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Arrays

Arrays. Jiafan Zhou. 1. Background. Programmer often need the ability to represent a group of values as a list List may be one-dimensional or multidimensional Java provides arrays and the collection classes The Vector class is an example of a collection class Consider arrays first. 2. c.

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Arrays

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  1. Arrays Jiafan Zhou 1

  2. Background • Programmer often need the ability to represent a group of values as a list • List may be one-dimensional or multidimensional • Java provides arrays and the collection classes • The Vector class is an example of a collection class • Consider arrays first 2

  3. c - … value 0 0 0 0 0 Example • Definitions char[] c; int[] value = new int[10]; • Causes • Array object variable c is un-initialized • Array object variable value references a new ten element list of integers • Each of the integers is default initialized to 0 3

  4. An array example int[] v = new int[10]; int i = 7; int j = 2; int k = 4; v[0] = 1; v[i] = 5; v[j] = v[i] + 3; v[j+1] = v[i] + v[0]; v[v[j]] = 12; System.out.println(v[2]); v[k] = stdin.nextInt(); int[] v = new int[10]; int i = 7; int j = 2; int k = 4; v[0] = 1; v[i] = 5; v[j] = v[i] + 3; v[j+1] = v[i] + v[0]; v[v[j]] = 12; System.out.println(v[2]); v[k] = stdin.nextInt(); int[] v = new int[10]; int i = 7; int j = 2; int k = 4; v[0] = 1; v[i] = 5; v[j] = v[i] + 3; v[j+1] = v[i] + v[0]; v[v[j]] = 12; System.out.println(v[2]); v[k] = stdin.nextInt(); int[] v = new int[10]; int i = 7; int j = 2; int k = 4; v[0] = 1; v[i] = 5; v[j] = v[i] + 3; v[j+1] = v[i] + v[0]; v[v[j]] = 12; System.out.println(v[2]); v[k] = stdin.nextInt(); int[] v = new int[10]; int i = 7; int j = 2; int k = 4; v[0] = 1; v[i] = 5; v[j] = v[i] + 3; v[j+1] = v[i] + v[0]; v[v[j]] = 12; System.out.println(v[2]); v[k] = stdin.nextInt(); int[] v = new int[10]; int i = 7; int j = 2; int k = 4; v[0] = 1; v[i] = 5; v[j] = v[i] + 3; v[j+1] = v[i] + v[0]; v[v[j]] = 12; System.out.println(v[2]); v[k] = stdin.nextInt(); int[] v = new int[10]; int i = 7; int j = 2; int k = 4; v[0] = 1; v[i] = 5; v[j] = v[i] + 3; v[j+1] = v[i] + v[0]; v[v[j]] = 12; System.out.println(v[2]); v[k] = stdin.nextInt(); int[] v = new int[10]; int i = 7; int j = 2; int k = 4; v[0] = 1; v[i] = 5; v[j] = v[i] + 3; v[j+1] = v[i] + v[0]; v[v[j]] = 12; System.out.println(v[2]); v[k] = stdin.nextInt(); int[] v = new int[10]; int i = 7; int j = 2; int k = 4; v[0] = 1; v[i] = 5; v[j] = v[i] + 3; v[j+1] = v[i] + v[0]; v[v[j]] = 12; System.out.println(v[2]); v[k] = stdin.nextInt(); Suppose 3 is extracted 8 is displayed 4

  5. ElementType [ ] id; Brackets Name of Type of indicate array list values in variable being list defined Array variable definition styles • Without initialization int [] a; int a[]; 5

  6. Nonnegative integer expression specifying the number of elements in the array ElementType id ElementType [n]; = new [ ] A new array of n elements Array variable definition styles • With initialization 6

  7. Where we’ve seen arrays • public static void main (String[] args) • Thus, the main() method takes in a String array as the parameter • Note that you can also define it as: • public static void main (String args[]) • or • public static void main (String[] foobar) 7

  8. Basic terminology • List is composed of elements • Elements in a list have a common name • Example: a[3] = 5; • The common name is ‘a’ • The list as a whole is referenced through the common name • List elements are of the same type — the base type • Elements of a list are referenced by subscripting (indexing) the common name 8

  9. Java array features • Subscripts are denoted as expressions within brackets: [ ] • Base (element) type can be any type • Size of array can be specified at run time • This is different that pure C! (for the most part, at least) • Index type is integer and the index range must be 0 ... n-1 • Where n is the number of elements • Just like Strings indexing! • Automatic bounds checking • Ensures any reference to an array element is valid • Data field length specifies the number of elements in the list • Array is an object • Has features common to all other objects • More on this later… 9

  10. Review of arrays • Creating an array: int[] foo = new int[10]; • Accessing an array: foo[3] = 7; System.out.print (foo[1]); • Creating an array: String[] bar = new String[10]; • Accessing an array: bar[3] = “qux”; System.out.println (bar[1]); 10

  11. Consider • Segment int[] b = new int[100]; b[-1] = 0; b[100] = 0; • Causes • Array variable to reference a new list of 100 integers • Each element is initialized to 0 • Two exceptions to be thrown • -1 is not a valid index – too small • 100 is not a valid index – too large • IndexOutOfBoundsException 11

  12. vertex vertex vertex p[0] p[0] p[0] p[0] p[1] p[1] p[1] p[1] p[2] p[2] p[2] p[2] p[0] p[0] p[0] p[1] p[1] p[1] p[2] p[2] p[2] p p p p null null null p p p Point: (4, 4) Point: (4, 4) Point: (4, 4) Point: (1, 0) Point: (1, 0) Point: (1, 0) Point: (1, 0) Point: (1, 2) Point: (1, 2) Point: (2, 2) Point: (2, 2) Point: (2, 2) Point: (0, 0) Point: (1, 0) Point: (1, 1) Point: (1, 1) Point: (2, 2) Point: (2, 2) Consider Point[] p = new Point[3]; p[0] = new Point(0, 0); p[1] = new Point(1, 1); p[2] = new Point(2, 2); p[0].setX(1); p[1].setY(p[2].getY()); Point vertex = new Point(4,4); p[1] = p[0]; p[2] = vertex; Point[] p = new Point[3]; p[0] = new Point(0, 0); p[1] = new Point(1, 1); p[2] = new Point(2, 2); p[0].setX(1); p[1].setY(p[2].getY()); Point vertex = new Point(4,4); p[1] = p[0]; p[2] = vertex; 12

  13. id references an array of n elements. id[0] has value exp0, id[1] has value exp1, and so on. Each expiis an expression that evaluates to type ElementType Explicit initialization • Syntax ElementType id exp , exp , ... exp [] = { } ; 0 1 -1 n 13

  14. Explicit initialization • Example String[] puppy = { “pika”, “mila”, “arlo”, “nikki” }; int[] unit = { 1 }; • Equivalent to String[] puppy = new String[4]; puppy[0] = “pika"; puppy[1] = “mila"; puppy[2] = “arlo"; puppy[3] = “nikki"; int[] unit = new int[1]; unit[0] = 1; 14

  15. Array members • Member length • Size of the array for (int i = 0; i < puppy.length; ++i) { System.out.println(puppy[i]); } • Note that length is a field, not a method! • I.e., it is not puppy.length() 15

  16. u[0] u[0] u[0] u[1] u[1] u[1] u u u Point: (0, 0) Point: (0, 0) Point: (0, 0) Point: (1, 1) Point: (1, 1) Point: (1, 1) v v Point: (4, 30) v[0] v[0] v[1] v[1] Array members Point[] u = { new Point(0, 0), new Point(1, 1)}; Point[] v = u.clone(); v[1] = new Point(4, 30); • Member clone() • Produces a shallow copy Point[] u = { new Point(0, 0), new Point(1, 1)}; Point[] v = u.clone(); v[1] = new Point(4, 30); 16

  17. u[0] u[0] u[1] u[1] u[0] u[1] u u u Point: (0, 0) Point: (0, 0) Point: (1, 1) Point: (1, 1) Point: (0, 0) Point: (10, 1) v v v[0] v[1] v[0] v[1] Array members • Member clone() • Produces a shallow copy Point[] u = { new Point(0, 0), new Point(1, 1)}; Point[] v = u.clone(); v[1].setX(10); Point[] u = { new Point(0, 0), new Point(1, 1)}; Point[] v = u.clone(); v[1].setX(10); 17

  18. Array - length = 5 - data = 1 2 3 4 5 1 2 3 4 5 + … How Java represents arrays • Consider int[] a = { 1, 2, 3, 4, 5 }; a 18

  19. a b null c 0 0 0 0 0 d 1 2 3 4 5 More about how Java represents Arrays int[] a; int[] b = null; int[] c = new int[5]; int[] d = { 1, 2, 3, 4, 5 }; a = c; d = c; • Consider int[] a; int[] b = null; int[] c = new int[5]; int[] d = { 1, 2, 3, 4, 5 }; a = c; d = c; - 19

  20. Consider that main() method again • public static void main (String args[]) • How does one pass in a parameter to the main method? public class MainParameters { public static void main (String args[]) { System.out.println ("Number of paramters to “ + "main(): " + args.length); if ( args.length > 0 ) { for ( int i = 0; i < args.length; i++ ) System.out.println ("parameter " + i + ": '" + args[i] + "'"); } } } 20

  21. Program Demo • MainParameters.java • Via Eclipse • Via the command line 21

  22. Searching for a value System.out.println("Enter search value (number): "); int key = stdin.nextInt(); int i; for (i = 0; i < data.length; ++i) { if (key == data[i]) { break; } } if (i != data.length) { System.out.println(key + " is the " + i + "-th element"); } else { System.out.println(key + " is not in the list"); } ++i System.out.println("Enter search value (number): "); int key = stdin.nextInt(); int i; if (key == data[i]) { break; if (i != data.length) { System.out.println(key + " is the " + i + "-th element"); } i = 0 i < data.length 22

  23. Searching for the minimum value • Segment int minimumSoFar = sample[0]; for (int i = 1; i < sample.length; ++i) { if (sample[i] < minimumSoFar) { minimumSoFar = sample[i]; } } 23

  24. Sorting • Problem • Arranging elements so that they are ordered according to some desired scheme • Standard is non-decreasing order • Why don't we say increasing order? • Major tasks • Comparisons of elements • Updates or element movement 24

  25. Iteration i // find the location of the ith smallest element int spot = i; for (int j = i+1; j < v.length; ++j) { if (v[j] < v[spot]) // is spot ok? // update spot with index of smaller element spot = j; } // spotisnowcorrect,swap elementsv[spot]andv[i] 25

  26. Binary search • Given a list, find a specific element in the list • List MUST be sorted! • Each time it iterates through, it cuts the search space in half • A binary search is MUCH faster than a sequential search 26

  27. Binary search vs. sequential search • Assume the array has n elements • Sequential search takes n iterations to find the element • Binary search takes log2n iterations to find the element • Consider a list of 1 million elements • Binary search takes about 20 iterations • Sequential search takes 1,000,000 iterations • Consider a list of 1 trillion elements • Binary search takes about 40 iterations • Sequential search takes 1,000,000,000,000 iterations 27

  28. Limitations of arrays • You can’t change their size once created • This can be a big problem! • So we will create a new class that will operate like an array: • We can store and get elements by index number • It will automatically increase in size as needed • And other fancy features… • Let’s call the class Vector • As we are basically writing the java.util.Vector class 28

  29. Properties of our Vector class • It needs to have an array to hold the values • As our internal array will often be bigger than the number of elements in the Vector, we need a size as well • More on what this means in a slide or two… • Not much else… 29

  30. Methods in our Vector class • Insert and remove elements into the Vector • Get an element from the Vector • Find the length • Print it out to the screen • What happens when the array field is full, and we want to add an element? • We will need to increase the size of the array • So we need a method to do that as well 30

  31. Our first take on our Vector class public class Vector { private Object array[]; private int size = 0; Vector() { array = new Object[100]; } Vector(int length) { array = new Object[length]; } } • What does this mean? • We’ll see that a bit later… • But briefly, it means the array can store any object 31

  32. Adding an element to our Vector public void add (Object o) { array[size++] = o; } • Pretty easy! • But what if the array is full? • We need a way to increase the capacity of the array 32

  33. Increasing the Vector’s array’s capacity private void increaseCapacity() { int oldSize = array.length; Object newArray[] = new Object[2*oldSize]; for ( int i = 0; i < oldSize; i++ ) newArray[i] = array[i]; array = newArray; } • And our new add() method: public void add (Object o) { if ( size == array.length ) increaseCapacity(); array[size++] = o; } 33

  34. Methods can be private as well • Notice that the increaseCapacity() method is called only by the add() method when necessary • It’s not ever going to be called by whomever is using our Vector • Thus, we will make it private • That means that only other Vector methods can call it 34

  35. Removing an element from a Vector public Object remove (int which) { Object ret = array[which]; for ( int i = which; i < array.length-1; i++ ) array[i] = array[i+1]; array[array.length-1] = null; size--; return ret; } 35

  36. Miscellaneous other methods public int size() { return size; } public Object get (int which) { return array[which]; } 36

  37. Our toString() method public String toString() { String ret = "["; for ( int i = 0; i < size; i++ ) { ret += array[i]; if ( i != size-1 ) ret += ", "; } ret += "]"; return ret; } 37

  38. Using our Vector • This code is in a separate class called VectorUsage public static void main (String[] args) { Vector v = new Vector(); for ( int i = 12; i < 30; i++ ) { v.add (String.valueOf(i)); } System.out.println (v); System.out.println (v.size()); String s = (String) v.get(5); System.out.println (s); v.remove (5); System.out.println (v); v.remove (5); System.out.println (v); } 38

  39. The “real” Vector class • Java provides a Vector class • In java.util • It contains all of the methods shown 39

  40. More on using the Vector class • To add a String object s to the end of a Vector v • v.add(s); • To get the String object at the end of the Vector v • String s = (String) v.get(v.size()-1); • To remove a String object from the end of a Vector v • String s = (String) v.remove(v.size()-1); • This both removes the object from the Vector and stores the removed value into s 40

  41. Multidimensional Array 41

  42. Multidimensional arrays • Many problems require information be organized as a two-dimensional or multidimensional list • Examples • Matrices • Graphical animation • Economic forecast models • Map representation • Time studies of population change • Microprocessor design 42

  43. m[0] m[1] m[2] m[2][0] m[2][1] m[2][2] m[2][3] m 0 0 0 0 0 0 0 0 0 0 0 0 m[0][0] m[0][1] m[0][2] m[0][3] m[1][0] m[1][1] m[1][2] m[1][3] Example • Segment int[][] m = new int[3][]; m[0] = new int[4]; m[1] = new int[4]; m[2] = new int[4]; • Produces When an array is created, each value is initialized! m 43

  44. m[0] m[1] m[2] m[2][0] m[2][1] m[2][2] m[2][3] m 0 0 0 0 0 0 0 0 0 0 0 0 m[0][0] m[0][1] m[0][2] m[0][3] m[1][0] m[1][1] m[1][2] m[1][3] Example • Alternative int[][] m = new int[3][4]; • Produces 44

  45. 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 Multidimensional array visualization • A multi-dimensional array declaration (either one): int[][] m = new int[3][4]; • How we visualize it: or 45

  46. Explicit Initialization • Segment int c[][] = {{1, 2}, {3, 4}, {5, 6}, {7, 8, 9}}; • Produces 46

  47. But what about adding variables? • The add method takes an Object as a parameter • public void add (Object o) { • Although we haven’t seen it yet, this means you can add any object you want to the vector • Primitive types (i.e. variables) are not objects • How can they be added? • The solution: wrapper classes! 47

  48. Wrapper Classes 48

  49. The Integer wrapper class • This is how you add an int variable to a Vector: int x = 5; Integer i = new Integer(x); vector.add (i); //… Integer j = (Integer) v.get(0); int y = j.intValue(); • Pretty annoying syntax – we’ll see how to get around it in a bit… 49

  50. More on wrapper classes • All the primitive types have wrapper classes • Usually, the names are just the capitalized version of the type • I.e. Double for double, Byte for byte, etc. • Two exceptions: int and char • int has Integer • char has Character 50

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