Array Sorting: Bubble and Selection Sort Algorithms

1. Agenda • About the homework EvensAndOdds • Array class • Array Sorting Bubble sort Selection sort • practice

2. Remarks on the homework • If you have 2 arrarys, one for even, one for odds, you will find some elements in both arrays have no data and initialized to 0. • How to make the program better?

3. Array class • Built in class and ready for use • Provided in the java.util.Arrays • Sort method(function) syntax Arrays.sort(<array name>); • BinarySearch syntax Arrays.binarySearch(<arry>, key); => for this to work right, precondition: Array has to be sorted in ascending order

4. Practice int[] bArry = {16, 2, 31, 9}; System.out.println("array before sorting: "); for(int dummy : bArry) System.out.print(dummy+ " "); System.out.println(" "); Arrays.sort(bArry); System.out.println("array before sorting: "); for(int dummy : bArry) System.out.print(dummy+ " ");

5. Bubble Sort Original data Compare the shaded pair. 4>2, so swap Swap completed Compare the shaded pair. No swap needed Compare the shaded pair. 5>1, so swap Swap completed. Compare the shaded pair. 5>3, so swap Swap completed

6. selection Sort Original data 1st pass: select smallest value in gray area just above.. It’s 1. Then 1 and 4 have now been swapped. 2nd pass: select smallest value in gray area just above.. It’s 2. No swap necessary since the 2 above is less than 3. 3rd pass: select smallest value in gray area just above.. It’s 3. Then 3 and 5 have now been swapped. 4th pass: select smallest value in gray area just above.. It’s 5. No swap necessary since the 4 is less than 5.

7. Swap method • Original values p = 5; q = 97; • Swapped values p = 97; q=5; • How? temp = p; p = q; q = temp; temp <= p is 5 5 p <= q is 97 97 q <= temp 5

8. selection Sort pseudocode for(arrayIndex = 0 to numItems -1 ) { for(subarrayIndex = arrayIndex to numItems-1) { if (items[subarrayIndex] < items[arrayIndex]) { swap items[arrayIndex] and items[arrayIndex] } } }

9. Practice Run the selection sort coded on the last slide with the Tester class on page 41. Turn in the work before the break