Arrays Useful methods

1. Java.util.Arrays class Arrays Useful methods

2. Use of ready methods to manipulate arrays. • java.lang.System class • java.util.Arrays class

3. Java Arrays – Copyingarraycopy() • The class java.lang.System contains a method arraycopy that copies array efficiently. int array1[] = new int[10]; int array2[] = new int[10]; //copy array1 into array2 System.arraycopy(array1, 0, array2, 0, 10); //copy last 5 elements in array1 into first 5 of array2 System.arraycopy(array1, 5, array2, 0, 5);

4. Example arraycopy(Object src, int srcPos, Object dest, int destPos, int length) • arraycopy(Object src, int srcPos, Object dest, int destPos, int length) class CopyArray{ public static void main(String [] args){ int [] array1 = {1,2,3,4,5,6,7,8,9,10}; int [] array2 = new int[10]; //copy array1 into array2 System.arraycopy(array1, 0, array2, 0, 10); for(int i = 0; i<10; i++) System.out.println(array2[i]); } } Output 1 2 3 4 5 6 7 8 9 10

5. Example class CopyArray{ public static void main(String [] args){ int [] array1 = {1,2,3,4,5,6,7,8,9,10}; int [] array2 = new int[10]; //copy array1 into array2 System.arraycopy(array1, 0, array2, 0, 10); //copy last 5 elements in array1 into first 5 of array2 System.arraycopy(array1, 5, array2, 0, 5); for(int i = 0; i<10; i++) System.out.println(array2[i]); } } Output 6 7 8 9 10 6 7 8 9 10

6. Java Arrays – Sorting • java.util.Arrays class contain lots of useful methods to deal manipulate and searching for searching for data in an array. • i.e. sorting array of integers into ascending order int myArray[] = {5, 4, 3, 2, 1}; java.util.Arrays.sort(myArray); //myArray now holds 1, 2, 3, 4, 5

7. ExampleSorting an array of integers import java.util.*; class sort{ public static void main(String [] args){ int [] array1 = {10, 5, 6, 1, 3}; // this will sort array1 in ascending order Arrays.sort(array1); for(int i = 0; i<5; i++) System.out.println(array1[i]); } } Output 1 3 5 6 10

8. ExampleSorting an array of strings import java.util.*; class sort{ public static void main(String [] args){ String [] array1 = {"ba", "ad", "acd", "efg", "eag"}; // this will sort array1 in ascending order Arrays.sort(array1); for(int i = 0; i<5; i++) System.out.println(array1[i]); } } Output acd ad ba eag efg

9. Java Arrays – fill int myArray[] = new int[5]; java.util.Arrays.fill(myArray, 10); //myArray now holds 10, 10, 10, 10, 10 java.util.Arrays.fill(myArray, 1,3, 20); //myArray now holds 10, 20, 20, 10, 10

10. Example import java.util.*; class CopyArray{ public static void main(String [] args){ int [] array1 = new int[5] //fill array1 the value 10 Arrays.fill(array1, 10); for(int i = 0; i<5; i++) System.out.println(array1[i]); } } Output 10 10 10 10 10

11. Example import java.util.*; class CopyArray{ public static void main(String [] args){ int [] array1 = new int[5] //fill array1 the value 10 Arrays.fill(array1, 10); //fill array1 from the position the indetx 1 to 2(3-1) with the value 10 Arrays.fill(array1, 1, 3, 20); for(int i = 0; i<5; i++) System.out.println(array1[i]); } } Output 10 20 20 10 10

12. Java Arrays – binary search int myArray[] = {1,2,3,4,5,6,7,8,9,10} java.util.Arrays.binarysearch(myArray, 10); //myArray now holds 10, 10, 10, 10, 10

13. Example binarySearch(Object[] a, Object key) import java.util.*; class binarysearch{ public static void main(String [] args){ int [] array1 = {10, 5, 6, 1, 3}; // this will sort array1 in ascending order Arrays.sort(array1); // prints 0 position of the 1st occurrence of 1 System.out.println(Arrays.binarySearch(array1,1)); //prints 1 position of the 1st occurrence of 3 System.out.println(Arrays.binarySearch(array1,3)); // 20 does not exists (prints a negative number) System.out.println(Arrays.binarySearch(array1,20)); } }

14. ExamplebinarySearch(Object[] a, int fromIndex, int toIndex, Object key) Import java.util.*; class binarysearch{ public static void main(String [] args){ int [] array1 = {10, 5, 6, 1, 3}; // this will sort array1 in ascending order Arrays.sort(array1); // prints 4 position of the 1st occurrence of 10 System.out.println(Arrays.binarySearch(array1,0,5,10)); //searching from position 1 to 3 positions System.out.println(Arrays.binarySearch(array1,1,4,10)); } } Output 4 -4

15. Example Binary Search for array of strings import java.util.*; class binarysearchString{ public static void main(String [] args){ String [] array1 = {“ba”, “ab”, “aa”, “def”, “daf”}; // this will sort array1 in ascending order Arrays.sort(array1); // prints 0 position of the 1st occurrence of “aa” System.out.println(Arrays.binarySearch(array1,”aa”)); //prints 1 position of the 1st occurrence of “ab” System.out.println(Arrays.binarySearch(array1,”ab”)); // “Golsmiths” not in array1 (prints a negative number) System.out.println(Arrays.binarySearch(array1, “Goldmisths”)); } }

16. Java.util.Arrays class • A number of useful methods to manipulate arrays are defined in this class. • For more information see the following link: http://docs.oracle.com/javase/6/docs/api/java/util/Arrays.html

17. Java Arrays • Advantages • Very efficient, quick to access and add to • Type-safe, can only add items that match the declared type of the array • Disadvantages • Fixed size, some overhead in copying/resizing • Can’t tell how many items in the array, just how large it was declared to be • Limited functionality, need more general functionality

18. Exercise • Write a program that stores the value 20 into all elements of an array is integers.

19. Exercise • Write a program that copies the first 5 element of an array of integer of length 10 into the last 5 element of another array of the same length.

20. Practise exercisesSwapAll.java • Write a code that takes two arrays of integers of the same length and swaps their entire contents.

21. Practise exercisesSameArray.java • Write a code that checks if two arrays of integers of the same length are the same.

22. Practise exercisesAscendingInt.java • Write a code that sorts an array of integer in ascending order.

23. Practise exercisesAscendingString.java • Write a code that sorts an array of strings in ascending order.

24. Practise exercises IntegerIn.java • Write a code that checks if an integer is contained in array of integers.

25. Practise exercises StringIn.java • Write a code that checks if a string is contained in array of strings.

26. Search an array of strings • Given the following declaration • String[] cities = {"Washington", "London", "Paris", "New York"}; • Write a program that uses a binary search to search for a particular city is contained in the array cities.

27. import java.util.*; public class CityBinarySearch { public static void main(String [] args) { String[] cities = new String[]{"Washington", "London", "Paris", "New York"}; //sorting array in java Arrays.sort(cities); //searching on sorted array in java using Arrays binarySearch() method if(Arrays.binarySearch(cities, "Paris") >=0 ){ System.out.println("Element found on sorted String "+ "array using binary search"); } } }

28. Practise exercisesMinGap.java • Write a code that takes an array of integers and prints out the minimum 'gap' between adjacent values in the array. The gap between two adjacent values in a array is defined as the second value minus the first value. If the length of the array is less 2, the program prints out 0.

29. Summary • Example of useful methods defined in • java.util.Arrays class • Practise exercise