Selection sort works by iteratively finding the minimum/maximum element from the unsorted portion of the list and placing it at the end/beginning of the sorted portion. It does this by scanning the unsorted portion to find the minimum/maximum element, swapping it with the element in the last position of the unsorted portion, and moving it to the sorted portion. This process is repeated until the entire list is sorted.

1. Selection Sort

2. Selection Sort: Idea
1. We have two group of items:
 sorted group, and
 unsorted group
1. Initially, all items are in the unsorted
group. The sorted group is empty.
 We assume that items in the unsorted
group unsorted.
 We have to keep items in the sorted group
sorted.

3. Selection Sort: Cont’d
1. Select the “best” (eg. smallest) item
from the unsorted group, then put the
“best” item at the end of the sorted
group.
2. Repeat the process until the unsorted
group becomes empty.

4. Selection Sort Example
35 65 30 60 20 scan 0-4,
smallest 20
swap 35 and 20
20 65 30 60 35 scan 1-4, smallest
30
swap 65 and 30
20 30 65 60 35 scan 2-4, smallest
35
swap 65 and 35
Chapter 10: Sorting
4

5. Selection Sort
5 1 3 4 6 2
Comparison
Data Movement
Sorted

6. Selection Sort
5 1 3 4 6 2
Comparison
Data Movement
Sorted

7. Selection Sort
5 1 3 4 6 2
Comparison
Data Movement
Sorted

8. Selection Sort
5 1 3 4 6 2
Comparison
Data Movement
Sorted

9. Selection Sort
5 1 3 4 6 2
Comparison
Data Movement
Sorted

10. Selection Sort
5 1 3 4 6 2
Comparison
Data Movement
Sorted

11. Selection Sort
5 1 3 4 6 2
Comparison
Data Movement
Sorted

12. Selection Sort
5 1 3 4 6 2
Comparison
Data Movement
Sorted

Largest

13. Selection Sort
5 1 3 4 2 6
Comparison
Data Movement
Sorted

14. Selection Sort
5 1 3 4 2 6
Comparison
Data Movement
Sorted

15. Selection Sort
5 1 3 4 2 6
Comparison
Data Movement
Sorted

16. Selection Sort
5 1 3 4 2 6
Comparison
Data Movement
Sorted

17. Selection Sort
5 1 3 4 2 6
Comparison
Data Movement
Sorted

18. Selection Sort
5 1 3 4 2 6
Comparison
Data Movement
Sorted

19. Selection Sort
5 1 3 4 2 6
Comparison
Data Movement
Sorted

20. Selection Sort
5 1 3 4 2 6
Comparison
Data Movement
Sorted

Largest

21. Selection Sort
2 1 3 4 5 6
Comparison
Data Movement
Sorted

22. Selection Sort
2 1 3 4 5 6
Comparison
Data Movement
Sorted

23. Selection Sort
2 1 3 4 5 6
Comparison
Data Movement
Sorted

24. Selection Sort
2 1 3 4 5 6
Comparison
Data Movement
Sorted

25. Selection Sort
2 1 3 4 5 6
Comparison
Data Movement
Sorted

26. Selection Sort
2 1 3 4 5 6
Comparison
Data Movement
Sorted

27. Selection Sort
2 1 3 4 5 6
Comparison
Data Movement
Sorted

Largest

28. Selection Sort
2 1 3 4 5 6
Comparison
Data Movement
Sorted

29. Selection Sort
2 1 3 4 5 6
Comparison
Data Movement
Sorted

30. Selection Sort
2 1 3 4 5 6
Comparison
Data Movement
Sorted

31. Selection Sort
2 1 3 4 5 6
Comparison
Data Movement
Sorted

32. Selection Sort
2 1 3 4 5 6
Comparison
Data Movement
Sorted

33. Selection Sort
2 1 3 4 5 6
Comparison
Data Movement
Sorted

Largest

34. Selection Sort
2 1 3 4 5 6
Comparison
Data Movement
Sorted

35. Selection Sort
2 1 3 4 5 6
Comparison
Data Movement
Sorted

36. Selection Sort
2 1 3 4 5 6
Comparison
Data Movement
Sorted

37. Selection Sort
2 1 3 4 5 6
Comparison
Data Movement
Sorted

38. Selection Sort
2 1 3 4 5 6
Comparison
Data Movement
Sorted

Largest

39. Selection Sort
1 2 3 4 5 6
Comparison
Data Movement
Sorted

40. Selection Sort
1 2 3 4 5 6
Comparison
Data Movement
Sorted
DONE!

41. 4240 2 1 3 3 4 0 -1 655843
40 2 1 43 3 4 0 -1 42 65583
40 2 1 43 3 4 0 -1 58 3 6542
40 2 1 43 3 65 0 -1 58 3 42 4
Selection Sort: Example

42. 4240 2 1 3 3 4 0 655843-1
42-1 2 1 3 3 4 0 65584340
42-1 2 1 3 3 4 655843400
42-1 2 1 0 3 4 655843403
Selection Sort: Example

43. 1
42-1 2 1 3 4 6558434030
42-1 0 3 4 6558434032
1 42-1 0 3 4 6558434032
1 420 3 4 6558434032-1
1 420 3 4 6558434032-1
Selection Sort: Example

44. Selection Sort: Analysis

Running time:
− Worst case: O(N2
)
− Best case: O(N2
)