <= targetPrice) count++;}System.out.println(count + " matches");

  25. Figure 8 Removing an Element from an Array

  26. Program Remove1.java public class Remove1 { public static void main(String[] args) { ConsoleReader console = new ConsoleReader(System.in); String[] staff = new String[5]; staff[0] = "Harry"; staff[1] = "Romeo"; staff[2] = "Dick"; staff[3] = "Juliet"; staff[4] = "Tom"; int staffSize = staff.length; print(staff, staffSize); System.out.println("Remove which element? (0 - 4)"); int pos = console.readInt();

  27. // overwrite the removed element with the last element staff[pos] = staff[staffSize - 1]; staffSize--; print(staff,staffSize); } /** Prints an array of strings @param s the string array @param sSize the number of strings in the array */ public static void print(String[] s, int sSize) { for (int i = 0; i < sSize; i++) System.out.println(i + ": ” + s[i]); } }

  28. Figure 9 Removing an Element from an Ordered Array Element

  29. Program Remove2.java public class Remove2 { public static void main(String[] args) { ConsoleReader console = new ConsoleReader(System.in); String[] staff = new String[5]; staff[0] = "Dick"; staff[1] = "Harry"; staff[2] = "Juliet"; staff[3] = "Romeo"; staff[4] = "Tom"; int staffSize = staff.length; print(staff, staffSize); System.out.println("Remove which element? (0 - 4)"); int pos =console.readInt(); // shift all elements above pos down

  30. for (int i = pos; i < staffSize - 1; i++) staff[i] = staff[i + 1]; staffSize--; print(staff, staffSize); } /** Prints an array of strings @param s the string array @param sSize the number of strings in the array */ public static void print(String[] s, int sSize) { for (int i = 0; i < sSize; i++) System.out.println(i + ": ” + s[i]); } }

  31. Figure 10 Inserting an Element in an Ordered Array

  32. Program Insert.java public class Insert { public static void main(String[] args) { ConsoleReader console = new ConsoleReader(System.in); String[] staff = new String[6]; staff[0] = "Dick"; staff[1] = "Harry"; staff[2] = "Juliet"; staff[3] = "Romeo"; staff[4] = "Tom"; int staffSize = staff.length - 1; print(staff, staffSize); System.out.print ("Insert before which element? (0 - 4)"); int pos = console.readInt();

  33. // shift all element after pos up by one for (int i = staffSize; i > pos; i--) staff[i] = staff[i - 1]; // insert new element into freed slot staff[pos] = "New, Nina"; staffSize++; print(staff, staffSize); }

  34. /** Prints an array of strings @param s the string array @param sSize the number of strings in the array */ public static void print(String[] s, int sSize) { for (int i = 0; i < sSize; i++) System.out.println(i + ": ” + s[i]); } }

  35. Parallel Arrays • Product dataAceAcro 740 $3499.0 score = 71ACMA P500 $3195.0 score = 64Maximus $2495.0 score = 72<-best buySummit $2995.0 score = 48 • Naive solution: 3 “parallel” arraysString[] names;double[] prices;int[] scores;

  36. Figure 11 Parallel Arrays

  37. Program BestData.java public class BestData { public static void main(String[] args) { final int DATA_LENGTH = 1000; String[] names = new String[DATA_LENGTH]; double[] prices = new double[DATA_LENGTH]; int[] scores = new int[DATA_LENGTH]; int dataSize = 0; // read data ConsoleReader console = new ConsoleReader(System.in);

  38. boolean done = false; while (!done) { System.out.println ("Enter name or leave blank when done:"); String inputLine = console.readLine(); if (inputLine == null || inputLine.equals("")) done = true; else if (dataSize < DATA_LENGTH) { names[dataSize] = inputLine; System.out.println("Enter price:"); inputLine = console.readLine(); prices[dataSize] = Double.parseDouble(inputLine); System.out.println("Enter score:"); inputLine = console.readLine(); scores[dataSize] = Integer.parseInt(inputLine); dataSize++; }

  39. else // array is full { System.out.println("Sorry, the array is full."); done = true; } } // compute best buy if (dataSize == 0) return; // no data double best = scores[0] / prices[0]; for (int i = 1; i < dataSize; i++) if (scores[i] / prices[i] > best) best = scores[i] / prices[i]; // print out products, marking the best buys final int COLUMN_WIDTH = 30;

  40. for (int i = 0; i < dataSize; i++) { System.out.print(names[i]); // pad with spaces to fill column int pad = COLUMN_WIDTH - names[i].length(); for (int j = 1; j <= pad; j++) System.out.print(” "); // print price and score System.out.print(” $” + prices[i] + ” score = ” + scores[i]);

  41. // mark if best buy if (scores[i] / prices[i] == best) System.out.print(” <-- best buy"); System.out.println(); } } }

  42. Eliminating parallel arrays • Parallel arrays are an indication of a missed opportunity for finding objects • class Product{ . . . private String name; private double price; private int score;} • Product[] products

  43. Figure 12 Eliminating Parallel Arrays

  44. Program BestProduct.java public class BestProduct { public static void main(String[] args) { final int DATA_LENGTH = 1000; Product[] data = new Product[DATA_LENGTH]; int dataSize = 0; // read data ConsoleReader console = new ConsoleReader(System.in); boolean done = false; while (!done) { Product p = readProduct(console); if (p == null) done = true;

  45. else if (dataSize < DATA_LENGTH) { data[dataSize] = p; dataSize++; } else // array is full { System.out.println("Sorry, the array is full."); done = true; } } // compute best buy if (dataSize == 0) return; // no data double best = data[0].getScore() / data[0].getPrice();

  46. for (int i = 1; i < dataSize; i++) { double ratio = data[i].getScore() / data[i].getPrice(); if (ratio > best) best = ratio; } // print out data, marking the best buys for (int i = 0; i < dataSize; i++) { printProduct(data[i]); if (data[i].getScore() / data[i].getPrice() == best) System.out.print(” <-- best buy"); System.out.println(); } }

  47. /** Reads a product from a console reader. @param in the reader @return the product read if a product was successfully read, null if end of input was detected */ public static Product readProduct(ConsoleReader in) { System.out.println ("Enter name or leave blank when done:"); String name = in.readLine(); if (name == null || name.equals("")) return null; System.out.println("Enter price:"); String inputLine = in.readLine(); double price = Double.parseDouble(inputLine); System.out.println("Enter score:"); inputLine = in.readLine(); int score = Integer.parseInt(inputLine); return new Product(name, price, score); }

  48. /** Prints a product description. @param p the product to print */ public static void printProduct(Product p) { final int COLUMN_WIDTH = 30; System.out.print(p.getName()); // pad with spaces to fill column int pad = COLUMN_WIDTH - p.getName().length(); for (int i = 1; i <= pad; i++) System.out.print(” "); System.out.print(” $” + p.getPrice() + ” score = ” + p.getScore()); } }

  49. Arrays of Objects • public class Polygon{ public Polygon(int n) { corners = new Point2D.Double[n]; cornersSize = 0; } public void add(int i,Point2D.Double p) { if (cornersSize < corners.length) { corners[cornersSize] = p; cornersSize++; } } public void draw(Graphics2D g2) {...}private Point2D.Double[] corners;} • x

  50. Figure 13 A Polygon

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