Arrays and Lists CSC 121 Fall 2014 Howard Rosenthal

Transcription

1 Arrays and Lists CSC 121 Fall 2014 Howard Rosenthal

2 Lesson Goals Understand what an array is Understand how to declare arrays Understand what reference variables are Understand how to pass arrays to methods Understand basic sorting Understand linear and binary searching Understand multi-dimensional arrays 2

3 Fundamentals (1) An array is an object that is used to store a list of values. It is made out of a contiguous block of memory that is divided into a number of cells. Each cell holds a value, and all the values are of the same type. Sometimes the cells of an array are called slots. In the example array pictured at right, each cell holds an int. The name of this array is data. The cells are indexed 0 through 9. Each cell can be accessed by using its index. For example, data[0] is the cell which is indexed by zero (which contains the value 23). data[5] is the cell which is indexed by 5 (which contains the value 14). 3

4 Fundamentals (2) The cells are in an array are numbered sequentially starting at zero. If there are N cells in an array, the indexes will be 0 through N-1 Sometimes the index is called a subscript. The expression data[5] is usually pronounced "data-sub-five The value stored in a cell of an array is sometimes called an element of the array. An array has a fixed number of cells, but the values placed in those cells (the elements) may change over time. Every cell of an array holds a value of the same type. So, for example, you can have an array of int, an array of double, and so on. 4

5 Declaring Arrays In order to use arrays, you must learn the basic syntax An array is a named sequence of contiguous memory locations capable of holding a collection of data of the same type You can declare an array in two ways: type[] name or type name[] The first format is more typically used Examples: int[] myarray double[] numbers Declaring the variable creates a reference address in memory 5

6 Reference Variables There are two types of variables in Java primitive and reference Primitive variables are byte, short, int, long, double, float, boolean and char Reference variables hold memory addresses Declaring an array in Java creates a reference variable that holds a memory address i.e. boolean[] truthful truthful is a reference variable that holds an address, not a boolean value truthful[0] would hold a boolean value at the first address in the array 6

7 Array Instantiation There are several ways to instantiate an array. Both require using the new operator Declare and instantiate in two statements type[] name; double[] earnings; name = new type[size] ; earnings = new double[6]; Declare and instantiate in one statement type[] name = new type[size]; double[] earnings = new double[6]; Declare, instantiate and initialize in one statement type[] name = v 0, v 1, v 2 v n-1 ; double[] earnings = 25.2, 35.9, 72.6, 12.8, 176.4, 99.0; 7

8 Array Instantiation Basic Rules The values in an array must all be of the same data type The first index in the array is 0 An array is referenced from name[0] name[size-1] The elements of an array are always initialized to 0 by type boolean arrays are initialized to false char initializes to the null character Any reference type initializes to null 8

9 Instant Check Write the statement(s) necessary to instantiate: - a boolean type honesty with size 7 - a short type days with size 365 9

10 Bound Checking in Arrays The length of an array is how many cells it has. An array of length N has cells indexed 0..(N-1) Indexes must be an integer type greater than or equal to 0. If an expression is always illegal it will be caught at compile time However, if an index is found to be illegal during execution an exception message will appear and the program is terminated 10

11 Bound Checking in Arrays - Examples Given: int[] data = new int[10]; Example Outcome data[ -1 ] always illegal index must be >= 0 data[ 10 ] data[ 1.5 ] data[ 0 ] data[ 9 ] data[ j ] illegal out of range (given the above declaration) always illegal must be an integer always OK OK (given the above declaration) can't tell (depends on the value of j) 11

12 Using an Expression as an Index class ArrayExample public static void main ( String[] args ) double[] val = new double[4]; val[0] = 0.12; val[1] = 1.43; val[2] = 2.98; int j = 3; System.out.println( "cell 4: " + val[ j ] ); System.out.println( "cell 3: " + val[ j-1 ] ); j = j-2; System.out.println( "cell 2: " + val[ j ] ); 12

13 Copying Values from Cell to Cell public class ArrayExample public static void main ( String[] args ) int[] vala = 12, 23, 45, 56 ; int[] valb = new int[4]; valb[ 0 ] = vala[ 0 ] ; valb[ 1 ] = vala[ 1 ] ; valb[ 2 ] = vala[ 2 ] ; valb[ 3 ] = vala[ 3 ] ; 13

14 Arrays and Methods(1) Passing an Array to a Method An array reference can be passed as a parameter to a method This allows all the current values of the array to be obtained for use by the method Example: Method Header: public static void swap(int[] x, int i, int j) In the body of Main: int[] list = 1,3,5,7,9; swap(list, 2, 4) sends the address of the array list to x, as well as the values of i and j Within the method x can reference 5 successive integers, the same as those in list. It is pointing to the same place. Therefore it can change that array. 14

15 Arrays and Methods(2) Passing an Array to a Method import java.util.scanner; public class ArraySwapper public static void swap(int[] x, int i, int j) int temp = x[i]; x[i] = x[j]; x[j] = temp; public static void main(string[] args) int [] list = 1,3,5,7,9; System.out.print("Before: "); for (int i=0; i<list.length; i++) System.out.print(list[i] + " "); swap(list,2,4); System.out.print("\nAfter:"); for (int i=0; i<list.length; i++) System.out.print(list[i] + " "); 15

16 Arrays and Methods (3) Passing an Array from a Method The value of the method can be a reference to an array created in the method If the method returns a reference then it must return the reference to an array of like type. However, you don t need to create a new array, just declare the reference variable. 16

17 Arrays and Methods (4) Passing an Array from a Method public class ArrayPassing public static double[] returnarray( ) double[] x; x = new double[3]; // Create an array of 3 elements x[0] = 2.3; x[1] = 3.4; x[2] = 4.5; return( x ); // Return the **reference** (location) of the array public static void main (String[] args) double [ ] a; a = returnarray(); for (int i=0; i < 3; i++) System.out.println(a[i]); 17

18 Using Arrays (1) Populating an Array public class ArrayDemo public static void main(string[] args) int[] anarray; // DECLARE a reference for an array of integers anarray = new int[10]; // CREATE an array of integers // assign a value to each array element for (int i = 0; i < anarray.length; i++) anarray[i] = i; // print a value from each array element for (int i = 0; i < anarray.length; i++) //anarray.length is variable equal to the size of the array System.out.print(anArray[i] + " "); System.out.println(); 18

19 Using Arrays (2) Populating an Array with a RunTime Array Length import java.util.*; public class ArrayDemo2 public static void main(string[] args) int size; int[] anarray; // DECLARE a reference for an array of integers Scanner keyboard = new Scanner(System.in); System.out.print("Input size of the array"); size= keyboard.nextint(); anarray = new int[size]; // CREATE an array of integers // assign a value to each array element for (int i = 0; i < anarray.length; i++) anarray[i] = i; // print a value from each array element for (int i = 0; i < anarray.length; i++) //anarray.length is variable equal to the size of the array System.out.print(anArray[i] + " "); System.out.println(); Deciding the size of the array in real-time allows the program to be more efficient and flexible it is data driven rather than hard-coded 19

20 Using Arrays (3) Finding The Maximum Number in an Array import java.util.*; public class ArrayMax public static void main(string[] args) int size, max_value; int[] anarray; // DECLARE an array of integers Scanner keyboard = new Scanner(System.in); System.out.print("Input size of the array: "); size= keyboard.nextint(); anarray = new int[size]; // CREATE an array of integers dynamically //read in an array for (int i=0; i < anarray.length; i++) System.out.println("Enter integer value for anarray(" +i +")"); anarray[i] = keyboard.nextint(); max_value= anarray[0]; for (int i = 1; i < anarray.length; i++) if (anarray[i] > max_value) max_value = anarray[i]; // print the maximum System.out.print("The maximum value of the array is: " +max_value); Now write a program to find the minimum value in an array. Then write a program that sums the numbers in an array 20

21 Using Arrays (4) Finding The Average of the Numbers in an Array import java.util.*; public class ArrayAverage public static void main(string[] args) int size, sum; long average; int[] anarray; // DECLARE an array of integers Scanner keyboard = new Scanner(System.in); System.out.print("Input size of the array: "); size= keyboard.nextint(); anarray = new int[size]; // CREATE an array of integers dynamically //read in an array for (int i=0; i < anarray.length; i++) System.out.println("Enter integer value for anarray(" +i +")"); anarray[i] = keyboard.nextint(); sum= 0; for (int i = 0; i < anarray.length; i++) sum = sum + anarray[i]; average = Math.round((double)(sum)/anArray.length); System.out.print("The average value of the array is: " +average); 21

22 Using Arrays (5) Sorting import java.util.*; public class ArraySorter public static void main(string []args) int size, swap; Scanner in = new Scanner(System.in); System.out.println("Input number of integers to sort"); size = in.nextint(); int array[] = new int[size]; System.out.println("Enter " + size + " integers"); for (int c = 0; c < size; c++) array[c] = in.nextint(); for (int c = 0; c < ( size - 1 ); c++) for (int d = 0; d < size - c - 1; d++) if (array[d] > array[d+1]) /* For descending order use < */ swap = array[d]; array[d] = array[d+1]; array[d+1] = swap; System.out.println("Sorted list of numbers"); for (int k = 0; k < size; k++) System.out.println(array[k]); 22

23 Using Arrays (6) - How Does the Insertion Method Work Assume the list is array[0] =15 array[1] = 14 array[2] = 13 array[3] = 12 after first round we have after second round after third round c d array[d] array[d+1] swap

24 Using Arrays (7) Search Linear Search Linear search is a method for searching one by one through a list until you find a match It is somewhat laborious For larger lists it becomes very inefficient Review ArrayLinearSearch.java for example Binary Search Requires that the list is ordered you can order the list first Simply keep dividing the list in half until you find the number Review ArrayBinarySearch.java 24

25 Two-Dimensional Arrays A two dimensional array is like a table or a matrix It has rows and columns In java, both the rows and columns are indexed starting at zero A two-dimensional array is actually an array of arrays As with one-dimensional arrays all values of the array must be of the same type The reference is always first by row and then column - array [row][column] Array [0][0] Array [0][1] Array [0][2] Array [0][3] Array [1][0] Array [1][1] Array [1][2] Array [1][3] Array [2][0] Array [2][1] Array [2][2] Array [2][3] Array [3][0] Array [3][1] Array [3][2] Array [3][3] Array [4][0] Array [4][1] Array [4][2] Array [4][3] 25

26 Declaring Two-Dimensional Arrays (1) Both the two-step and one step methods are available Two-steps: int[][] table; table = new int[2][3] One step: int[][] table = new int[2][3] It can be processed by row or by column Either way it looks like this, with all values initialized to 0:

27 Declaring Two-Dimensional Arrays (2) You can also explicitly declare a matrix by rows: int[][] gradetable = 99, 42, 74, 83, 100, 90, 91, 72, 88, 95, 88, 61, 74, 89, 96, 61, 89, 82, 98, 93, 93, 73, 75, 78, 99, 50, 65, 92, 87, 94, 43, 98, 78, 56, 99 Using this method you can create rows with uneven numbers of entries (advanced subject) row Col

28 Simple Example of a Useful Table Student Week The table has 7 rows and 5 columns the headers are not part of this table 28

29 Finding the Number of Rows and Columns of Arrays With Unequal Length The length of a two-dimensional array is its number of rows Array.length The number of columns in a row is Array[row].length In matrix below: Array.length = 5 Array[0].length = 4; Array[1].length = 3; Array[2].length = 4; Array[3].length = 2; Array[4].length = 4; Array [0][0] Array [0][1] Array [0][2] Array [0][3] Array [1][0] Array [1][1] Array [1][2] Array [2][0] Array [2][1] Array [2][2] Array [2][3] Array [3][0] Array [3][1] Array [4][0] Array [4][1] Array [4][2] Array [4][3] 29

30 Declaring An Array With Unequal Column Entries int [][] array = new int [4][]; array[0] = new int[5]; array[1] = new int[1]; array[2] = new int[1]; array[3] = new int[4]; Remember We are declaring an Array of Arrays 30

31 Declaring Two-Dimensional Arrays With Uneven Numbers of Columns during Population You can also explicitly declare a matrix by rows: int[][] gradetable = 99, 42, 74, 90, 91, 72, 88, 95, 88, 61, 89, 82, 98, 93, 73, 75, 78, 99, 50, 65, 92, 87, 94, 43, 98 Using this method you can create rows with uneven numbers of entries (advanced subject) Table.length = 7 Table[0].length = 3 Table[1].length = 5 Table[2].length = 1 Table[3].length = 4 Table[4].length = 5 Table[5].length = 5 Table[6].length = 2 row Col

32 Printing a Two-Dimensional Array class ArrayPrint2D public static void main( String[] args ) // declare and construct a 2D array int[][] myarray = 1, 9, 4, 6, 0, 2,4,5, 0, 1, 2, 3 ; // print out the array for ( int row=0; row < myarray.length; row++ ) System.out.print("Row " + row + ": "); for ( int col=0; col < myarray[row].length; col++ ) System.out.print( myarray[row][col] + " "); System.out.println(); 32

33 Programming Exercises Class (1) Exercise 1 Array Data Write two methods that read data from the console and store the data in an array: a. The method int readdata(int[] x) reads a list of at most 100 integers into the array x. A sentinel, -999, terminates the list. The method returns the size of the list. b. The method int[] readdata reads and returns a list of integers. The list is preceded by the number of the items in the list. For example, the data indicates that there are six items in the list. The leading 6 is not included in the list. Test both these methods within a single program that includes a method: void printlist(int [] x, int n) that displays x(0) through x(n-1) 33

34 Programming Exercises Class (2) Exercise 14 Max Sort Implement a method void maxsort(int[] x, int size) // that sorts the partially filled array x. The method maxsort( ) first determines the largest value in x and swaps that value with x[size-1]; then maxsort( ) finds the next largest value and swaps that value with x[size- 2], and so on. Include an auxiliary method int max(int[] x, int i) that returns the index of the largest element between x[0] and x[i] inclusive. Test your methods in a program. 34

35 Programming Exercises Lab (1) Exercise 9 Largest and Smallest Design a method that determines the largest and smallest values stored in an integer array x. Your method should return these values in an array of length 2. Use the following algorithms: Initialize variables currentbig and currentsmall to the larger and smaller values of x[0] and x[1]. Process the rest of the list two elements at a time. Compare the larger of the two elements to currentbig and replace currentbig if necessary. Compare the smaller of the two elements to currentsmall and replace currentsmall if necessary. Test your method in a program and include a method that reads a list, terminated by -999, into an array. 35

36 Programming Exercises Lab (2) Exercise 15 Selection Sort Implement a method void selectionsort(int[ ] x, int size) // (size <= x.length) that sorts the partially filled array x. The method selectionsort( ) first determines the smallest value in x and swaps that value with x[0]; then selectionsort( ) finds the next smallest value and swaps that with x[1] and so on. Include an auxiliary method int min(int[ ] x, int i) that returns the index of the smallest element between x[i] and x[size-1], inclusive. Test your methods in a program. 36