Selection Sort and Quick Sort

1
Selection Sort and Quick Sort

• Instructor Mainak Chaudhuri
• mainakc_at_cse.iitk.ac.in

2
Selection sort

• class SelectionSort
• public static void main (String arg)
• int size 20
• double array new doublesize
• Initialize (array, size) // not shown
• PrintArray (array, size) // not shown
• Sort (array, size)
• PrintArray (array, size) // not shown

3
Selection sort

• public static void Sort (double array, int
  size)
• int i, j, minIndex
• for (i0iltsize-2i)
• minIndex i
• for (ji1jltsizej)
• if (arrayj lt arrayminIndex)
• minIndex j
• Swap (array, minIndex, i)
• // Invariant array0 to arrayi is
  sorted

4
Selection sort

• public static void Swap (double array, int
  p, int q)
• double temp
• temp arrayp
• arrayp arrayq
• arrayq temp
• // end class (How many comparisons?)

5
Selection sort

• Better than bubble sort by a constant factor
• Less number of assignments
• Asymptotically both are O(n2)

6
Quick sort

• Pick a pivot element and put it in its place
• All elements to its left are smaller and all to
  its right are bigger
• Recursively sort the left half and the right half
• Best case is O(nlogn), same as merge sort
• Worst case is O(n2)
• Recall that the worst case time for merge sort is
  O(nlogn)

7
Quick sort

• class QuickSort
• public static void main (String arg)
• int n 20
• double array new doublen
• Initialize (array, n) // not shown
• Sort (array, 0, n-1)
• // continued on next slide

8
Quick sort

• public static void Sort (double array, int
  start, int end)
• int pivot
• if (start lt end)
• if (start end-1)
• if (arraystart gt arrayend)
• Swap (array, start, end)
• else
• pivot Partition (array, start,
  end)
• Sort (array, start, pivot-1) //
  left half
• Sort (array, pivot1, end) //
  right half
• // continued in next slide

9
Quick sort

• public static int Partition (double array,
  int start, int end)
• int pivot start
• int downstream start1
• int upstream end
• while (true)
• while ((downstream lt end)
  (arraydownstream lt arraypivot))
• downstream
• // continued in next slide

10
Quick sort

• while ((upstream gt start)
  (arrayupstream gt arraypivot))
• upstream--
• if (downstream lt upstream)
• Swap(array, downstream, upstream)
• else
• break
• Swap (array, pivot, upstream)
• return upstream
• // continued in next slide

11
Quick sort

• public static void Swap (double array, int
  p, int q)
• double temp
• temp arrayp
• arrayp arrayq
• arrayq temp
• // end class