Notes

Outline

COM1100 Fundamentals of Computer Science –Winter 2000 Lecture 19 02/28/00

Lecture today Array Sort an array Make all elements in an array in ascending or descending order Sort algorithm Selection Sort Bubble Sort

Selection Sort In a selection sort, the smallest value in a list is initially selected from the complete list of data and exchanged with the first element in the list. After this first selection and exchange, the next smallest element in the revised list is selected and exchanged with the second element in the list. Since the smallest element is already in the first position in the list, this second pass need only consider the second through the last elements.

Example of Selection Sort

Selection Sort Implementation We will implement three functions void swap(int& x, int& y); int smallestElement(int list[], int size, int startIndex); int SelectionSort(int list[], int size);

Function swap() // swap the values of two integer variable void swap(int& x, int& y) { int temp; temp = x; x = y; y = temp; }

Function smallestIndex // return the index of the smallest value // startIndex indicates the starting element int smallestElement(int list[], int size, int startIndex) { int smallIndex = startIndex; for (int i = startIndex + 1; i < size; i++) { if (list[smallIndex] > list[i]) smallIndex = i; } return smallIndex; }

Function SelectionSort // This function will sort a given array in ascending order // and it will also return how many exchanges needed in the process int SelectionSort(int list[], int size) { int move = 0;    int smallIndex;   for(int i = 0; i < size; i++) { smallIndex = smallestElement(list, size, i); if ( smallIndex != i) { move++; swap(list[i], list[smallIndex]); } } return move; }

Function call int list[]= {690, 307, 32, 155, 426}; cout << "The moves :" << SelectionSort(list,5)  << endl; for(int i = 0; i < 5; i++) cout << list[i] << "  " ; cout << endl;

Bubble Sort In a Bubble Sort, we need N-1 passes. On each pass we visit each element, and check whether that element and its immediate neighbor are in order. If they are out of order we switch them before going on. After the first pass through the list at least the largest element will be put in its proper place. After the second pass, at least the largest two elements will be in the right place, and so on. We repeat the process until the whole list is sorted.

Bubble Sort Exchange adjacent pairs of elements that are out of order Largest value “sinks” to the bottom each pass, while smaller values gradually “bubble” up to the top

Example of BubbleSort Initial array(list) : 690   307   32   155   426 Pass 1 690   307  | 32   155   426 307   690   32 |  155   426 307     32   690  155 | 426 307     32   155   690   426 | 307     32   155   426   690 Pass 2 307     32 | 155   426   690  32     307  155 | 426   690  32     155  307   426   690 (already in order)

Function BubbleSort() // This function will sort a given array in ascending order // and it will also return how many exchanges made in the process int BubbleSort(int list[], int size) { int move = 0; // move will record how many exchanges needed // totally, there will be size -1 passes for(int i = 0; i < size -1; i++) { //in each pass, there will be size-1 comparison for(int j =1; j < size; j++) { // check if exchange is needed if (list[j] < list[j-1]){ swap(list[j], list[j-1]); move++;   } } } return move; }

Function call int list[]= {690, 307, 32, 155, 426}; cout << "The moves :" << BubbleSort(list,5)  << endl; for(int i = 0; i < 5; i++)      cout << list[i] << "  " ; cout << endl;